Method and apparatus for synchronizing optical and electron beam pulses

Abstract

A method for tuning temporal overlap between a pulsed charged particle beam and a pulsed optical beam includes directing pulses of the pulsed charged particle beam to impinge an area of a screen. A property of the screen changes as a result of interaction with the pulsed charged particle beam. The method also includes temporally scanning a delay between the pulsed optical beam and the pulsed charged particle beam. The method further includes while temporally scanning the delay, repeatedly measuring the property of the screen. The method further includes determining, based on the repeated measurements of the property of the screen, a temporal relationship between the pulsed charge particle beam and the pulsed optical beam. The method further includes tuning, based on the temporal relationship, the delay between the pulsed optical beam and the pulsed charged particle beam.

Description

METHOD AND APPARATUS FOR SYNCHRONIZING OPTICAL ANDELECTRON BEAM PULSESGOVERNMENT SUPPORT

[0001] This invention was made with government support under 1935994 awarded by the National Science Foundation. The government has certain rights in the invention.TECHNICAL FIELD

[0002] The disclosed embodiments relate generally to synchronizing optical and electron beam pulses, and more specifically for synchronizing optical and electron beam pulses for generating x-ray pulses.BACKGROUND

[0003] Electron accelerators are critical for applications ranging from bright x-ray production to medical therapies. The integration of ultrafast lasers with electron beams can improve the performance and expand the application scope of electron accelerators, enabling compact x-ray sources, attosecond x-ray pulses, and compact high-energy particle beams.

[0004] Accordingly, in order to fully leverage the opportunities of electron accelerators, precise and stable spatiotemporal overlap between a relativistic electron beam and a high power laser is required. Traditional electrooptic sampling may not have the sensitivity due to the small Coulomb field of an electron beam / bunch.SUMMARY

[0005] In accordance with some embodiments, a method for tuning temporal overlap between a pulsed charged particle beam and a pulsed optical beam includes directing pulses of the pulsed charged particle beam to impinge an area of a screen. A property of the screen changes as a result of interaction with the pulsed charged particle beam. The method also includes temporally scanning a delay between the pulsed optical beam and the pulsed charged particle beam. The method further includes while temporally scanning the delay, repeatedly measuring the property of the screen. The method further includes determining, based on the repeated measurements of the property of the screen, a temporal relationship between the pulsed charge particle beam and the pulsed optical beam. The method further includes tuning, based onthe temporal relationship, the delay between the pulsed optical beam and the pulsed charged particle beam.

[0006] Further, in accordance with some embodiments, a method for tuning spatial overlap between a pulsed charged particle beam and a pulsed optical beam includes directing pulses of the pulsed charged particle beam to impinge an area of a screen includes directing pulses of the pulsed charged particle beam to impinge an area of a screen. A property of the screen changes as a result of interaction with the pulsed charged particle beam. The method also includes spatially scanning the pulsed optical beam across the area of the screen to obtain a spatial overlap. The method further includes while spatially scanning the spatial overlap, repeatedly measuring the property of the screen. The method further includes determining, based on the repeated measurements of the property of the screen, a spatial relationship between the pulsed charge particle beam and the pulsed optical beam. The method further includes tuning, based on the spatial relationship, the spatial overlap between the pulsed optical beam and the pulsed charged particle beam.

[0007] Further, in accordance with some embodiments, a device for tuning temporal and spatial overlap between a pulsed charged particle beam and a pulsed optical beam includes a screen. The property of the screen changes as a result of interaction with a pulsed charged partial beam. The device also includes a camera configured to produce an image of an area of the screen where the pulsed charged particle beam impinges and a light beam interacts. The device further includes a computer system. The computer system includes one or more processors and memory storing instructions for determining, based on the image of the screen, a temporal relationship and / or a spatial relationship between the pulsed optical beam and the pulsed charged particle beam using any of the above methods.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention will be more fully understood by referring to the following Detailed Description in conjunction with the not-to scale Drawings, of which:

[0009] FIGS. 1 A-1C are schematic diagrams illustrating a light source (e.g., a firee- electron laser), in accordance with some embodiments.

[0010] FIGS. 2A-2D illustrate an example instrumental setup for spatiotemporal overlap diagnostics, in accordance with some embodiments.

[0011] FIGS. 3A-3D illustrates an example laser-screen calibration for coarsely aligning pulsed laser beam with a yttrium aluminium garnet (YAG) screen, in accordance with some embodiments.

[0012] FIG. 4 illustrates an example coarse tuning of temporal overlap based on a SMA cable test, in accordance with some embodiments.

[0013] FIG. 5A-5B illustrate an example fine tuning of spatial overlap, in accordance with some embodiments.

