Laser pulse generation system

Abstract

Systems, methods, and apparatus are described for a laser pulse generation system including at least one electron linear accelerator configured to accelerate electrons from an electron source, a first undulator arranged along a first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission, an optical cavity at least partially overlapping with the first undulator and configured to confine the first electromagnetic emission within the optical cavity along a first optical path, and a second undulator arranged along a second electron beam path different from the first electron beam path, the second undulator being configured to cause electrons moving along the second electron beam path to emit a second electromagnetic emission.

Description

[0001] Attorney Docket No. 58110-0002W01

[0002] LASER PULSE GENERATION SYSTEM

[0003] CROSS-REFERENCE TO RELATED APPLICATIONS

[0004] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 690,036, filed on September 03, 2024, the contents of which are hereby incorporated by reference.

[0005] TECHNICAL FIELD

[0006] This specification relates to laser systems.

[0007] BACKGROUND

[0008] High power lasers are an important class of energy sources for delivering a large amount of energy concentrated in space and / or time. Progress in laser technology has resulted in lasers having very high average power, for example 100 kW or higher, but which usually operate in continuous-wave or quasi-continuous-wave operation mode, where the peak power is relatively low. On the other hand, high pulse energy or extremely high peak power lasers, for example, petawatt class lasers, generally only operate with low repetition rates, e.g., a few kHz or less.

[0009] For many applications in advanced manufacturing and fabrication, space, and other fields, having both high average power and pulse energy are needed. For example, extreme ultraviolet (EUV) lithography is a lithography technology for manufacturing state-of-the-art integrated circuits that uses short wavelength ultraviolet radiation (e.g., approximately 13 nm) to transfer a pattern from a mask to a resist layer on a wafer. Generating EUV radiation typically involves using an infrared (IR) laser source, e.g., a carbon dioxide (CO2) laser source, to initiate a pulsed plasma source that emits 13 nm light. To support high throughput manufacturing, e.g., >100 wafers exposed per hour, the lithography system should have kilowatt-level of average EUV power. Considering the conversion efficiency from IR to EUV is typically only a few percent, the IR pump source should provide lOOkW level of average power output. At the same time, to produce the hot-and-dense plasma that has a temperature sufficiently high to emit EUV light, the IR pump laser source should be pulsed into, e.g., 1-Joule-level of pulses in 10-100 nanosecond duration. Such light sources are generally costly and energy inefficient. Moreover, the infrared laser source can have limited Attorney Docket No. 58110-0002W01 tunability. As such, the plasma process is often limited to materials compatible to the midinfrared wavelengths of the CO2 laser source, e.g., tin, which can further produce contamination issues in the manufactured integrated circuits.

[0010] SUMMARY

[0011] This specification describes technologies for a laser pulse generation system. These technologies generally involve a free electron laser-based system as a high average power, high pulse energy, energy efficient, small scale, and tunable light source. In some implementations, the system can be used as a laser source for a secondary processing system. For example, rather than using the free electron laser (FEL)-based system as a direct laser source, the technology described can be used as a pump source for a plasma-based system for producing EUV wavelengths, where the FEL-based laser pulse generation system can be more compact in the micron wavelength range (e.g., mid-IR emission) than an FEL-based system for directly generating EUV wavelengths. This can reduce an overall size of the laser system, e.g., by shortening the length of linear accelerator to a few meters.

[0012] In general, one innovative aspect of the subject matter described in this specification can be embodied in a laser pulse generation system including at least one electron linear accelerator configured to accelerate electrons from an electron source, a first undulator arranged along a first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission, an optical cavity at least partially overlapping with the first undulator and configured to confine the first electromagnetic emission within the optical cavity along a first optical path, a second undulator arranged along a second electron beam path different from the first electron beam path, the second undulator being configured to cause electrons moving along the second electron beam path to emit a second electromagnetic emission, at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along a second optical path different from the first optical path, at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path, at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path, and a pulse Attorney Docket No. 58110-0002W01 shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission.

[0013] The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In particular, one embodiment includes all the following features in combination. In some implementations, the first electron beam path and the second electron beam path are aligned along an overlap portion, and the at least one electron linear accelerator includes a first electron linear accelerator at a first node, and a second electron linear accelerator at a second node, where the first node is located along the overlap portion of the first electron beam path and the second electron beam path, and where the second node is located along the second electron beam path.

[0014] In some implementations, the at least one electron linear accelerator is arranged in the first electron beam path, second electron beam path, or both, between the electron source and at least one of the first undulator and the second undulator.

[0015] In some implementations, the at least one electron linear accelerator is configured to accelerate electrons along at least one of the first electron beam path and the second electron beam path to increase energy of the electrons. In some implementations, the at least one electron kicker coupled to the electron linear accelerator includes a first electron kicker arranged to divert electrons from the first electron linear accelerator to the first undulator along the first electron beam path or to the second undulator along the second electron beam path, and a second electron kicker arranged to divert electrons from the first undulator or the second undulator to the at least one electron linear accelerator along the overlap portion of the first electron beam path and the second electron beam path.

[0016] In some implementations, the at least one electron linear accelerator is configured to increase the energy of the electrons prior to the first undulator or the second undulator such that an exit electron energy of electrons at the second electron kicker meets an acceptance condition. The acceptance condition can include an exit electron energy of electrons at the second electron kicker that is within a threshold deviation of an entry electron energy of electrons at the first electron kicker.

[0017] In some implementations, the at least one electron linear accelerator includes an energy recovery linear accelerator configured to decelerate electrons along the overlap portion to decrease energy of the electrons. The energy recovery linear accelerator can be Attorney Docket No. 58110-0002W01 configured to capture energy from the decelerated electrons, and provide the captured energy to the RF field of the energy recovery linear accelerator.

[0018] In some implementations, the system further includes an energy recovery module arranged along the overlap portion and configured to decelerate electrons along the overlap portion to decrease energy of the electrons.

[0019] In some implementations, the pulse shaper is configured to shape laser pulses from a pulse train comprising at least one of the first electromagnetic emission and the second electromagnetic emission, wherein a frequency of the pulse train is selectable by the electron source. The pulse shaper can include a switch configured to divert a first portion of the pulse train along a first path and a second portion of the pulse train along a second, different path, where the first path and the second path comprise different, interleaved paths, and one or more optical elements arranged at converging point of the first and second paths, the one or more optical elements being configured to recombine the first portion of the pulse train and the second portion of the pulse train. The pulse shaper can be configured to receive, as input, the pulse train, and provide, as output, a macropulse comprising a plurality of micropulses of the pulse train.

[0020] In some implementations, the system further includes a controller, where the controller is in data communication with and configured to provide control signals to the at least one electron linear accelerator, electron source, first undulator, second undulator, at least one electron kicker, at least one optical coupler, and pulse shaper. The controller can be configured to provide control signals to cause the operations of generating the first electromagnetic emission confined in the optical cavity along the first optical path, including providing the electron beam comprising electrons along the first electron beam path, accelerating the electrons by the electron linear accelerator and along the first electron beam path, causing, by the first undulator arranged along the first electron beam path, the electrons along the first electron beam path to emit the first electromagnetic emission along the first optical path, confining, by the optical cavity, the first electromagnetic emission within the optical cavity about at least one pass of the optical cavity. The operations further include generating, using first electromagnetic emission, the second electromagnetic emission including providing, along the second optical path different from the first optical path, the first electromagnetic emission, selectively providing the electrons from the electron linear Attorney Docket No. 58110-0002W01 accelerator along the second electron beam path different from the first electron beam path, wherein at least a portion of the second electron beam path overlaps with the second optical path, and causing, by the second undulator arranged along the second electron beam path, the electrons along the second electron beam path to emit second electromagnetic emission along the second optical path, and providing at least one of the first electromagnetic emission and the second electromagnetic emission as the laser pulse output.

[0021] In some implementations, the operations further include providing, the laser pulse output to a pulse shaper, generating, by the pulse shaper and from at least one of the first electromagnetic emission and the second electromagnetic emission, a shaped pulse, and providing the shaped pulse. In some implementations, the operations of providing the shaped pulse includes providing, as input to at least one secondary system, the laser pulse output. Providing, as input to the at least one secondary system can include providing the shaped pulse as input to a plasma generation system. Providing, as input to the at least one secondary system can include dividing the shaped pulse into two or more divided outputs and providing each of the two or more divided outputs as inputs to a respective secondary system of two or more secondary systems.

[0022] In general, another innovative aspect of the subject matter described in this specification can be embodied in a pulse shaping device including a switch configured to divert a first portion of an electromagnetic (EM) pulse train along a first path and a second portion of the EM pulse train along a second, different path, where the first path and the second path comprise different, interleaved paths, and one or more optical elements arranged at a location where the first and second paths converge, the one or more optical elements being configured to recombine the first portion of the pulse train and the second portion of the pulse train.

[0023] In general, another innovative aspect of the subject matter described in this specification can be embodied in a laser pulse generation system including an electron linear accelerator configured to accelerate electrons from an electron source, a first electron beam path including a first undulator arranged along the first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission, a first optical path including an optical cavity at least partially overlapping with the first undulator and configured to confine the extracted first Attorney Docket No. 58110-0002W01 electromagnetic emission within the optical cavity, a second electron beam path including: a second undulator arranged along the second electron beam path and configured to cause electrons moving along the second electron beam path through the second undulator to emit a second electromagnetic emission, a second optical path different from the first optical path including at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along the second optical path, and a pulse shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission, at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path, and at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path.

[0024] In general, another innovative aspect of the subject matter described in this specification can be embodied in a laser pulse generation system including at least one electron linear accelerator configured to accelerate electrons from an electron source, a first undulator arranged along a first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission, an optical cavity at least partially overlapping with the first undulator and configured to confine the first electromagnetic emission within the optical cavity along a first optical path, a second undulator arranged along a second electron beam path different from the first electron beam path, the second undulator being configured to cause electrons moving along the second electron beam path to emit a second electromagnetic emission, at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along a second optical path different from the first optical path, at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path, and at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path. Attorney Docket No. 58110-0002W01

[0025] In general, another innovative aspect of the subject matter described in this specification can be embodied a laser pulse generation system including at least one electron linear accelerator configured to accelerate electrons from an electron source, an undulator arranged along a first electron beam path and configured to cause electrons moving along the electron beam path through the undulator to emit electromagnetic emission, an optical cavity at least partially overlapping with the undulator and configured to confine the electromagnetic emission within the optical cavity along a first optical path, at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to a second optical path, and a pulse shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission.

[0026] The technology described in this specification can be implemented so as to realize one or more of the following advantages. In some implementations, the system can be used a laser source for a secondary processing system. For example, the laser pulse generation system can be compatible with existing plasma-based systems in EUV scanners, e.g., tin- based plasma systems, as a laser pulse source for the plasma generation process.

[0027] As described in further detail below, the electromagnetic emission is tunable (e.g., infrared to visible wavelength emission) to provide additional benefits. For example, as a laser pump source for EUV applications, having tunability of the laser wavelength, e.g., tunability outside standard 10.6 microns produced by CO2 lasers, can provide higher conversion efficiency from IR to EUV wavelengths. It can also be compatible with other seed materials in addition to tin, where the material can be selected in part based on compatibility with a process, reduce contaminants, etc. for example, the system can be compatible with Xe-based plasma systems. In certain applications, tunability gives the ability to produce multiple working wavelengths from a single system, which can be highly desirable.