[0014] FIG. 6A-6B illustrate an example fine tuning of temporal overlap, in accordance with some embodiments.

[0015] FIG. 7A-7E illustrate an example feedback correction mechanism over a period, in accordance with some embodiments.

[0016] Figures 8A-8C collectively provide a flow chart of a method for tuning spatiotemporal overlap between a pulsed charged particle beam and a pulsed optical beam, in accordance with some embodiments of the present disclosure.

[0017] Generally, the sizes and relative scales of elements in Drawings may be set to be different from actual ones to appropriately facilitate simplicity, clarity, and understanding of the Drawings. For the same reason, not all elements present in one Drawing may necessarily be shown in another.DETAILED DESCRIPTION

[0018] Various embodiments of this application are direct to methods and apparatus for independently tuning spatial and temporal overlap (e.g., spatiotemporal overlap diagnostics) between a pulsed charged particle beam (e.g., a relativistic electron bunch) and a pulsed optical beam (e.g., a pulsed laser). For example, such spatiotemporal overlap diagnostics can be used to achieve a spatiotemporal overlap between a few-micron sized pulsed charged particle beam and a few-micron sized pulsed optical beam. In some embodiments, the pulsed charged particle beam includes a sub -pi co second relativistic electron bunch that is generated using a 30 MeV electron beam from an X-band linac focused to a 10 pm spot. In some embodiments, the pulsed optical beam includes a near-infrared (near-IR) pulsed laser. In some embodiments, the spatiotemporal overlap diagnostics is performed using an instrumental setup that is cost- effective and accessible in a form factor for use in compact, relatively low-energy, low-charge accelerators. In some embodiments, the instrumental setup for the spatiotemporal overlapdiagnostics includes a cerium-doped yttrium aluminum garnet (Ce:YAG) screen (e.g., on a translation stage) and a near-IR and / or visible camera. The Ce: YAG screen is positioned at the intersection of the pulsed charged particle beam and the pulsed optical beam to obtain their spatial and temporal overlap. This is achieved by measuring changes in transient absorption (e.g., transmitted infrared intensity) of the Ce: YAG screen using the camera. In particular, the instrumental setup is compact and capable of quantifying spatial correlation with micron-level precision (e.g., a few pm on the order of the size of the pulsed optical beam) and temporal correlation with sub-picosecond precision, all while using nanojoules of laser energy and few- pC electron bunch charges. In some embodiments, the instrumental setup includes an optical bench (e.g., equipped with interferometer(s), spectrometer(s), optical imaging system(s), laser system(s), etc.) In some embodiments, the instrumental setup is applicable to pulsed charged particle beams (e.g., electron beams) with energies down to microjoule-level, and uses standard scintillator screens common in electron accelerators. In some embodiments, the spatiotemporally overlapped pulsed charged particle beam and pulsed optical beam are used to produce x-rays via inverse Compton scattering (ICS) (e.g., using the free-electron lasers discussed below in reference to FIGS. 1 A-1C). In some embodiments, the spatiotemporal overlap diagnostics quantifies the spatial and temporal overlap by measuring the long-term spatiotemporal stability of an ICS x-ray source operating at 1 kHz.

[0019] For the purposes of this disclosure and the appended claims, the use of the terms “substantially”, “approximately”, “about” and similar terms in reference to a descriptor of a value, element, property or characteristic at hand is intended to emphasize that the value, element, property, or characteristic referred to, while not necessarily being exactly as stated, would nevertheless be considered, for practical purposes, as stated by a person of skill in the art. These terms, as applied to a specified characteristic or quality descriptor means “mostly”, “mainly”, “considerably”, “by and large”, “essentially”, “to great or significant extent”, “largely but not necessarily wholly the same” such as to reasonably denote language of approximation and describe the specified characteristic or descriptor so that its scope would be understood by a person of ordinary skill in the art. In one specific case, the terms “approximately”, “substantially”, and “about”, when used in reference to a numerical value, represent a range of plus or minus 20% with respect to the specified value, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2% with respect to the specified value. As a non-limiting example, two values being “substantially equal” to one another implies that the difference between the two values may be within the range of + / - 20%of the value itself, preferably within the + / - 10% range of the value itself, more preferably within the range of + / - 5% of the value itself, and even more preferably within the range of + / - 2% or less of the value itself.

[0020] The use of these term in describing a chosen characteristic or concept neither implies nor provides any basis for indefiniteness and for adding a numerical limitation to the specified characteristic or descriptor. As understood by a skilled artisan, the practical deviation of the exact value or characteristic of such value, element, or property from that stated falls and may vary within a numerical range defined by an experimental measurement error that is typical when using a measurement method accepted in the art for such purposes.

[0021] For example, a reference to an identified direction or vector or line or plane being substantially parallel to a referenced line or plane is to be construed as such a direction or vector or line or plane that is the same as or very close to that of the referenced line or plane (with angular deviations from the referenced line or plane that are considered to be practically typical in related art, for example between zero and fifteen degrees, preferably between zero and ten degrees, more preferably between zero and 5 degrees, even more preferably between zero and 2 degrees, and most preferably between zero and 1 degree). For example, a reference to an identified direction or vector or line or plane being substantially perpendicular to a referenced line or plane is to be construed as such a direction or vector or line or plane the normal to the surface of which lies at or very close to the referenced line or plane (with angular deviations from the referenced line or plane that are considered to be practically typical in related art, for example between zero and fifteen degrees, preferably between zero and ten degrees, more preferably between zero and 5 degrees, even more preferably between zero and 2 degrees, and most preferably between zero and 1 degree).

[0022] Other specific examples of the meaning of the terms “substantially”, “about”, and / or “approximately” as applied to different practical situations may have been provided elsewhere in this disclosure.

[0023] An embodiment of a system generally may include electronic circuitry (for example, a computer processor) at least governing an operation of the embodiment and controlled by instructions stored in a memory, to perform specific data collection / processing and calculation steps as disclosed above. The memory may be random access memory (RAM), read-only memory (ROM), flash memory or any other memory, or combination thereof, suitable for storing control software or other instructions and data. Those skilled in the art should wouldreadily appreciate that instructions or programs defining the operation of the present embodiment(s) may be delivered to a processor in many forms, including, but not limited to, information permanently stored on non-writable storage media (e.g. read-only memory devices within a computer, such as ROM, or devices readable by a computer I / O attachment, such as CD-ROM or DVD disks), information alterably stored on writable storage media (e.g. floppy disks, removable flash memory and hard drives) or information conveyed to a computer through communication media, including wired or wireless computer networks. In addition, while the invention may be embodied in software, the functions necessary to implement a method of the invention may optionally or alternatively be embodied in part or in whole using firmware and / or hardware components, such as combinatorial logic, Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware or some combination of hardware, software and / or firmware components.

[0024] The invention as recited in claims appended to this disclosure is intended to be assessed in light of the disclosure as a whole. Various changes in the details, steps and components that have been described may be made by those skilled in the art within the principles and scope of the invention.

[0025] While the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. Accordingly, the invention should not be viewed as being limited to the disclosed embodiment s).

[0026] FIGS. 1 A-1C are schematic diagrams illustrating a light source 100 (e.g., a free- electron laser), in accordance with some embodiments. Note that, for brevity, only the most pertinent aspects of light source 100 are discussed in detail below.

[0027] In some embodiments, light source 100 produces x-rays. In some embodiments, light source 100 produces hard x-rays (e.g., x-rays having energies above 1 keV). In some embodiments, light source 100 produces soft x-rays or extreme ultraviolet light. In some embodiments, as described below, the light (e.g., x-rays) produced by light source 100 is fully spatially- and temporally-coherent (e.g. light source 100 produces light with coherence properties similar to those of conventional lasers used for generating light in optical, ultraviolet, infrared, and other wavelengths). In some embodiments, light source 100 generates light by interacting a relativistic electron beam with an electromagnetic field (e.g., either from a UVlaser, in the case of inverse Compton scattering, as described below, or from an undulator). Note, however, that for embodiments in which an undulator is used, coherent light can be generated from light source 100 using a much shorter undulator than conventional FELs (e.g., 10 meters as opposed to 100 meters). Thus, light source 100 is sometimes referred to as a compact x-ray free-electron laser (CXFEL).

[0028] Starting with FIG. 1C, an electron bunch is generated and initially accelerated using an electron photoinjector 102. For example, in some embodiments, a 4 MeV beam (e.g., electron bunch, note that the terms beam and bunch are used synonymously through the present disclosure) is generated by a 4.5 cell x-band photoinjector, which comprises a solenoid and an RF gun. The photoinjector is followed (e.g., downstream) by one or more linear accelerator (LINAC) sections (LINAC sections 104a- 104c, respectively), powered by one or more klystrons (klystrons 106a-106b). For example, in some embodiments, three 35 cm long LINAC sections 104a- 106c accelerate the beam to 35 MeV.

[0029] Note that, as shown, RF power from a single klystron 106 may be applied to several different components (e.g., klystron 106b powers both LINAC section 104b and LINAC section 104c as well as RF deflector cavity and accelerator cavity 124, whereas klystron 106a powers both the initial acceleration of the electron bunch and LINAC 105c). Further, a phase shift may be applied by phase shifters 108 (e.g., phase shifters 108a-108d) to the power supplied by the various klystrons 106 to the various components. RF loads 128 (e.g., RF loads 128a- 128b) are included and positioned where necessary for load balancing and control.