[0028] In some implementations, the systems and methods described in this specification can be implemented to achieve sufficiently high average power and high pulse energy, e.g., energy within the macropulse, to generate electromagnetic emission that are shorter wavelength than the EUV range, e.g., less than 10'8meters. For example, the techniques described can be applied to produce plasmas that can emit soft x-ray wavelengths (e.g., 10'9 Attorney Docket No. 58110-0002W01 meters), or can be used in a laser wake field accelerator (LWFA) to accelerate electrons to GeV (e.g., 109eV) levels to generate output laser pulses in the X-ray wavelength range or shorter (e.g., 10'9meters, 10‘10meters).

[0029] The systems described herein can be more compact than existing mid-infrared, high- power systems. With estimated size less than a few meters in maximum dimension, it is more compact than current CO2 systems. In some implementations, the systems described here can be fit in a single commercial shipping container, e.g., preassembled prior to shipping to its final destination.

[0030] The system can be configured to emit in the infrared while additionally meeting power requirements, e.g., 100 kW to ~MW range, to be used as a pump for laser-produced- plasma (LPP) source for EUV applications. The dual undulator design of the system described in this specification enables high average power as well as high pulse energy, e.g., Joule-level energy output, concentrated in a short-interval macropulse, e.g., 20-100 nanoseconds macropulse, where both are needed as input for LPP-based systems.

[0031] The dual undulator design of the system described in this specification can have a high extraction efficiency, e.g., 10-20%, of the accelerated electron beam, and can have higher system-level energy efficiencies with lower power requirements than comparable CO2 laser-based systems.

[0032] As described in further detail below, in certain implementations, a first, seed generation mode uses an untapered undulator to build up photon beam from noise, where the extraction efficiency of the seed generation mode is limited by FEL saturation, usually 1-2% or less. Much higher extraction efficiency, e.g., 15%-20%, is achieved in a second, shot mode where a tapered undulator is used together with the seed generated by the seed mode. By using low power electron beams in the seed generation mode and a higher, e.g., 5x, lOx, 15x, 20x or higher, electron beam in the shot mode, the overall energy extraction efficiency of the system approaches the level reached in the shot mode.

[0033] In addition, the system can deploy an energy recovery module to recover energy in the spent electrons leaving the undulators, therefore improving the overall system efficiency. The ultimate efficiency of the system can also depend on the energy recovery efficiency, e.g., recovery of energy from decelerated electrons. The energy recovery efficiency can be greater than 95%, for example, when using superconducting energy recovery linear Attorney Docket No. 58110-0002W01 accelerators (ERLs) and from reusing energy captured from recovered electrons. For example, by converting recovered energy into electricity to power operations of the system, or by capturing the recovered energy into the RF field of the ERL.

[0034] In some instances, the electron beam power during seed generation mode can be less than about 20% of the electron beam power during the shot mode. In an example case where energy recovery efficiency is about 90%, and 200 kW IR power is needed, the system can use 1 MW of electron beam power in the shot mode with 20% extraction efficiency to produce such IR power, while wasting about 100 kW due to energy recovery losses. For the seed generation mode, 250 kW electron beam power is used to produce 5kW of seed with 2% extraction efficiency, while wasting about 25 kW due to energy recovery losses. Overall, an example system as configured can produce 200 kW of output EM emission while wasting about 125 kW of power, with an overall efficiency of about 61%. Additionally, in instances where additional power, e.g., -200 kW of additional power, is used for system-level requirements, e.g., for RF cavity loss, cooling system, magnets, and electronic system, and the like, the overall efficiency can be about 40%.

[0035] In certain implementations, the system includes a pulse shaper to shape the output laser pulses. By using a pulse shaper, the system can increase an energy density of a macropulse without being limited by a frequency repetition rate of the electron source. Additionally, a pulse shaper can be used to generate a range of pulse shapes of the output of the system and can be used to adjust power output of the pulses of the system, e.g., to provide higher energy within a given time duration.

[0036] Although described herein primarily as a laser pulse generation system for providing laser pulses to plasma-based EUV applications, the systems and methods described in this specifications can be used for other applications where high average power, high energy, tunability (IR to visible) of wavelength output can be advantageous. For example, the systems and methods described here can be used for atmospheric science, e.g., atmospheric measurements, communications, space and / or satellite applications, or other applications where a high-power mid-infrared light source may be applicable. Additionally, the flexibility to tune the wavelength output of the system ranging from IR to visible wavelengths can be advantageous for applications benefiting from a spectrum of high average power, high energy wavelengths produced by the same system. Attorney Docket No. 58110-0002W01

[0037] The details of one or more embodiments of the subject matter of this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

[0038] BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. l is a schematic of an example laser pulse generation system.

[0040] FIGS. 2A and 2B are schematics of an example of a first stage of the laser pulse generation system.

[0041] FIGS. 3A and 3B are schematics of an example of a second stage of the laser pulse generation system.

[0042] FIG. 4 is a schematic of a pulse shaper of the laser pulse generation system.

[0043] FIG. 5 is a flow diagram of an example process of the laser pulse generation system.

[0044] FIG. 6 is a flowchart of an example process for generating the first electromagnetic emission confined in an optical cavity along a first optical path.

[0045] FIG. 7 is a flowchart of an example processing for generating, using the first electromagnetic emission, the second electromagnetic emission.

[0046] FIG. 8 is a flowchart of an example process 800 for generating a plasma using a laser pulse generation system.

[0047] FIG. 9 is a schematic of another example laser pulse generation system.

[0048] Like reference numbers and designations in the various drawings indicate like elements.

[0049] DETAILED DESCRIPTION

[0050] A laser pulse generation system includes a two-stage free electron laser configuration to form a first, seed stage and a second, shot stage, where the first stage is used for a “seed generation” mode operation of the system and the second stage is used for a “shot” mode operation of the system. In the first stage, in seed generation mode, the system operates as a free electron laser (FEL) oscillator generating an electromagnetic (EM) emission which is confined to an optical cavity. In the second stage, in shot mode, the system operates as a high-gain, single pass FEL amplifier where the accumulated EM emission from the first stage is amplified to provide high-powered laser emission. Attorney Docket No. 58110-0002W01

[0051] In general, the components described in this specification of the laser pulse generation system related to the electron generation, propagation, and recovery are invacuum components. For example, the electron gun, electron linear accelerators, undulators, electron deflection components, etc., can be all maintained within vacuum conditions. Moreover, the electron beam paths through which the electrons move through the laser pulse generation system are additionally contained within vacuum conditions. In some implementations, vacuum conditions are ultra-high vacuum (UHV) conditions, e.g., pressures below about 1x1 O’6Torr.

[0052] In some implementations, a vacuum system includes multiple vacuum chambers connected in vacuum with one or more pumping stages. The pumping stages can include, for example, turbo pumps, diaphragm pumps, ion pumps, cryogenic pumps, or the like. The vacuum chambers and / or pumping stages can be isolated from each other, e g., using isolation valves, gate valves, etc. The vacuum levels can be monitored using one or more pressure gauges, e.g., ion gauges. In some implementations, some or all of the vacuum chambers and / or in-vacuum components can be temperature regulated, e.g., cryogenically cooled. The vacuum system can include, for example, liquid nitrogen cooling, liquid helium- based cooling, or another form of cryogenic cooling. For instance, the undulators, linear accelerators, etc., of the laser pulse generation system may be cryogenically cooled during operation. In some instances, some or all of the components can be temperature-regulated using non-cryogenic cooling, e.g., water-cooling, gas-based cooling, etc.

[0053] In some implementations, some or all of the optical components described in this specification of the laser pulse generation system related to the EM emission generation and propagation are out-of-vacuum components. For example, the one or more optical components forming the optical cavity, modulators, mirrors, and pulse shaper, can be outside of the vacuum system of the laser pulse generation system. In some implementations, EM emission emitted by the electrons passing through the undulators can be coupled out of vacuum, e.g., using fiber optics and / or free-space optics.

[0054] In some implementations, the in-vacuum components and out-of-vacuum components of the laser pulse generation system can be arranged in different levels, e.g., vertically stacked, to compact the arrangement of the components of the laser pulse generation system. In some examples, a first set of the in-vacuum components can be arranged in a different Attorney Docket No. 58110-0002W01 level from a second set of the in-vacuum components, e.g., the first set of in-vacuum components defining a first sub-path and a second set of in-vacuum components defining a second sub-path can be physically arranged in different levels.

[0055] FIG. 1 shows an example laser pulse generation system 100 for providing pulses of laser radiation, e.g., to drive a plasma source. Laser pulse generation system 100 includes an electron feed path 101, which feeds an electron beam to one of two undulators arranged along parallel electron sub-paths. The two parallel sub-paths merge at an electron return path 103. Together, the electron feed path 101, the two parallel sub-paths 108 and 110, and the electron return path 103 form a closed loop along which electrons are accelerated and decelerated to stimulate and amplify laser emission as described below. Although depicted in FIG. 1 as a closed loop, other embodiments may include open loop configuration, e.g., as described in further detail below with reference to FIG. 9.

[0056] A first of the undulators, undulator 116, facilitates gain within an optical cavity 130. The second undulator, undulator 118, serves to amplifier laser emission from the optical cavity. Each of the undulators 116 and 118 is arranged in a respective one of the parallel subpaths 108 and 110, the operation of which is described in further detail below.

[0057] The system 100 includes an electron source 102 which injects electrons into the electron feed path lOlElectron source 102 includes an electron gun and an injector configured to inject electrons at a given energy, e g., about 2-10 megaelectron volts (MeV), e g., 5 MeV, into the electron feed path 101. In some examples, the electron source 102 is a modular component and is arranged with respect to the system 100 to couple electrons into the system 100, e.g., into an energy recovery linear accelerator 104, along an electron beam path.

[0058] The system 100 also includes two electron linear accelerators 104, 106, and electron kickers 112 and 114, e.g., magnetic-based electron beam steerers, arranged along the electron feed path 101 and electron return path 103, where the electron feed path 101 and electron return path 103 form an overlap portion of the electron path through which electrons travel regardless if the electrons are directed to sub-path 108 or sub-path 110. An electron kicker, e.g., also referred to as a “kicker magnet,” includes dipole magnets that can be used to switch a particle beam between two different paths, e.g., by switching on and off electrical current to the dipole magnets to produce a magnetic field to steer the particle beam. Electron kicker Attorney Docket No. 58110-0002W01

[0059] 112 is arranged between the electron feed path 101 and the two parallel electron sub-paths 108 and 110. Electron kicker 114 is arranged between the two parallel electron sub-paths 108 and 110 and the electron return path 103. Linear accelerator 104, is arranged along the electron feed path 101 between the electron source 102 and electron kicker 112. Electron linear accelerator 106 is arranged along the electron sub-path 110 between the electron kicker 112 and the undulator 118. An electron beam dump 142, which receives decelerated electrons diverted from the electron return path 103, is arranged along the electron beam path and following the electron recovery linear accelerator (ERL) 104. Decelerated electrons are expended at the electron beam dump 142 after passing through the ERL 104.

[0060] The laser pulse generation system 100 includes an optical cavity 130 that is at least partially overlapping with the undulator 116 and configured to confine the electromagnetic (EM) emission, e.g., laser pulse train, emitted by electrons passing through the undulator 116 within the optical cavity 130 along a first optical path 132. The optical cavity 130 includes a switchable optical coupler 136 arranged with respect first optical path 132 to selectively direct EM emission from the optical cavity 130 along first optical path 132 to a second optical path 138.

[0061] The system 100 includes an amplifying optical path, e.g., second optical path 138, where a portion of the second optical path 138 is aligned with the undulator 118 arranged along the second electron sub-path 110. The laser pulse generation system 100 includes a pulse shaper 140 arranged along the second optical path 138 between the undulator 118 and an output 152 of the system 100. The pulse shaper 140 is configured to receive a pulse train including multiple micropulses of the EM emission from the first optical path 130 and second optical path 138. Further details of the pulse shaper 140 are discussed below with reference to FIG. 4.

[0062] The system includes and is in data communication with a controller 150, e.g., one or more data processing apparatuses. The controller is configured to provide control signals to one or more components of the laser pulse generation system to control operation of the laser pulse generation system, as described in further detail below. Although described herein as a controller 150 configured to provide control signals to the components of the laser pulse generation system, the operations described with reference to controller 150 can be performed by multiple controllers in data communication with the laser pulse generation Attorney Docket No. 58110-0002W01 system 100. For example, a first controller can be configured to provide control signals to the electron source 102 and a different, second controller can be configured to provide control signals to the undulator 116.

[0063] Laser pulse generation system 100, as depicted in FIG. 1, includes linear accelerator 104 and linear accelerator 106. As configured in system 100, linear accelerator 104 is an energy recovery linear accelerator (ERL) 104, where the operation of ERL 104 includes both (1) a linear accelerator to accelerate electrons to increase the energy of the electrons, and (2) an energy recovery module to decelerate returning electrons along electron return path 103 to capture energy from the decelerating electrons and return the captured energy back into the electric field of the ERL 104, thereby improving the overall system efficiency. In some instances, ERL 104 is a single module performing both functions (1) and (2), e.g., using controlled timing of injected and returning electrons with respect to the electric field of the ERL 104 to select for acceleration or deceleration. In some instances, ERL 104 can include separate modules, each performing a function (1) or (2), e.g., a linear accelerator and a separate energy recovery module arranged along the electron beam path at stage B.

[0064] In either case, both ERL 104 and linear accelerator 106 accelerate the electrons to increase the energy of the electrons passing through the electron linear accelerator arranged such that the electrons at return nodes, e.g., nodes J and K, into the electron beam path are within a threshold range of a target electron energy. For example, at nodes J and K where electrons along two different sub-paths of the electron beam path are joined together, the electrons in each sub-path have respective energies within a target range of the target electron energy, for example, between about 38 MeV and 42 MeV, e.g., about 40 MeV.

[0065] In some implementations, system 100 does not include electron linear accelerator 106. For example, in instances in which system efficiency is less of a concern, the linear accelerator 106 can be omitted from the system 100 or unused during operation of the system 100. In such cases, the electrons progress from electron kicker 112 into the undulator 118 along sub-path 110.

[0066] In some implementations, linear accelerator 104 is not an energy recovery linear accelerator and is not configured during operation of system 100 to perform energy recovery of electrons returning along return path 103. For example, in instances in which system efficiency is less of a concern and / or cooling of components of the system to Attorney Docket No. 58110-0002W01 superconducting temperatures is not available or desired, linear accelerator 104 may not be an energy recovery linear accelerator or the linear accelerator 104 can be configured to not perform energy recovery of electrons, e.g., the energy recovery functionality may be unused during operation of system 100. In some instances, as discussed in further detail with reference to FIG. 9 below, energy recovery of electrons along electron return path 103 can be performed along second, different path by an energy recovery module, e.g., where electron paths 101 and 103, and sub-paths 108, 110 do not form a closed loop.

[0067] The ERL 104 of the laser pulse generation system 100 is arranged with respect to the system such that electrons from the electron source 102 are injected into the ERL 104 along the electron feed beam path 101 and the accelerated electrons exiting the ERL 104 are directed along the electron feed beam path 101 into one of two undulators 116, 118 along the first electron sub-path 108 or second electron sub-path 110. The linear accelerator 106 of the laser pulse generation system 100 is arranged with respect to the system such that electrons from the first sub-path 108 pass through the linear accelerator 106 along the second sub-path 108 and prior to entering the second undulator 118. Further details of the first and second sub-paths 108, 110 are described below.

[0068] In some implementations, the electron linear accelerators, e.g., ERL 104 and linear accelerator 106, of system 100 are configured to accelerate the electrons passing through the electron linear accelerator such that electrons are maintained at a targeted nominal electron energy, for example at about 40 MeV, at each return node of the electron beam path, e.g., where electrons on different sub-paths are joined together. At points along the electron beam path, electrons may lose energy. For example, when electrons pass through an undulator, e.g., undulators 116 and 118, located along the electron beam path. In response, the electron linear accelerator 106 is configured to increase the energy of the electrons by an energy boost value equivalent to an additional energy loss value that the electron will lose when passing through undulator 118 along the second sub-path 110, when compared to the electron energy loss in undulator 116 when it moves along the first sub-path 108. For example, as described by equation (1):

[0069] E0 =Ea ~EA (1) where Eois a target energy value of the electron at return node following the undulator, Eais the energy of the electron exiting the electron linear accelerator s) and prior Attorney Docket No. 58110-0002W01 to the undulator along the electron beam path, and EAis the energy lost by the electron passing through the undulator. For example, in instances in which 20% of the electron energy is extracted by the undulator and converted to electromagnetic emission, the energy of the electrons at the exit node of the undulator is much lower than the target energy value. In some examples, EAis about 0.25 MeV, 0.5 MeV, 0.75 MeV, 1 MeV, 2 MeV, 5 MeV, 10 MeV or more. In some examples, E is a fractional amount of the target energy value, e.g., 0.01%, 0.025%, 0.05%, 0.1%, 0.25%, 0.5%, 1%, 2%, or more.

[0070] In some implementations, a EAvalue depends in part on the properties of the electrons and on the undulator configuration, where the E value is different for each of the two undulators of the laser pulse generation system. As such, each electron linear accelerator, e.g., ERL 104 and linear accelerator 106, can be configured in combination to accelerate an electron by a same or different energy amount such that the energy of the electron exiting each electron linear accelerator can have the same or different Eavalue.

[0071] The electrons exiting the undulator can be configured out of phase with the RF field in ERL 104, therefore can be decelerated such that the energy from the electrons is captured by the field and reused by the system 100. For example, a path length of the electron return path 103 can be selected such that the electrons are 180 degrees out-of-phase with the RF field of the ERL 104. The electrons are therefore decelerated by maximum amount when encountering the RF field of the ERL 104 and release energy back to the RF field as they pass through the ERL 104. Remaining energy is ultimately released into the beam dump 142.

[0072] In general, an electron acceleration capacity of the linear accelerator depends in part on a length of the electron linear accelerator 104, 106. For example, the energy recovery linear accelerator (ERL) 104 can be about 0.25 meters, 0.5 meters, 1 meter, 1.5 meters, 2 meters, 2.5 meters long such that an electron entering the ERL 104 at an energy of about 5- 10 MeV is accelerated such that the electron energy exiting the ERL 104 reaches what is needed to generate the targeted EM emission in the undulators, as described with respect to Equation 2 and the description thereof below. For example, the electron is accelerated such that the electron energy exiting the ERL 104 to about 40 MeV. Such energies can be used to generate EM emission within the infrared spectrum, e.g., with wavelengths around few microns. In some instances, a shorter length (e.g., lower energy acceleration) of the ERL 104 is desirable, e.g., in order to reduce physical space occupied by the ERL and requirements of Attorney Docket No. 58110-0002W01 cooling the ERL to superconducting temperature, which can at times be more than 100+ meters long when the target electron energy of electrons exiting the ERL is large, for example, about 500-1000 MeV, which can be used to generate EUV radiation, e.g., using the system described herein.

[0073] The electron linear accelerators 104, 106 of the system 100 are RF frequency accelerators, such that there is a 1 -to- 1 relationship between the timing and bunches of the electron pulse train exiting the linear accelerators 104, 106 and the EM emission pulse train, e.g., laser pulse train. For example, a frequency and number of pulses in the electron pulse train from the linear accelerators 104, 106 corresponds to a frequency and a number of pulses of the EM emission pulse train, e.g., laser pulse train, provided to the pulse shaper 140. As such, a frequency (e.g. repetition rate) of the EM emission can be determined by the RF frequency of the linear accelerators of the electrons of the electron beam.

[0074] The laser pulse generation system includes electron kickers 112, 114. Electron kickers selectively re-route electrons (A) from the electron feed path 101 along a first electron sub-path 108 or a second electron sub-path 110 and (B) re-route electrons from the first electron sub-path 108 or second electron sub-path 110 along a return path 103 to the ERL 104, e.g., to be reused in the system 100.

[0075] Controller 150 is configured to provide instructions, e.g., control signals, to the electron kickers 112, 114 which cause the electron kickers 112, 114 to switch a respective state and deflect the electron beam path. Electron kicker 112 is coupled to the ERL 104 such that the controller 150 can provide control signals that cause the electron kicker 112 to selectively divert electrons from the ERL 104 to the first electron sub-path 108 or the second electron sub-path 110. Electron kicker 114 is coupled to the ERL 104 such that the controller 150 can provide control signals that cause the electron kicker 114 selectively divert electrons from the first electron sub-path 108 or the second electron sub-path 110 to the ERL 104. In a first set of selected positions of the electron kicker 112, 114, electrons travel through the first stage of the system 100 through the ERL 104 and through the first undulator 116 along the first electron sub-path 108 and then return along the return path 103. In a second set of selected positions of the electron kickers 112, 114, electrons travel through a second stage of the system through the ERL 104 and through the second undulator 118 along the second electron sub-path 110 and then return along return path 103. Attorney Docket No. 58110-0002W01

[0076] Laser pulse generation system 100 includes two undulators, e.g., undulator 1 16 and undulator 118. Each undulator 116, 118 is arranged along a respective electron beam subpath such that an electron passing through the respective undulator emits electromagnetic radiation from the resonant interaction of the electron of the electron beam with the undulator. Equation (2) describes a relationship between the undulator properties and a wavelength of the electromagnetic emission emitted by the electron passing through the undulator: is the electromagnetic emission wavelength,uis the undulator period, y is the relativistic factor of the electron beam energy, and K is undulator deflection parameter, proportional to the magnetic field strength of the undulator. The properties of the undulator period and magnetic field strength for each of undulator 116, 118 may be the same or different. The properties of each undulator can be selected based on, for example, a target wavelength of the electromagnetic emission wavelength and an electron beam energy of the electrons entering the undulator. As will be described in more detail below, undulator 116 is used in an oscillator configuration, where the K value is kept constant. For example, a K value can be about 0.5-3, more typically in the 1-2 range. In some implementations, undulator 116 can include a slightly negative taper, with K values increasing towards the end of undulator, e.g., by a few percent. Undulator 118 is used in seeded, high power amplification mode, where a positive tapering is needed. For example, K values for undulator 118 can vary in the range of 0-4, where 0 refers to the drift section of the undulator where no magnets are present. In some implementations, undulator 118 has a first section located along the beginning of the undulator length with a constant K value, e.g., in the range of about 0.5-4, or more typically, in the range of 1-2.5. The first section is followed by one or more additional sections along the undulator length with varying K values, having a general trend of values decreasing towards the end of the undulator length. In some implementations, at the end of undulator length for undulator 118, a K value can decrease by 30% to 40% when compared to the initial K value. In some other implementations, the K value can decrease by a value ranging between about 10% to 60%. Attorney Docket No. 58110-0002W01

[0077] Each undulator 116 and 1 18 of the laser pulse generation system 100 is arranged along a respective electron sub-path 108, 110. As depicted in FIG. 1, laser pulse generation system 100 includes the first stage including sub-path 108 for the “seed generation mode” e.g., a free electron laser oscillator, and the second stage including sub-path 110 is for the “shot mode,” e.g., a free electron laser seeded amplifier.