[0030] In some embodiments, a diffraction grating 110 (e.g., diffraction crystal, such as the silicon gratings described below) is located between two of the LINAC sections (or, at least, after a first LINAC section). For example, in some embodiments, the diffraction grating 110 is located between LINAC section 104a and LINAC section 104b. The diffraction grating 110 is arranged in a transmission geometry with respect to the path of the electron bunch (e.g., the direction of propagation of the electron bunch). In some embodiments, the diffraction grating 110 diffracts the electron beam at a tunable energy with a maximum of 12 MeV.

[0031] Referring now to FIGS. 1 A-1B, light source 100 includes a variety of electron optics for patterning and shaping the electron bunch downstream of the LINAC sections 104. The electron optics of light source 100 includes three main sections: a nano-pattern imaging section 112, an emittance exchange (EEX) section 114, and an inverse Compton scattering (ICS)interaction section 116. Note, however, that in some embodiments, the ICS interaction section 116 is replaced with an undulator (e.g., an undulator less than 20 m in length).

[0032] The nano-patterning imaging section 112 is downstream of LINAC section 104c and, in some embodiments, comprises two quadrupole triplets 118 (e.g., quadrupole triplet 118a and quadrupole triplet 118b) forming a telescope system.

[0033] In some embodiments, the diffraction grating 110 produces a diffraction pattern transverse to the direction of propagation of the electron bunch and the emittance exchange section 114 transforms (e.g., swaps) the diffraction pattern into a direction parallel to the direction of propagation of the electron bunch. To that end, the EEX section 114 comprises four bend magnets 120a-120d, an RF deflector cavity and accelerator cavity (collectively 124) that are independently phased and powered, along with sextupoles magnets 122a-122-c and octopole magnets 126 for aberration correction.

[0034] After EEX section 114, the ICS interaction section 116 starts with a focusing triplet 130 that reduces the beam size at the ICS interaction point 132 (e.g., to approximately a micron) before colliding the electron beam with ICS laser field from an inverse Compton scattering laser 138 (e.g., light from the inverse Compton scattering laser 138 is piped in and redirected to be nearly parallel with the electron beam at the ICS interaction point 132). The collision of the electron beam with the ICS laser field produces x-rays (or other light) 136. Downstream the ICS interaction point 132, two dipoles 134a- 134b respectively bend the beam into a beam dump (e.g., by 30 degrees horizontally and 90 degrees, respectively, into a vertical beam dump). In some embodiments, the collision of the electron beam with the ICS laser field is within a magnet field of dipole magnet 134a. ICS interaction section 116 is an example of a light-generating apparatus. An undulator (not shown) is another example of a light-generating apparatus.

[0035] FIGS. 2A-2C illustrate an example instrumental setup 200 for spatiotemporal overlap diagnostics, in accordance with some embodiments. In some embodiments, the instrumental setup 200 is used to tune spatial and temporal overlap between a pulsed charged particle beam (e.g., a relativistic electron bunch) and a pulsed optical beam (e.g., a pulsed laser) for rapid and productive accelerator alignment. In some embodiments, the spatiotemporally overlapped pulsed charged particle beam and pulsed optical beam are used to produce x-rays via ICS (e.g., light source 100 discussed above in reference to FIGS. 1A-1C).

[0036] FIG. 2A is a schematic diagram of a partial view of the instrumental setup 200 (e.g., ICS interaction point 132) used to facilitate interactions between a pulsed charged particle beam (e.g., a pulsed electron beam 202) and a pulsed optical beam (e.g., a pulsed laser beam 204). In some embodiments, the instrumental setup 200 includes a screen (e.g., a YAG screen 206) having one or more holes (e.g., an array of holes 224 in FIG. 2C). The instrumental setup 200 further includes a SubMiniature version A (SMA) cable 208. The SMA cable 208 is attached (e.g., in a physical contact) or positioned closely to a hole of the YAG screen 206. The instrumental setup 200 further includes a line of sight 210 a camera 216 (in FIG. 2B) that produces images showing interactions between the pulsed electron beam 202, the pulsed laser beam 204, and the YAG screen 206. The instrumental setup 200 further includes a first lens 212- 1 (e.g., a focusing lens) and a second lens 212-2 (e.g., a collimating lens). The first lens 212-1 is configured to focus the pulsed laser beam 204 onto the YAG screen 206 through a first optical mirror 214-1, and the second lens 212-2 is configured to collimate the pulsed laser beam 204 that passes through the YAG screen 206 and is reflected by a second optical mirror 214-2. In some embodiments, the YAG screen 206 is positioned at a normal incidence along the propagation axis of the pulsed electron beam 202, and the electron-irradiated region of the YAG screen 206 is probed with the counter propagating pulsed laser beam 204. In some embodiments, the YAG screen 206 includes a laser-machined Ce: YAG screen. In some embodiments, the pulsed laser beam 204 is at near-IR (e.g., 1030 nm). In some embodiments, the YAG screen 206 includes Ce:YAG.