[0078] FIG. 2A depicts an example of the first stage of the laser pulse generation system 100 as configured for seed generation mode. Undulator 116 is arranged along the first electron sub-path 108 between the electron kickers 112 and 114 along the first sub-path 108 and configured to cause electrons moving along the first electron sub-path 108 through the undulator 116 to emit electromagnetic (EM) emission. A wavelength of the EM emission can be, for example, between visible wavelengths and infrared wavelengths, e.g., between 380 nm to up about 1 millimeter. In some examples, the wavelength of the EM emission is mid-IR wavelength, e.g., 2.5 microns. In some implementations, undulator 116 is a straight, untapered undulator. In some examples, undulator 116 has a period of 1.4 centimeters and a length of 2.0 meters. In other examples, undulator 116 can have a period in the range of 0.5 to 8 centimeters, and a length of 1 to 4 meters.

[0079] In some implementations, electrons along the first electron sub-path 108 from the ERL 104 and through the undulator 116 can have energies between about 25 MeV to about 50 MeV. Electrons passing through undulator 116 can have an energy loss that is less than about 5 MeV, 3 MeV, 2 MeV, 1 MeV, 0.5 MeV, or less. An electron beam power can be less than about 250 kW, 150 kW, 100 kW, 50 kW, 25kW, 10 kW, or less.

[0080] The laser pulse generation system 100 includes an optical cavity 130 that is at least partially overlapping with the undulator 116 and configured to confine the EM emission emitted by electrons passing through the undulator 116 within the optical cavity 130 along a first optical path 132. As depicted in FIG. 2B, the undulator 116 and optical cavity 130 for a laser oscillator cavity including at least two mirrors, e.g., mirrors 134a, 134b. EM emission emitted by electrons 202 passing through the undulator 116 builds from quantum noise to a seed-level of EM emission over a number of cycles of the EM emission oscillating in the optical cavity 130. The EM emission 204 can be dumped out of the cavity and used as a seed for the second stage of the system 100, e.g., the shot mode operation. Attorney Docket No. 58110-0002W01

[0081] The optical cavity 130 includes a set of high-reflectivity mirrors, e.g., mirror 134, where a reflectivity value is selected to obtain a lowest loss of EM emission from the optical cavity. For example, for IR wavelengths, mirrors with interference coatings can reach >99% reflectivity. In some examples, the optical cavity 130 includes at least two mirrors. The round-trip length of the optical cavity, e.g., distance traveled by a photon, determines the length of the pulse train that will be sent to undulator 118 for high power amplification. A round-trip length of the cavity can be selected to match the capability of the pulse shaper and the desired total energy level in the pulse train. The round-trip cavity length can be, for example, about 30 meters long, where a time for the photon to complete a roundtrip through the optical cavity is about 100 nanoseconds. This can allow for 200 micropulses when using a 2GHZ RF linear accelerator, such that an output macropulse energy is about 1-2 J. In some implementations, a round-trip cavity length is realized by using a set of mirrors arranged such that the EM emission bounces multiple times between the mirrors.

[0082] The optical cavity 130 includes a switchable optical coupler 136 arranged with respect first optical path 132. Controller 150 is configured to provide control signals to the switchable optical coupler 136 to selectively direct EM emission from the optical cavity 130 along the first optical path 132 to a second optical path 138. In some examples, the switchable optical coupler 136 is an acoustic-optical modulator. In some examples, as depicted in FIG. 1, the switchable optical coupler 136 consists of a polarization analyzer 135 and a polarization modulator 144. The polarization modulator 144 adjusts the polarization of the EM emission in response to control signals from controller 150, and the polarization analyzer 135 subsequently can switch the EM emission between a reflection condition and a transmission condition.

[0083] In some implementations, controller 150 can provide control signals to the optical coupler 136 that cause the optical coupler to switch between two operating states of the optical cavity: (1) closed-loop oscillation mode within a high reflectivity optical cavity 130 and minimum loss, e.g., from ~1% to 10%. and (2) a dump mode where the EM emission is all diverted out of the optical cavity along the second optical path 138 by the optical coupler 136. Further details of the laser pulse generation process is described below with reference to FIGS. 5-7. Attorney Docket No. 58110-0002W01

[0084] In some implementations, during a seed mode operation of the laser pulse generation system 100, electrons of the electron beam cycle through one or more cycles of the first stage of the system 100 including the electron feed path 101, first electron sub-path 108, and electron return path 103. During each cycle, electrons of the electron beam pass through the undulator 116 which causes the electrons to emit EM emission that overlaps with any electromagnetic radiation in the optical cavity 130. As such, the undulator can act as a gain medium where the EM emission confined in the optical cavity 130 increases in optical energy as additional cycles of the electrons through the undulator 116 along the first electron beam sub-path 108. The EM emission is amplified with each cycle through the undulator 116 by the synchronized, co-propagating electrons of the electron beam along first electron sub-path 108. In some examples, a number of cycles can be one cycle. In some examples, a number of cycles can be, 2 cycles, 3 cycles, 4 cycles, 5 cycles, 6 cycles, or more. In some instances, a number of cycles can be selected based on a target energy for each of the pulses of EM emission of the optical pulse train. For example, a number of cycles is selected to obtain a target energy of about 50 microjoules for each micropulse of the multiple micropulses forming the optical pulse train propagating along the first optical path 132. In some instances, a number of cycles can be selected based on a target energy for a macropulse formed of the multiple micropulses of the EM emission pulse train propagating along the first optical path 132. For example, a number of cycles is selected to obtain a target energy of about 10 millijoules for the macropulse.

[0085] As used herein, the term “micropulse” refers to a single pulse of an EM emission pulse train, where the EM emission pulse train includes multiple pulses, e.g., 200 pulses. A number of micropulses in an EM pulse train corresponds to a number of pulses in the electron pulse train as output by the electron linear accelerator, e.g., ERL 104. A “macropulse” as used herein refers to the aggregate of the micropulses, e.g., the shaped pulse composed of the multiple pulses of the EM emission pulse train.

[0086] Referring back to FIG. 1, after a target number of cycles through the first electron sub-path, the controller 150 provides control signals to the electron kicker 112 and optical coupler 136 causing the electron kicker 112 to divert electrons along the second electron subpath 110 and causing the optical coupler 136 to divert EM emission from the first optical path 130 to the second optical path 138, e.g., to initiate “shot” operation mode. The controller Attorney Docket No. 58110-0002W01

[0087] 150 can include a timing circuit or timing program such that the switching of the operating states of the electron kicker 112 and the optical coupler 136 is performed such that the EM emission from the first optical path 132 to the second optical path 138 and the electrons along the electron sub-path 110 pass through undulator 118 simultaneously, amplifying of the EM emission with the additional emitted EM emission from the electrons.

[0088] FIG. 3A depicts an example of a second stage of the laser pulse generation system configured for shot operation mode. EM emission 204 from the first optical path 132 is directed by the optical coupler 136 along the second optical path 138. As depicted in FIG. 3 A, an optical element, e.g., a mirror 156, arranged between the optical coupler 136 and the undulator 118 directs the EM emission 204 from along the second optical path 138 and along the portion of the second optical path 138 that is aligned with undulator 118. Undulator 118 is arranged along the second electron sub-path 110 and configured to cause electrons moving along the second electron sub-path 110 through the undulator 118 to emit electromagnetic (EM) emission, e.g., as depicted in FIG. 3B. The EM emission can be mid-IR wavelength emission. EM emission 204 from the optical cavity 130 passes through the undulator 118 while electrons 302 passing through the undulator 118 emit EM emission, amplifying the EM emission 204 such that amplified EM emission 304 exits the undulator 118 and propagates towards the pulse shaper 140.

[0089] In some implementations, undulator 118 is a tapered undulator. The tapered undulator can have a variation of undulator properties, e.g., magnetic field strength, periodicity of the magnetic field strength, or a combination of both. The variation of the undulator properties can be selected, for example, to compensate for electron energy loss at the electron travels along the undulator 118.

[0090] In some implementations, the tapering of the tapered undulator is positive tapering, where the K value, e.g., magnetic field strength, decreases along the undulator length. Positive tapering can be used to compensate for beam energy loss, compensate for energy chirp of the e-beam, increase power after saturation of a high-gain FEL preserving a resonance condition, or any combination of these. Further details of taper undulators can be found in N.M. Kroll et al. IEEE J. Quantum Electronics, QE-17, 1436 (1981). Attorney Docket No. 58110-0002W01

[0091] In some implementations, the tapering of the tapered undulator is negative tapering, where the K value is increased along undulator length. Negative tapering can be used, for example, to increase power in an oscillation mode.

[0092] In some implementations, an electron energy of an electron prior to entering the undulator 118 is higher than an electron energy of an electron prior to entering the undulator 116. Electrons passing through undulator 116 can have an energy loss that is greater than about 1 MeV, 3 MeV, 5 MeV, 10 MeV, 15 MeV or greater. An electron beam power can be greater than about 50 kW, 100 kW, 250 kW, 500 kW, 1000 kW, 1500 kW or more.

[0093] The difference in electron energy Eacan be selected, for example, due to an increased loss of energy of the electron when passing through undulator 118 as compared to the loss of energy of the electron when passing through undulator 116. For example, an energy of the electrons along the second electron sub-path 110 and through the undulator 118 can have energies between about 25 MeV to about 60 MeV. In some implementations, undulator 118 is configured to operate at higher power values than the undulator 116, e.g., in order to extract higher energy EM emission from the electrons.

[0094] The laser pulse generation system 100 includes a pulse shaper 140 arranged along the second optical path 138 between the undulator 118 and an output 152 of the system 100. The pulse shaper 140 is configured to receive an EM emission pulse train including multiple micropulses of the EM emission from the first optical path 130 and second optical path 138, e g., the amplified EM emission pulse train. The EM emission pulse train has finite duration as described above and is referred to in this specification as a macropulse. For example, 200 micropulses separated by 0.5 ns, e.g., 2 GHz RF. A total duration of the pulse train can have a total pulse length, e.g., 100 ns long. In this case, the pulse train can be described as a 100 ns macropulse with 200 micropulses in it. The macropulse duration and intensity profile in time can have a flat top comb structure and can be further manipulated for better performance in the application, e.g., using a pulse shaper 140. The pulse shaper can receive the macropulse from undulator 118 as input and provide another macropulse with different time characteristics.

[0095] FIG. 4 depicts an example of a pulse shaper of the laser pulse generation system 100. A pulse shaper 400, e.g., pulse shaper 140, includes two interleaved paths, e.g., a first path 402 and a second path 404, and an optical switch 406. The optical switch 406 is arranged to Attorney Docket No. 58110-0002W01 receive the EM emission from the undulator 118, e.g., EM emission 304, and selectively direct the EM emission along the first path 402 or second path 404.

[0096] In some implementations, the optical switch 406 is an acoustic-optical modulator. In some implementations, as depicted in FIG. 4, the optical switch 406 includes a polarization analyzer 407 and a polarization modulator 418, for example an electro-optical modulator (EOM). The polarization analyzer 407 and modulator 418 together can be operable to direct the EM emission pulse train to either path following instructions from controller 150. Controller 150 is configured to provide control signals to the optical modulator 418 to cause the optical switch 406 to selectively direct a portion of the micropulses of the EM emission pulse train along the first path 402 and a second portion of the micropulses of the pulse train along the second path 404. In some examples, the first path 402 and the second path 404 are different lengths, e.g., where a time for a pulse to travel the first path 402 is different than a time for the pulse to travel the second path 404.

[0097] In some implementations, the pulse shaper includes two mirrors, e.g., mirrors 408, 410, arranged with respect to each other at respective angles to form a multi -bounce cavity to form the first path 402. For example, a distance X between the mirrors and a number of bounces n creates a path length of X*n for the first path 402.

[0098] In some implementations, changing the spacing of the mirrors 408, 410 or the tile / angle of the mirrors relative to each other can adjust the overall path length of the first path 402, and generates a time delay of the first portion of the pulse train relative to the second portion of the pulse train. For example, controller 150 can be configured to provide control signals to one or more adjustable stages (not shown) retaining the mirrors 408, 410 to cause the adjustable stages to adjust an orientation of one or both of the mirrors 408, 410.

[0099] In some implementations, controller 150 can update a length and shape, e.g., amplitude over the duration of the pulse, of the condensed pulse train by adjusting a timing of the optical switch selecting to direct pulses of the pulse train along the first path 402 and the second path 404. At recombination element 412, e g., recombination optics, where the first and second portions of the pulse train each propagating along a respective first path 402 and second path 404 recombine, a length of the condensed pulse train is shortened relative to an original pulse length of the pulse train. In some other implementations, the optical switch Attorney Docket No. 58110-0002W01 can be configured such that the first part of the EM emission pulse train is directed along the shorter path to generate an elongated macropulse.

[0100] In some implementations, the pulse shaper 400 includes optical elements, e.g., mirrors 414, 416, to recombine the pulses of the pulse train propagating along each of the first path 402 and second path 404. For example, the first and second paths can be recombined using optical elements, e.g., mirrors, to direct the first and second portions of the pulse train along a same beam path.

[0101] In some implementations, the pulse shaper 400 includes following optics, e.g., recombination element 412, to couple of the condensed macropulse aligned with a target region. For example, to condense a spot size and align the shaped pulses out of the laser pulse generation system 100 and aligned with a plasma target. In some implementations, the pulse shaper 400 is configured to compress the pulse train to achieve improved laser produced plasma source (LPP) output power. For example, the LPP source may have a limit on its repetition rate, such that a higher output power requires a higher pulse energy. This in turn can require a higher macropulse energy from the pump source, but still having a same pulse duration, e.g., 50 nanoseconds. The micropulse energy from the undulator is potentially limited by the accelerator technology and may also reach its limit, e.g., at 10 - 20 mJ per micropulse. In such cases, the system can be configured to produce 200 micropulses using the dual undulator configuration described above, in a macropulse having 2 J of pulse energy, with a 100 nanosecond duration pulse. The pulse shaper can be configured to switch in the during the duration of the macropulse, e.g., dividing the pulse in two parts, sending 100 micropulses to both paths and delaying the first path by exactly 50 nanoseconds. When recombined, the new macropulse will be 2 J of pulse energy compressed in 50 nanoseconds, allowing the LPP source to produce increased EUV energy per pulse.

[0102] In some implementations, a nested version of the pulse shaper can be configured to include multiple optical switches, with corresponding paths, to allow the split and recombination of more segments from the incoming macropulse, therefore allowing more pulse shaping flexibility.

[0103] In some implementations, a pulse shaper, e.g., pulse shaper 400, can be a separate apparatus that can be configured to receive an input pulse train from an EM emission generation system, e.g., a different system from system 100, and produce a shaped pulse Attorney Docket No. 58110-0002W01 output, e.g., as described with reference to FIG. 4. For example, the pulse shaper can be configured to receive an input pulse train from a laser source, e.g., a CO2 laser source, and output a shaped pulse. In another example, the pulse shaper can be configured to receive an input pulse train from an EM emission system producing an X-ray pulse train, an EUV pulse train, a visible wavelength pulse train, or another wavelength of EM emission, and produce a shaped pulse as output. The pulse shaper can be configured with corresponding optics compatible with the wavelength of light of the input pulse train.

[0104] In general, the ‘shape’ of a pulse refers to the distribution of EM emission, e g., light emission, in time, space, or other characteristics, for example polarization. In some implementations, the pulse shaper can be used to compress or expand a pulse train in time. In some implementations, the pulse shaper can be used to produce multiple sub-beams that are separated in space. For example, the pulse shaper can split an input pulse train to two or more shorter pulses and provide them to downstream secondary systems in a spatially separated pattern, e.g., at different locations in the secondary system, or from different angles.

[0105] In some implementations, different polarization adjustments can be applied at one or more of the paths in the pulse shaper so that the shaped pulse output includes a mixture of different polarization states. In some implementations, the shaping of the pulse occurs simultaneously in selected ones of time, space, polarization, and other characteristics of the EM emission so that the output shaped pulse can have a multitudes of properties, for example, time duration, beam size, beam propagation angles, and polarizations, which are significantly different from the input pulse.

[0106] In some implementations, the laser pulse generation system 100 does not include a pulse shaper. As an alternative, or in addition to the pulse shaper, the system can include one or more optical elements, e.g., focusing or condensing optics, to couple the output 152 of the second optical path out of the laser pulse generation system 100.

[0107] Laser pulse generation system is operable to produce, as output 152, laser pulses.

[0108] The system operating at a given repetition rate, for example 100 kHz, provides the targeted high average power. In such cases, the system produces laser pulses repetitively at a given interval. For example, for a 100 kHz repetition rate, the laser system produces pulses with a 10 microsecond interval between pulses. The system generates one macropulse within each interval, and each macropulse consists of multiple micropulses, in the form of a pulse train. Attorney Docket No. 58110-0002W01

[0109] FIGS. 5, 6, and 7 describe processes of generating one macropulse in the laser pulse generation system. It is noted that these described processes produce one macropulse, and that the processes can be repeated using the system to produce multiple macropulses at a given time interval. For convenience, the processes 500, 600, and 700 below will be described as being performed by a system including a controller including one or more computers, located in one or more locations, and programmed appropriately in accordance with this specification. For example, a controller of a laser pulse generation system, e.g., controller 150 of the laser pulse generation system 100 of FIG. 1, appropriately programmed, can perform the processes 500, 600, and 700.

[0110] FIG. 5 is a flowchart of an example process 500 for generating a laser pulse by the laser pulse generation system 100. The system generates (502) a first electromagnetic (EM) emission confined in an optical cavity along a first optical path. For example, the first EM emission is confined in optical cavity 130 along the first optical path 132.

[0111] FIG. 6 is a flowchart of an example process 600 for generating the first electromagnetic (EM) emission confined in an optical cavity along a first optical path. As described above with reference to FIG. 1, the process 600 is an example of a “seed generation mode” using a first stage of the laser pulse generation system. The system provides (504), by an electron source, an electron beam including electrons along a first electron beam path. For example, as depicted in FIG. 1 at stage A, controller 150 provides control signals to the electron source 102 which causes electron source to inject electrons into the electron feed path 101. For example, an injection energy of the electrons from the electron source 102 is about 5 MeV.

[0112] The system accelerates (506) the electrons by an electron linear accelerator and along the first electron beam path. For example, as depicted in FIG. 1 at stage B, controller 150 provides control signals to the electron linear accelerator, e.g., ERL 104, which causes the ERL 104 to accelerate the electrons passing through the ERL 104 along the electron feed path 101 and increase an energy of the electrons along the electron feed path 101. For example, an energy of the electrons exiting the ERL 104 along the electron feed path 101 is about 40 MeV. The electrons exiting the ERL form an electron pulse train having, for example, 1000 bunches, each having about 50 pC of charge. The time interval between the Attorney Docket No. 58110-0002W01 bunches are determined by the RF frequency of the ERL 104, for example, the time interval between bunches is 0.5 nanosecond for a RF frequency of 2 GHz.

[0113] The system causes (508), by a first undulator arranged along the first electron beam path, the electrons along the first electron beam path to emit the first electromagnetic emission along the first optical path. For example, the controller 150 provides control signals to the electron kicker 112 to cause the electron kicker 112, e.g., as depicted at stage C of FIG. 1, to deflect electrons from the electron feed path 101 through the first electron sub-path 108. At stage D of FIG. 1, the undulator 116 arranged along the first electron sub-path 108 generates an oscillatory magnetic field that cause electrons passing through the undulator 116 to emit EM emission along the first optical path, e.g., at stage E of FIG. 1.

[0114] The controller 150 provides control signals to electron kicker 114 to cause the electron kicker at stage J to deflect the electrons from the first electron sub-path 108 to electron return path 103. Electrons returning at stage K through the electron return path 103 are recycled back into the electron feed path 101 by decelerating the electrons through the ERL 104. At times, a fraction of the decelerated electrons are dumped, e.g., using electron dump 142.

[0115] The first EM emission emitted by the electrons passing through undulator 116 has a wavelength as defined by equation (2), for example, in the mid-infrared (mid-IR). In some examples, the wavelength of the EM emission is 2.5 microns. The system confines (510), by the optical cavity, the first electromagnetic emission within the optical cavity for at least one pass of the optical cavity. At stage E of FIG. 1, the controller 150 provides control signals to optical coupler 136 to cause the optical switch to select a first mode of operation, e.g., to reflect incident electrons back into the optical cavity 130 such that the EM emission is confined within the optical cavity 130 along the first optical path 132.

[0116] Controller 150 can provide control signals to the components of system 100 to enable the seed generation mode of the laser pulse generation system 100 for a number of cycles. For example, controller 150 can provide control signals to the electron kickers 112, 114 and to optical coupler 136 to cause the electron kickers 112, 114 to select the electron path including the electron feed path 101, first electron sub-path 108, and electron return path 103, and to optical coupler 136 to select a first, optical confinement mode. The number of cycles can be, for example, 5 cycles of the electrons through the undulator 116. For example, a Attorney Docket No. 58110-0002W01 first cycle starts when a first electron bunch arrives at undulator 1 16, generating a first EM emission micropulse and ends when N electron bunches generate the N micropulses, e.g., where N is the total number of EM emission micropulses in the optical cavity. A second cycle starts when the first micropulse of the pulse train returns to the undulator 116 and is synced with the (N+l)th electron bunch and is amplified and the second cycle ends with the Nth micropulse being amplified by the (N+N)th electron bunch. Each additional cycle proceeds similarly. A time of the seed generation mode depends in part on a number of cycles of the electrons through the first electron sub-path 108. For example, for 5 cycles at a round trip time of 100 nanoseconds, the seed generation mode can be enabled for about 500 nanoseconds.