[0037] FIG. 2B is a schematic diagram of another partial view of the instrumental setup 200 used to facilitate interactions between a pulsed charged particle beam (e.g., the pulsed electron beam 202) and a pulsed optical beam (e.g., the pulsed laser beam 204). As shown in FIG. 2 A, the instrumental setup 200 further includes a camera 216 configured to obtain images of the pulsed laser beam 204 that transmits through the YAG screen 206. In particular, the camera 216 is configured to capture both fluorescence of the YAG screen 206 resulting from incident electrons and intensity of laser transmission through the YAG screen 206. The instrumental setup 200 further includes a 1030 nm pulsed laser 218 to generate the pulsed laser beam 204 at 1030 nm. The instrumental setup 200 further includes an optical stage 220 configured to scan a delay between the pulsed laser beam 204 and the pulsed electron beam 202. In particular, the delay is used to determine temporal relationship between the pulsed laser beam 204 and the pulsed electron beam 202. In some embodiments, the instrumental setup 200 further includes a triple quadrupole lens 222 (e.g., the focusing triplet 130 in FIG. 1 A), In someembodiments, the camera 216 is positioned approximately 650 mm to 750 mm from the YAG screen 206 for optimize the capture of fluorescence and / or transmitted laser images. In some embodiments, the camera 216 includes a charge-coupled device (CCD) camera. In some embodiments, the camera 216 includes an IR (e.g., near-IR) camera.

[0038] FIG. 2C illustrates an example of the YAG screen 206 (e.g., a Ce: YAG screen) for measuring spatial overlap between the pulsed electron beam 202 and the pulsed laser beam 204. In some embodiments, the YAG screen 206 includes an array of holes 224 with different diameters (e.g., increasing or decreasing diameters from one side to another of the YAG screen 206). In particular, the array of holes 224 of the YAG screen 206 are used to coarsely tune spatial overlap between the pulsed electron beam 202 and the pulsed laser beam 204. For example, in some embodiments, the array of holes 224 include nine holes (e.g., 0.01 mm, 0.02 mm, 0.30 mm, 0.05 mm, 0.07 mm, 0.10 mm, 0.20 mm, 0.50 mm, and 1.0 mm from the left to the right as illustrated in FIG. 2C). Those nine holes are used for estimating the size of the pulsed electron beam 202 and roughly aligning the pulsed laser beam 204 to a nominal interaction point. The YAG screen 206 also includes a calibration hole 226 for testing the SMA cable 208. The calibration hole 226 is designed to accommodate a stripped end of the SMA cable 208, which is positioned parallel to the plane of the YAG screen 206, with its center extended to the center of the calibration hole 226. In some embodiments, the diameter of the calibration hole 226 is approximately 1 mm. In some embodiments, the calibration hole 226 is used for both spatial and temporal overlap diagnostics, as discussed below. In some embodiments, the size of the YAG screen 206 is approximately 24.5 mm x 10.5 mm. In some embodiments, the shape of the YAG screen 206 can vary (e.g., square, rectangular, circular, or other shape).

[0039] FIG. 2D illustrates an example image 230 captured by the camera 216 that shows a fluorescence spot 232 of the pulsed electron beam 202. The image 230 also includes a profile 234 of the calibration hole 226 (e.g., resulting from a portion of the reflected pulsed electron beam 202).

[0040] In some embodiments, the spatiotemporal overlap diagnostics for the pulsed electron beam 202 and the pulsed laser beam 204 includes (i) coarse tuning of spatiotemporal overlap and (ii) fine tuning of spatiotemporal overlap. In some embodiments, the coarse tuning of spatiotemporal overlap is performed prior to the fine tuning of spatiotemporal overlap. In some embodiments, temporally scanning temporal overlap (e.g., temporal delays) between thepulsed electron beam 202 and the pulsed laser beam 204 is performed after spatial overlap is tuned.