[0117] A final energy of each pulse of the EM emission pulse train confined in the optical cavity 130 along the first optical path 132 can depend in part on a number of cycles of the seed generation mode. For example, is a seed generation mode includes 5 cycles for a pulse train including 200 micropulses, each micropulse can have an energy of about 50 microjoules and a total macropulse energy of about 10 millijoules. A final EM emission beam power depends on the final energy of the EM emission pulse train as well as the repetition rate of the pulse train, which is defined by the electron source 102. Using the 100 kHz repetition rate as defined by the electron source 102 in this example, the EM emission beam power can be about 1 kW.

[0118] Referring now to FIG. 5, the system generates (512), using the first electromagnetic emission, a second electromagnetic emission. As described above with reference to FIG. 1, the second electromagnetic emission is the amplified EM emission pulse train, e.g., shot mode operation.

[0119] FIG. 7 is a flowchart of an example process for generating, using the first electromagnetic emission, the second electromagnetic emission. The process 700 is an example of a “shot mode” using a second stage of the laser pulse generation system. The system provides (514) along a second optical path different from the first optical path, the first electromagnetic emission. For example, at stage F of FIG. 1, controller 150 provides control signals to the optical coupler 136 which causes the optical coupler to select a second, transmission mode of operation, thereby dumping the first EM emission from optical cavity 130 along second optical path 138. Attorney Docket No. 58110-0002W01

[0120] The system selectively provides (516) the electrons from the electron linear accelerator along a second electron beam path different from the first electron beam path, where at least a portion of the second electron beam path overlaps with the second optical path. For example, controller 150 provides control signals to electron kicker 112 and 114 to cause electron kickers 112, 114 to divert electrons along an electron beam path including electron feed path 101, second electron sub-path 110, and electron return path 103.

[0121] For example, at stage G of FIG. 1, the electrons diverted by electron kicker 112 pass through linear accelerator 106. The controller 150 provides control signals to the linear accelerator 106 that cause the linear accelerator 106 to accelerate the electrons passing through the linear accelerator 106 and increase the energy of the electrons. As described above, the increase in energy of the electrons can depend in part on an energy loss of the electrons passing through undulator 118. For example, the linear accelerator 106 may be configured to increase an energy of the electrons to about 45 MeV to counteract an energy loss of about 5 MeV when passing through the undulator 118. An output electron pulse train of the linear accelerator 106 can have fewer number of bunches than the output of the electron pulse train of the ERL 104 in the seed mode, e.g., about 200 bunches, but with a higher charge per bunch, for example about 1 nC. In other words, in seed mode, the EM emission pulse train includes N micropulses circulating in the optical cavity and passes through the undulator M times. Each time the N micropulses circulate through the undulator 116, a new electron bunch is required to amplify the pulse train, so MxN electron bunches are needed. In shot mode, the N micropulses make a single pass through undulator 118, such that a number of electron bunches required is equal to N. The output electron pulse train of the linear accelerator 106 and into the second electron beam sub-path 110, can have a higher electron beam power than the electron beam power exiting the ERL 104 and into the electron beam sub-path 108. For example, the output electron pulse train from the linear accelerator 106 can have an electron beam power of about 1000 kW. This can be achieved by the controller 150 directing the electron source 102 to produce higher electron charges to provide to the shot mode, and when deployed into shot mode, the system can add additional energy boost from linear accelerator 106.

[0122] The system causes (518), by a second undulator arranged along the second electron beam path, the electrons along the second electron beam path to emit second electromagnetic Attorney Docket No. 58110-0002W01 emission along the second optical path. For example, at stage H of FIG. 1, controller 150 can be configured to provide control signals to cause the electron kickers 112, 114 to divert the electrons from the electron feed path 101 and the optical coupler 136 to dump the EM emission from the optical cavity along second optical beam path 138 simultaneously. For example, controller 150 can include timing circuitry operable to time a maximum spatial overlap of the electrons passing through undulator 118 with the EM emission along the portion of the second optical path 138 that overlaps with undulator 118, e.g., to maximize an amplification of the first EM emission with the emission from the electrons passing through the undulator 118.

[0123] The EM emission pulse train from the optical cavity is amplified as it passes through the undulator 118 by the electrons passing through the undulator 118. In shot mode, the electrons as well as the EM emission pulse train pass through in a single-pass. A mode duration of the shot mode can be, for example, about 100 nanoseconds. A final energy of each micropulse of the EM emission pulse train can be, for example, about 10 mJ, with a final macropulse of the EM emission pulse train can be, for example, 2 Joules.

[0124] Referring now to FIG. 5, the system provides (520) at least one of the first electromagnetic emission and the second electromagnetic emission along the second optical path to a pulse shaper. For example, at Stage I of FIG. 1, the amplified EM emission, e.g., laser pulse train, is provided to the pulse shaper 140, e.g., as described in further detail with reference to FIG. 4.

[0125] The system generates (522), by the pulse shaper and from at least one of the first electromagnetic emission and the second electromagnetic emission, a shaped pulse. For example, as depicted in FIG. 4, controller 150 provides control signals to the optical switch 406 to cause the optical switch 406 to divert a first portion of the EM emission pulse train along a first optical path 402 and divert a second portion of the EM emission pulse train along a second optical path 404, e.g., by adjusting a polarization of the optical modulator.

[0126] In some implementations, controller 150 provides control signals to one or more adjustable stages retaining mirrors 408, 410 to adjust a path length of the first path 402. By adjusting a path length of first path 402, the system can further select a pulse shape of the shaped pulse output 152 from the system 100. Attorney Docket No. 58110-0002W01

[0127] The system provides (524) the shaped pulse. For example, as depicted in FIG. 1, an output 152 of the pulse shaper 140 is coupled out of the system 100. The output 152 can be coupled out of the system 100 using free-space optics or fiber-coupling.

[0128] In some implementations, the output 152 is provided as input to a secondary system. For example, the output 152 can be provided as an input to a plasma generation system that produces a plasma source that emits shorter wavelengths, e.g., EUV wavelengths. FIG. 8 is a flowchart of an example process 800 for generating a plasma using a laser pulse generation system. The system generates (802) a shaped pulse and provides (804) the shaped pulse to a plasma chamber including a target material, e.g., tin, xenon, or another plasma seed material. The system generates (806), from the heating of the target material with the shaped pulse, a hot plasma in the plasma chamber, where the plasma emits a target EUV radiation, e.g., a plasma having adequate temperature and density to emit the target EUV radiation. The system collects (808) the EUV radiation from the produced plasma source and delivers the EUV radiation to perform a lithography process. For example, the system collects, using EUV-compatible optics, the EUV radiation and provides the collected EUV radiation downstream to a lithography apparatus to perform a lithography process.

[0129] In some implementations, the laser pulse generation system includes only the seed stage, e.g., as depicted in FIG. 2A, or is configured, during operation, to only operate in seed generation mode. In such cases, the laser pulse generation system includes only includes sub-path 108 for the “seed generation mode” e g., a free electron laser oscillator, or only operates, during operation, in seed generation mode. For example, similarly to the description of seed generation mode with reference to FIGS. 1 and 2A, the system will build up EM emission within the optical cavity 130 to a target value and then provide the EM emission from the optical cavity as the output emission to a pulse shaper 140 or directly provide the EM emission as output from the system. The switchable optical coupler 136 can be used to selectively dump the collected EM emission from within the optical cavity, e.g., once the EM emission has reached a target value.

[0130] In some implementations, the laser pulse generation system includes only the shot stage. The shot stage of the laser pulse generation system can be combined with a seed generation laser system which uses technologies other than free electron lasers. For example, when the laser pulse generation system is configured to operate at a wavelength where an Attorney Docket No. 58110-0002W01 alternate laser technology, e.g., solid-state laser, fiber laser, gas laser, etc., can be used, or when the laser pulse generation system can be combined with nonlinear media to generate seeding pulses with adequate temporal and spatial characteristics with sufficient power, the seed stage of the laser pulse generation system can be replaced by such laser systems. In such cases, utilizing these alternative laser systems can reduce the size, cost, or complexity of full laser pulse generation system. In some implementations, the output 152 is divided and used as input to multiple secondary systems, e.g., two or more secondary systems. For example, the output 152 can be divided to drive respective multiple plasma sources in multiple lithography apparatuses. The division of output 152 can be performed using various multiplexing schemes, e.g., division in amplitude by partial transmission optics, diffraction gratings, or division in time domain by optical modulators.

[0131] Multiple example laser pulse generation systems are described above. Further implementations are possible. For instance, in some cases, energy recovery of electrons along return path 103 can be performed along a second, different path by an energy recovery module, e.g., where the electron feed path and the electron return path do not form a closed loop. FIG. 9 is a schematic of an example laser pulse generation system 900 implementing such an energy recovery module.

[0132] As depicted in FIG. 9, an electron feed path 901 and an electron return path 903 do not form a closed loop as in the prior example including electron feed path 101 and electron return path 103 depicted in FIG. 1. Instead, electrons returning along electron return path 103 at stage K are recovered along in second, different path to an energy recovery module 911, where decelerated electrons are collected by electron beam dump 142. Electrons at stage K are decelerated through energy recovery module 911, where recovered energy from the energy recovery module 911 is converted to usable energy, e.g., electrical power. Optionally, the converted usable energy is provided back into the system 100 to power operations of the system 100, e.g., provided 913 to power operation of the linear accelerator 904. Converting energy from decelerated electrons into usable power back to the system can, as noted above, increase the overall system efficiency of the laser pulse generation system 900.

[0133] These and other embodiments are possible. For example, in some cases, a laser pulse generation system is not configured to include, or to perform during operation, energy Attorney Docket No. 58110-0002W01 recovery of electrons along return path 103, e.g., does not include an ERL. Additionally, in some cases, a laser pulse generation system is not configured to include, or to perform during operation, energy boosting of the electrons by a linear accelerator in the shot mode operation, e.g., prior to entering undulator 118. In some cases, a laser pulse generation system is not configured to, or does not perform during operation, both the energy recovery of electrons along return path 103 and energy boosting of the electrons during shot mode operation prior to entering undulator 118. In such cases, the configuration of the laser pulse generation system may optimize for cost of operation, power requirements of operation etc., versus system overall efficiency.

[0134] The subject matter and the actions and operations described in this specification can be implemented in digital electronic circuitry, in tangibly-embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. The subject matter and the actions and operations described in this specification can be implemented as or in one or more computer programs, e.g., one or more modules of computer program instructions, encoded on a computer program carrier, for execution by, or to control the operation of, data processing apparatus. The carrier can be a tangible non-transitory computer storage medium. Alternatively, or in addition, the carrier can be an artificially-generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, which is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. The computer storage medium can be or be part of a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination of one or more of them. A computer storage medium is not a propagated signal.

[0135] The term “data processing apparatus” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. Data processing apparatus can include special-purpose logic circuitry, e.g., an FPGA (field programmable gate array), an ASIC (application-specific integrated circuit) , or a GPU (graphics processing unit). The apparatus can also include, in addition to hardware, code that creates an execution environment for Attorney Docket No. 58110-0002W01 computer programs, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them.

[0136] A computer program can be written in any form of programming language, including compiled or interpreted languages, or declarative or procedural languages; and it can be deployed in any form, including as a stand-alone program, e.g., as an app, or as a software module, component, engine, subroutine, or other unit suitable for executing in a computing environment, which environment may include one or more computers interconnected by a data communication network in one or more locations.

[0137] A computer program may, but need not, correspond to a file in a file system. A computer program can be stored in a portion of a file that holds other programs or data, e.g., one or more scripts stored in a markup language document, in a single file dedicated to the program in question, or in multiple coordinated files, e g., files that store one or more modules, sub-programs, or portions of code.

[0138] The processes and logic flows described in this specification can be performed by one or more computers executing one or more computer programs to perform operations by operating on input data and generating output. The processes and logic flows can also be performed by special-purpose logic circuitry, e.g., an FPGA, an ASIC, or a GPU, or by a combination of special-purpose logic circuitry and one or more programmed computers.

[0139] Computers suitable for the execution of a computer program can be based on general or special-purpose microprocessors or microcontrollers or a combination of them, or any other kind of central processing unit. Generally, a central processing unit will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a central processing unit for executing instructions and one or more memory devices for storing instructions and data. The central processing unit and the memory can be supplemented by, or incorporated in, special-purpose logic circuitry.

[0140] Generally, a computer will also include, or be operatively coupled to, one or more mass storage devices, and be configured to receive data from or transfer data to the mass storage devices. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) Attorney Docket No. 58110-0002W01 receiver, or a portable storage device, e.g., a universal serial bus (USB) flash drive, to name just a few.

[0141] To provide for interaction with a user, the subject matter described in this specification can be implemented on one or more computers having, or configured to communicate with, a display device, e.g., a LCD (liquid crystal display) monitor, or a virtual- reality (VR) or augmented-reality (AR) display, for displaying information to the user, and an input device by which the user can provide input to the computer, e.g., a keyboard and a pointing device, e.g., a mouse, a trackball or touchpad. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback and responses provided to the user can be any form of sensory feedback, e.g., visual, auditory, speech, or tactile feedback or responses; and input from the user can be received in any form, including acoustic, speech, tactile, or eye tracking input, including touch motion or gestures, or kinetic motion or gestures or orientation motion or gestures. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user’s device in response to requests received from the web browser, or by interacting with an app running on a user device, e.g., on a smartphone or electronic tablet. Also, a computer can interact with a user by sending text messages or other forms of message to a personal device, e.g., a smartphone that is running a messaging application, and receiving responsive messages from the user in return.

[0142] This specification uses the term “configured to” in connection with systems, apparatus, and computer program components. That a system of one or more computers is configured to perform particular operations or actions means that the system has installed on it software, firmware, hardware, or a combination of them that in operation cause the system to perform the operations or actions. That one or more computer programs is configured to perform particular operations or actions means that the one or more programs include instructions that, when executed by data processing apparatus, cause the apparatus to perform the operations or actions. That special-purpose logic circuitry is configured to perform particular operations or actions means that the circuitry has electronic logic that performs the operations or actions. Attorney Docket No. 58110-0002W01

[0143] In addition to the embodiments of the attached claims and the embodiments described above, the following numbered embodiments are also innovative.

[0144] Embodiment 1 is a laser pulse generation system comprising: at least one electron linear accelerator configured to accelerate electrons from an electron source; a first undulator arranged along a first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission; an optical cavity at least partially overlapping with the first undulator and configured to confine the first electromagnetic emission within the optical cavity along a first optical path; a second undulator arranged along a second electron beam path different from the first electron beam path, the second undulator being configured to cause electrons moving along the second electron beam path to emit a second electromagnetic emission; at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along a second optical path different from the first optical path; at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path; at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path; and a pulse shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission.

[0145] Embodiment 2 is the system of embodiment 1, wherein the first electron beam path and the second electron beam path are aligned along an overlap portion, and wherein the at least one electron linear accelerator comprises: Attorney Docket No. 58110-0002W01 a first electron linear accelerator at a first node; and a second electron linear accelerator at a second node, wherein the first node is located along the overlap portion of the first electron beam path and the second electron beam path, and wherein the second node is located along the second electron beam path.

[0146] Embodiment 3 is the system of embodiment 1 or 2, wherein the at least one electron linear accelerator is arranged in the first electron beam path, second electron beam path, or both, between the electron source and at least one of the first undulator and the second undulator.

[0147] Embodiment 4 is the system of any one of the preceding embodiments, wherein the at least one electron linear accelerator is configured to accelerate electrons along at least one of the first electron beam path and the second electron beam path to increase energy of the electrons.

[0148] Embodiment 5 is the system of any one of the preceding embodiments wherein the at least one electron kicker coupled to the electron linear accelerator comprises: a first electron kicker arranged to divert electrons from the first electron linear accelerator to the first undulator along the first electron beam path or to the second undulator along the second electron beam path; and a second electron kicker arranged to divert electrons from the first undulator or the second undulator to the at least one electron linear accelerator along the overlap portion of the first electron beam path and the second electron beam path.

[0149] Embodiment 6 is the system of any one of the preceding embodiments, wherein the at least one electron linear accelerator is configured to increase the energy of the electrons prior to the first undulator or the second undulator such that an exit electron energy of electrons at the second electron kicker meets an acceptance condition. Attorney Docket No. 58110-0002W01

[0150] Embodiment 7 is the system of embodiment 6, wherein the acceptance condition includes an exit electron energy of electrons at the second electron kicker that is within a threshold deviation of an entry electron energy of electrons at the first electron kicker.

[0151] Embodiment 8 is the system of any one of the preceding embodiments, wherein the at least one electron linear accelerator comprises an energy recovery linear accelerator configured to decelerate electrons along the overlap portion to decrease energy of the electrons.

[0152] Embodiment 9 is the system of embodiment 8, wherein the energy recovery linear accelerator is configured to capture energy from the decelerated electrons; and provide the captured energy to the RF field of the energy recovery linear accelerator.

[0153] Embodiment 10 is the system of any one of the preceding embodiments, further comprising an energy recovery module arranged along the overlap portion and configured to decelerate electrons along the overlap portion to decrease energy of the electrons.

[0154] Embodiment 11 is the system of any one of the preceding embodiments, wherein the pulse shaper configured to shape laser pulses from a pulse train comprising at least one of the first electromagnetic emission and the second electromagnetic emission, wherein a frequency of the pulse train is selectable by the electron source.

[0155] Embodiment 12 is the system of any one of the preceding embodiments, wherein the pulse shaper comprises: a switch configured to divert a first portion of the pulse train along a first path and a second portion of the pulse train along a second, different path, wherein the first path and the second path comprise different, interleaved paths; and one or more optical elements arranged at converging point of the first and second paths, the one or more optical elements being configured to recombine the first portion of the pulse train and the second portion of the pulse train. Attorney Docket No. 58110-0002W01

[0156] Embodiment 13 is the system of any one of the preceding embodiments, wherein the pulse shaper is configured to receive, as input, the pulse train, and provide, as output, a macropulse comprising a plurality of micropulses of the pulse train.

[0157] Embodiment 14 is the system of any one of the preceding embodiments, further comprising a controller, wherein the controller is in data communication with and configured to provide control signals to the at least one electron linear accelerator, electron source, first undulator, second undulator, at least one electron kicker, at least one optical coupler, and pulse shaper.

[0158] Embodiment 15 is the system of embodiment 14, wherein the controller is configured to provide control signals to cause the operations of generating the first electromagnetic emission confined in the optical cavity along the first optical path, the generating comprising: providing the electron beam comprising electrons along the first electron beam path; accelerating the electrons by the electron linear accelerator and along the first electron beam path; causing, by the first undulator arranged along the first electron beam path, the electrons along the first electron beam path to emit the first electromagnetic emission along the first optical path; and confining, by the optical cavity, the first electromagnetic emission within the optical cavity about at least one pass of the optical cavity; generating, using first electromagnetic emission, the second electromagnetic emission comprising: providing, along the second optical path different from the first optical path, the first electromagnetic emission; selectively providing the electrons from the electron linear accelerator along the second electron beam path different from the first electron beam path, wherein at least a portion of the second electron beam path overlaps with the second optical path; and causing, by the second undulator arranged along the second electron beam Attorney Docket No. 58110-0002W01 path, the electrons along the second electron beam path to emit second electromagnetic emission along the second optical path; and providing at least one of the first electromagnetic emission and the second electromagnetic emission as the laser pulse output.

[0159] Embodiment 16 is a method for generating a laser pulse comprising: generating a first electromagnetic emission confined in an optical cavity along a first optical path, the generating comprising: providing an electron beam comprising electrons along a first electron beam path; accelerating the electrons by an electron linear accelerator and along the first electron beam path; causing, by a first undulator arranged along the first electron beam path, the electrons along the first electron beam path to emit the first electromagnetic emission along the first optical path; and confining, by the optical cavity, the first electromagnetic emission within the optical cavity about at least one pass of the optical cavity; generating, using first electromagnetic emission, a second electromagnetic emission comprising: providing, along a second optical path different from the first optical path, the first electromagnetic emission; selectively providing the electrons from the electron linear accelerator along a second electron beam path different from the first electron beam path, wherein at least a portion of the second electron beam path overlaps with the second optical path; and causing, by a second undulator arranged along the second electron beam path, the electrons along the second electron beam path to emit second electromagnetic emission along the second optical path; and providing at least one of the first electromagnetic emission and the second electromagnetic emission as the laser pulse output. Attorney Docket No. 58110-0002W01

[0160] Embodiment 17 is the method of embodiment 16, further comprising: providing, the laser pulse output to a pulse shaper; generating, by the pulse shaper and from at least one of the first electromagnetic emission and the second electromagnetic emission, a shaped pulse; and providing the shaped pulse.

[0161] Embodiment 18 is the method of embodiment 17, wherein providing the shaped pulse comprises: providing, as input to at least one secondary system, the laser pulse output.

[0162] Embodiment 19 is the method of embodiment 18, wherein providing, as input to the at least one secondary system comprises: providing the shaped pulse as input to a plasma generation system.

[0163] Embodiment 20 is the method of embodiment 18, wherein providing, as input to the at least one secondary system comprises: dividing the shaped pulse into two or more divided outputs; and providing each of the two or more divided outputs as inputs to a respective secondary system of two or more secondary systems.

[0164] Embodiment 21 is a pulse shaping device comprising: a switch configured to divert a first portion of an electromagnetic (EM) pulse train along a first path and a second portion of the EM pulse train along a second, different path, wherein the first path and the second path comprise different, interleaved paths; and one or more optical elements arranged at a location where the first and second paths converge, the one or more optical elements being configured to recombine the first portion of the pulse train and the second portion of the pulse train.