[0041] In some embodiments, the instrumental setup 200 is configured to perform coarse tuning of spatiotemporal overlap for achieving a coarse alignment between the pulsed electron beam 202 and the pulsed laser beam 204. For example, tuning spatial overlap is achieved by coarsely adjusting the relative positioning of the beams on the YAG screen 206 corresponding to the array of holes 224. In another example, tuning temporal overlap is achieved by coarsely adjusting the delay time of the pulsed laser beam 204. In some embodiments, coarse tuning of temporal overlap is conducted prior to performing fine tuning of temporal overlap. This can be achieved by identifying temporal overlap at low temporal resolution using real-time electronic (e.g., current) signals that are monitored on an oscilloscope. For example, as shown in FIG. 2C, the calibration hole 226 is designed to accommodate a stripped end of the SMA cable 208, thereby inducing a current signal when an electron bunch of the pulsed electron beam 202 passes through or near the calibration hole 226 and strikes the SMA cable 208. Similarly, when the pulsed laser beam 204 passes through or near the calibration hole 226 and strikes the SMA cable 208, it induces another current signal via ablation and ionization. Each current signal (e.g., induced by either the pulsed electron beam 202 or the pulsed laser beam 204) is detected and displayed on the oscilloscope as voltage spikes, which correspond to the timing of the respective beam to an external trigger of the oscilloscope. Accordingly, the temporal delay between the pulsed electron beam 202 and the pulsed laser beam 204 is adjusted (e.g., coarsely tuned) by modifying the delay time of the pulsed laser beam 204. In some embodiments, the temporal resolution of this coarse tuning process may be limited by the bandwidth of the oscilloscope (e.g., 25 picoseconds).

[0042] In some embodiments, the instrumental setup 200 is configured to facilitate fine tuning of spatiotemporal overlap between the pulsed electron beam 202 and the pulsed laser beam 204. In particular, fine tuning of spatiotemporal overlap leverages transient absorption induced in the YAG screen 206. During the measurement, each pulse of the pulsed electron beam 202 is directed to impinge an area (e.g., in a region away from the array of holes 224) of the YAG screen 206, such that an optical property (e.g., as measured in transmission or reflection) of the YAG screen 206 changes as a result of interaction with the pulsed electron beam 202. For example, the impinged pulses of the pulsed electron beam 202 temporary decrease near-IR transmission through the YAG screen 206. In some embodiments, the pulsed laser beam 204 that transmits through the YAG screen 206 is loosely focused onto the camera216, which detects relative changes in transmitted light intensity as the spatial and temporal alignment (e.g., position) of the electron beam pulses and the laser beams are adjusted. In some embodiments, spatially scanning the pulsed laser beam 204 across the YAG screen 206 introduces changes in light intensity that follow an inverse Gaussian curve, where the center of the inverse Gaussian curve corresponds to a maximum spatial overlap. In some embodiments, spatially scanning the pulsed laser beam 204 is repeatedly performed along both horizontal and vertical axes (e.g., a comprehensive two-dimensional (2D) scan). In some embodiments, temporally scanning the pulsed laser beam 204 by adjusting a temporal delay (e.g., Dt) introduces changes in light intensity that follow an error function. In particular, the optical stage 220 (e.g., a mechanical delay stage) is used in fine tuning of temporal overlap by scanning a temporal delay (e.g., Dt) between the pulsed electron beam 202 and the pulsed laser beam 204.

[0043] In some embodiments, the spatiotemporal overlap diagnostics includes tuning temporal overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and a pulsed optical beam (e.g., pulsed laser beam 204) using the instrumental setup 200. The instrumental setup 200 is configured to direct pulses of the pulsed charged particle beam to impinge an area of a screen (e.g., YAG screen 206). A property (e.g., optical property such as transmission, reflection, absorption, etc.) of the screen changes as a result of interaction with the pulsed charged particle beam. The instrumental setup 200 is configured to temporally scan a delay between the pulsed optical beam and the pulsed charged particle beam. For example, the optical stage 220 is used to measure, adjust, and / or minimize a temporal delay (e.g., Dt) between the pulsed electron beam 202 and the pulsed laser beam 204. The instrumental setup 200 is configured to, while temporally scanning the delay, repeatedly measure the property of the screen. For example, a pump-probe measurement technique is used to repeatedly measure the property of the screen as a function of the temporal delay, e.g., at a predetermined time interval that allows the camera 216 to capture and average multiple optical signals for enhanced precision and signal fidelity. In particular, the camera 216 is configured to produce images of the area of the screen where the pulsed charged particle beam impinges and a light beam interacts. The instrumental setup 200 is configured to determine, based on the repeated measurements of the property of the screen, a temporal relationship between the pulsed charge particle beam and the pulsed optical beam. For example, the instrumental setup 200 includes a computer system for performing signal / image processing for collected measurement data points (e.g., images detected by the camera 216, voltage spikes detected by an oscilloscope). The instrumental setup 200 is configured to tune (e.g., minimize), based on the temporal relationship, the delay betweenthe pulsed optical beam and the pulsed charged particle beam (e.g., to achieve maximal overlap between pulses of the optical beam and pulses of the charged particle beam). In some embodiments, the screen is at least partially transparent to the pulsed optical beam and the property is measured in transmission of the pulsed optical beam.