[0165] Embodiment 22 is a laser pulse generation system comprising: an electron linear accelerator configured to accelerate electrons from an electron source; Attorney Docket No. 58110-0002W01 a first electron beam path comprising a first undulator arranged along the first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission; a first optical path comprising: an optical cavity at least partially overlapping with the first undulator and configured to confine the extracted first electromagnetic emission within the optical cavity; a second electron beam path comprising: a second undulator arranged along the second electron beam path and configured to cause electrons moving along the second electron beam path through the second undulator to emit a second electromagnetic emission; a second optical path different from the first optical path comprising at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along the second optical path; and a pulse shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission; at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path; and at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path.

[0166] Embodiment 23 is a laser pulse generation system comprising: at least one electron linear accelerator configured to accelerate electrons from an electron source; a first undulator arranged along a first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission; an optical cavity at least partially overlapping with the first undulator and configured Attorney Docket No. 58110-0002W01 to confine the first electromagnetic emission within the optical cavity along a first optical path; a second undulator arranged along a second electron beam path different from the first electron beam path, the second undulator being configured to cause electrons moving along the second electron beam path to emit a second electromagnetic emission; at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along a second optical path different from the first optical path; at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path; and at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path.

[0167] Embodiment 24 is a laser pulse generation system comprising: at least one electron linear accelerator configured to accelerate electrons from an electron source; an undulator arranged along a first electron beam path and configured to cause electrons moving along the electron beam path through the undulator to emit electromagnetic emission; an optical cavity at least partially overlapping with the undulator and configured to confine the electromagnetic emission within the optical cavity along a first optical path; at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to a second optical path; and a pulse shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission.

[0168] While this specification contains many specific implementation details, these should not be construed as limitations on the scope of what is being claimed, which is defined by the claims themselves, but rather as descriptions of features that may be specific to particular implementations of particular inventions. Certain features that are described in this Attorney Docket No. 58110-0002W01 specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially be claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claim may be directed to a subcombination or variation of a subcombination.

[0169] Similarly, while operations are depicted in the drawings and recited in the claims in a particular order, this by itself should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system modules and components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

[0170] Particular implementations of the subject matter have been described. Other implementations are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In some cases, multitasking and parallel processing may be advantageous.

Claims

Attorney Docket No. 58110-0002W01What is claimed is:

1. A laser pulse generation system comprising: at least one electron linear accelerator configured to accelerate electrons from an electron source; a first undulator arranged along a first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission; an optical cavity at least partially overlapping with the first undulator and configured to confine the first electromagnetic emission within the optical cavity along a first optical path; a second undulator arranged along a second electron beam path different from the first electron beam path, the second undulator being configured to cause electrons moving along the second electron beam path to emit a second electromagnetic emission; at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along a second optical path different from the first optical path; at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path; at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path; and a pulse shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission.

2. The system of claim 1, wherein the first electron beam path and the second electron beam path are aligned along an overlap portion, and wherein the at least one electron linear accelerator comprises: a first electron linear accelerator at a first node; and a second electron linear accelerator at a second node,Attorney Docket No. 58110-0002W01 wherein the first node is located along the overlap portion of the first electron beam path and the second electron beam path, and wherein the second node is located along the second electron beam path.

3. The system of claims 1 or 2, wherein the at least one electron linear accelerator is arranged in the first electron beam path, second electron beam path, or both, between the electron source and at least one of the first undulator and the second undulator.

4. The system of claim 2, wherein the at least one electron linear accelerator is configured to accelerate electrons along at least one of the first electron beam path and the second electron beam path to increase energy of the electrons.

5. The system of claims 2 or 4, wherein the at least one electron kicker coupled to the electron linear accelerator comprises: a first electron kicker arranged to divert electrons from the first electron linear accelerator to the first undulator along the first electron beam path or to the second undulator along the second electron beam path; and a second electron kicker arranged to divert electrons from the first undulator or the second undulator to the at least one electron linear accelerator along the overlap portion of the first electron beam path and the second electron beam path.

6. The system of claim 5, wherein the at least one electron linear accelerator is configured to increase the energy of the electrons prior to the first undulator or the second undulator such that an exit electron energy of electrons at the second electron kicker meets an acceptance condition.

7. The system of claim 6, wherein the acceptance condition includes an exit electron energy of electrons at the second electron kicker that is within a threshold deviation of an entry electron energy of electrons at the first electron kicker.Attorney Docket No. 58110-0002W018. The system of claim 2, wherein the at least one electron linear accelerator comprises an energy recovery linear accelerator configured to decelerate electrons along the overlap portion to decrease energy of the electrons.

9. The system of claim 8, wherein the energy recovery linear accelerator is configured to capture energy from the decelerated electrons; and provide the captured energy to an RF field of the energy recovery linear accelerator.

10. The system of claim 2, further comprising an energy recovery module arranged along the overlap portion and configured to decelerate electrons along the overlap portion to decrease energy of the electrons.

11. The system of claim 1, wherein the pulse shaper configured to shape laser pulses from a pulse train comprising at least one of the first electromagnetic emission and the second electromagnetic emission, wherein a frequency of the pulse train is selectable by the electron source.

12. The system of claim 11, wherein the pulse shaper comprises: a switch configured to divert a first portion of the pulse train along a first path and a second portion of the pulse train along a second, different path, wherein the first path and the second path comprise different, interleaved paths; and one or more optical elements arranged at converging point of the first and second paths, the one or more optical elements being configured to recombine the first portion of the pulse train and the second portion of the pulse train.

13. The system of claims 11 or 12, wherein the pulse shaper is configured to receive, as input, the pulse train, and provide, as output, a macropulse comprising a plurality of micropulses of the pulse train.

14. The system of claim 1, further comprising a controller, wherein the controller is in data communication with and configured to provide control signals to the at least oneAttorney Docket No. 58110-0002W01 electron linear accelerator, electron source, first undulator, second undulator, at least one electron kicker, at least one optical coupler, and pulse shaper.

15. The system of claim 14, wherein the controller is configured to provide control signals to cause the operations of generating the first electromagnetic emission confined in the optical cavity along the first optical path, the generating comprising: providing the electron beam comprising electrons along the first electron beam path; accelerating the electrons by the electron linear accelerator and along the first electron beam path; causing, by the first undulator arranged along the first electron beam path, the electrons along the first electron beam path to emit the first electromagnetic emission along the first optical path; and confining, by the optical cavity, the first electromagnetic emission within the optical cavity about at least one pass of the optical cavity; generating, using first electromagnetic emission, the second electromagnetic emission comprising: providing, along the second optical path different from the first optical path, the first electromagnetic emission; selectively providing the electrons from the electron linear accelerator along the second electron beam path different from the first electron beam path, wherein at least a portion of the second electron beam path overlaps with the second optical path; and causing, by the second undulator arranged along the second electron beam path, the electrons along the second electron beam path to emit second electromagnetic emission along the second optical path; and providing at least one of the first electromagnetic emission and the second electromagnetic emission as the laser pulse output.

16. A method for generating a laser pulse comprising: generating a first electromagnetic emission confined in an optical cavity along a firstAttorney Docket No. 58110-0002W01 optical path, the generating comprising: providing an electron beam comprising electrons along a first electron beam path; accelerating the electrons by an electron linear accelerator and along the first electron beam path; causing, by a first undulator arranged along the first electron beam path, the electrons along the first electron beam path to emit the first electromagnetic emission along the first optical path; and confining, by the optical cavity, the first electromagnetic emission within the optical cavity about at least one pass of the optical cavity; generating, using first electromagnetic emission, a second electromagnetic emission comprising: providing, along a second optical path different from the first optical path, the first electromagnetic emission; selectively providing the electrons from the electron linear accelerator along a second electron beam path different from the first electron beam path, wherein at least a portion of the second electron beam path overlaps with the second optical path; and causing, by a second undulator arranged along the second electron beam path, the electrons along the second electron beam path to emit second electromagnetic emission along the second optical path; and providing at least one of the first electromagnetic emission and the second electromagnetic emission as the laser pulse output.

17. The method of claim 16, further comprising: providing, the laser pulse output to a pulse shaper; generating, by the pulse shaper and from at least one of the first electromagnetic emission and the second electromagnetic emission, a shaped pulse; and providing the shaped pulse.

18. The method of claim 17, wherein providing the shaped pulse comprises: providing, as input to at least one secondary system, the laser pulse output.Attorney Docket No. 58110-0002W0119. The method of claim 18, wherein providing, as input to the at least one secondary system comprises: providing the shaped pulse as input to a plasma generation system.

20. The method of claims 18 or 19, wherein providing, as input to the at least one secondary system comprises: dividing the shaped pulse into two or more divided outputs; and providing each of the two or more divided outputs as inputs to a respective secondary system of two or more secondary systems.

21. A pulse shaping device comprising: a switch configured to divert a first portion of an electromagnetic (EM) pulse train along a first path and a second portion of the EM pulse train along a second, different path, wherein the first path and the second path comprise different, interleaved paths; and one or more optical elements arranged at a location where the first and second paths converge, the one or more optical elements being configured to recombine the first portion of the pulse train and the second portion of the pulse train.

22. A laser pulse generation system comprising: an electron linear accelerator configured to accelerate electrons from an electron source; a first electron beam path comprising a first undulator arranged along the first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission; a first optical path comprising: an optical cavity at least partially overlapping with the first undulator and configured to confine the extracted first electromagnetic emission within the optical cavity; a second electron beam path comprising:Attorney Docket No. 58110-0002W01 a second undulator arranged along the second electron beam path and configured to cause electrons moving along the second electron beam path through the second undulator to emit a second electromagnetic emission; a second optical path different from the first optical path comprising at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along the second optical path; and a pulse shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission; at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path; and at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path.

23. A laser pulse generation system comprising: at least one electron linear accelerator configured to accelerate electrons from an electron source; a first undulator arranged along a first electron beam path and configured to cause electrons moving along the first electron beam path through the first undulator to emit a first electromagnetic emission; an optical cavity at least partially overlapping with the first undulator and configured to confine the first electromagnetic emission within the optical cavity along a first optical path; a second undulator arranged along a second electron beam path different from the first electron beam path, the second undulator being configured to cause electrons moving along the second electron beam path to emit a second electromagnetic emission; at least one optical element arranged with respect to the second undulator and configured to direct at least one of the first electromagnetic emission and the second electromagnetic emission along a second optical path different from the first optical path;Attorney Docket No. 58110-0002W01 at least one electron kicker coupled to the electron linear accelerator and configured to selectively divert electrons from the electron linear accelerator to the first electron beam path or the second electron beam path; and at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to the second optical path.

24. A laser pulse generation system comprising: at least one electron linear accelerator configured to accelerate electrons from an electron source; an undulator arranged along a first electron beam path and configured to cause electrons moving along the electron beam path through the undulator to emit electromagnetic emission; an optical cavity at least partially overlapping with the undulator and configured to confine the electromagnetic emission within the optical cavity along a first optical path; at least one optical coupler in the first optical path and configured to selectively divert electromagnetic emission from the first optical path to a second optical path; and a pulse shaper arranged in the second optical path and configured to shape laser pulses from at least one of the first electromagnetic emission and the second electromagnetic emission.

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