[0044] In some embodiments, the spatiotemporal overlap diagnostics includes tuning spatial overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and a pulsed optical beam (e.g., pulsed laser beam 204) using the instrumental setup 200. Similarly, the instrumental setup 200 is configured to direct pulses of the pulsed charged particle beam to impinge an area of a screen (e.g., YAG screen 206). A property (e.g., optical property such as transmission, reflection, absorption, etc.) of the screen changes as a result of interaction with the pulsed charged particle beam. The instrumental setup 200 is configured to spatially scan the pulsed optical beam across an area of a screen (e.g., YAG screen 206) to obtain a spatial overlap. For example, an optical stage (e.g., translation stage, rotation stage, piezoelectric stage, lens mount, etc.) is used to measure, adjust, and / or optimize spatial overlap between the pulsed electron beam 202 and the pulsed laser beam 204. The instrumental setup 200 is configured to, while spatially scanning the spatial overlap, repeatedly measuring the property of the screen, e.g., at a predetermined space interval that allows the camera 216 to capture a series of optical signals as a function of spatial positions. In particular, the camera 216 is configured to produce images of the area of the screen where the pulsed charged particle beam impinges and a light beam interacts. The instrumental setup 200 is configured to determine, based on the repeated measurements of the property of the screen, a spatial relationship between the pulsed charge particle beam and the pulsed optical beam. For example, the instrumental setup 200 includes a computer system for performing signal / image processing for collected measurement data points (e.g., images detected by the camera 216, voltage spikes detected by an oscilloscope). The instrumental setup 200 is configured to tune (e.g., optimize), based on the spatial relationship, the spatial overlap between the pulsed optical beam and the pulsed charged particle beam. In some embodiments, the screen is at least partially transparent to the pulsed optical beam and the property is measured in transmission of the pulsed optical beam.

[0045] In some embodiments, the spatiotemporal overlap diagnostics includes a feedback correction mechanism (e.g., discussed below in FIGS. 7A-7E) to correct the alignment including spatial and temporal overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and a pulsed optical beam (e.g., pulsed laser beam 204) using the instrumental setup 200. The feedback correction mechanism is critical for maintaining precisespatiotemporal overlap between the pulsed charged particle beam and the pulsed optical beam, as long-term drift can gradually degrade their alignment. Without correction, misalignment between the pulsed charged particle beam and the pulsed optical beam would occur, thereby reducing the interaction between the beams and compromising the generation of x-ray pulses.

[0046] In some embodiments, the feedback correction mechanism includes: (i) coarsely or finely tuning temporal overlap between the pulsed optical beam and the pulsed charged particle beam below a temporal threshold and (ii) coarsely or finely tuning spatial overlap between the pulsed optical beam and the pulsed charged particle beam below a spatial threshold. For example, in some embodiments, in accordance with a determination that temporal relationship (e.g., determined by fitting measured data points to an error function) between the pulsed optical beam and the pulsed charged particle beam does not meet temporal criteria, the instrumental setup 200 is configured to tune, based on the temporal relationship, temporal delay between the pulsed optical beam and the pulsed charged particle beam to minimize the temporal delay. In another example, in some embodiments, in accordance with a determination that the spatial relationship (e.g., determined by fitting measured data points to an inverse Gaussian distribution) does not meet spatial criteria, the instrumental setup 200 is configured to tune, based on the spatial relationship, spatial overlap between the pulsed optical beam and the pulsed charged particle beam to optimize the spatial overlap. In some embodiments, the temporal criteria include a predetermined temporal threshold in a scale of femtosecond (e.g., a few femtoseconds). In some embodiments, the spatial criteria include a predetermined positional threshold defined by root mean square (RMS) size (e.g., a few pm to tens of pm) of the pulsed charged particle beam. In some embodiments, the feedback correction mechanism is automatically conducted in real-time or at a predetermined intervals (e.g., per minute, per 30 minutes, per hour, per day, etc.).

[0047] Instrumental Setup

[0048] In some embodiments, the instrumental setup is configured to measure and tune spatiotemporal overlap between a pulsed charged particle beam (e.g., a pump source such as the pulsed electron beam 202) and a pulsed optical beam (e.g., a probe laser such as the pulsed laser beam 204) based on a pump-probe measurement technique.

[0049] In some embodiments, the pulsed electron beam 202 is produced by a photoinjector followed by three accelerating cavities that operate in the X-band at a radio frequency (RF) of 9300 MHz. The photoinjector includes a photocathode laser, e.g., a Yb:YAGLight Conversion Pharos laser producing 10.5 uJ of fourth harmonic (275 nm) incident light. The total photo-charge of the pulsed electron beam 202 is 28 pC for an ultraviolet (UV) laser (e.g., with a quantum efficiency of 1.84e-5) with the final kinetic energy after acceleration being 31.8 MeV. In some embodiments, the pulsed electron beam 202 includes a 28 pC, 31.8 MeV electron produced by a UV photoinjector and accelerated through an X-band linear accelerator with four accelerating modules and driven by two scandinova klystrons. The accelerator operates at 1 kHz repetition rate. The photocathode laser is a Yb:YAG Light Conversion Pharos laser producing 6 pj of fourth harmonic (275 nm) light, incident on the photocathode. The UV beam is shaped to a flat-top profile on the cathode with a diameter of 800 pm.

[0050] In some embodiments, the pulsed laser beam 204 acts as a probe laser and includes an attenuated thin-disk Yb:YAG laser (Trumpf DIRA-200) which is attenuated to 6 uJ of pulse energy. In particular, the pulsed laser beam 204 has a pulse duration of 1.5 ps. A lens of focal length f = 228mm focuses the pulsed laser beam 204 to a spot size of 10 pm at the interaction point between the pulsed electron beam 202 and the pulsed laser beam 204 on the YAG screen 206.

[0051] Coarse Tuning: Calibration

[0052] In some embodiments, the spatiotemporal overlap diagnostics includes a calibration step to coarsely align a pulsed optical beam (e.g., pulsed laser beam 204) with a screen (e.g., YAG screen 206) using the instrumental setup 200, prior to performing the coarse and / or fine tuning of spatiotemporal overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and the pulsed optical beam (e.g., pulsed laser beam 204). As shown in FIGS 2A and 2B, the intersection of the pulsed electron beam 202 and the pulsed laser beam 204 is at a finite angle. Accordingly, the beams can be focused and overlapped at a single point within the chamber for maximize the efficiency of the ICS process when producing x-rays.

[0053] FIGS. 3A-3D illustrates an example laser-screen calibration 300 for coarsely aligning the pulsed laser beam 204 with the YAG screen 206, in accordance with some embodiments. During the laser-screen calibration 300, the instrumental setup 200 is configured to scan the YAG screen 206 along the propagation axis of the pulsed laser beam 204. This scanning process identifies the optimal focal position of the pulsed laser beam 204 by detecting the intensity distribution of two-photon fluorescence within the YAG material (e.g., fluorescence signal induced by a two-photon excitation). In particular, the two-photon fluorescence signal serves as an indicator of the focal plane of the pulsed laser beam 204 forprecise alignment between the pulsed laser beam 204 and the YAG screen 206: the position that maximizes the optical intensity of the two-photon fluorescence concurrently minimizes the laser beam radius on the YAG screen 206.

[0054] FIG. 3 A illustrates a zoomed-out 2D fluorescence image 302 of the YAG screen 206 detected by the camera 216. In particular, the zoomed-out 2D fluorescence image 302 includes a two-photon fluorescence signal 306 whose optical intensity reaches a maximum when the YAG screen 206 is positioned at the focus of the pulsed laser beam 204.

[0055] FIG. 3B illustrates a z-scan 304 of the YAG screen 206 detected by the camera 216. In particular, the z-scan 304 is obtained by scanning the the YAG screen 206 through the direction of propagation of the pulsed laser beam 204. As shown in FIG. 3B, the optical intensity of the two-photon fluorescence signal 306 is measured along the z-scan 304 and fitted to a Lorentzian function to determine the focal position.

[0056] FIGS. 3C and 3D illustrate a first zoomed-in 2D fluorescence image 308 and a second zoomed-in 2D fluorescence image 310, respectively. In particular, the first zoomed-in 2D fluorescence image 308 is detected at the focus and the second zoomed-in 2D fluorescence image 310 is detected out of the focus. In some circumstances, it is difficult to directly measure the size of the pulsed laser beam 204 from the two-photon fluorescence signal 306. For example, the spot size of the pulsed laser beam 204 (e.g., estimated to be 10 pm based on the lens focal length and the beam’s quality factor M2) is smaller than the spatial resolution of the camera 216. In this situation, the spot size can be determined by scanning the optical intensity of two-photon fluorescence signal 306 within a region of interest (ROI) as a function of position and fitting the scanned curve (e.g., z-scan 304) to a Lorentzian function. Accordingly, the peak 313 of the fitted Lorentzian function 312 the corresponds to the optimal focus.

[0057] Coarse Tuning: Spatial

[0058] In some embodiments, the spatiotemporal overlap diagnostics includes coarse tuning of spatial overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and a pulsed optical beam (e.g., pulsed laser beam 204) using the instrumental setup 200. In some embodiments, the coarse tuning of spatial overlap is conducted after the laser-screen calibration 300 is performed (e.g., FIGS. 3A-3D). In particular, with the plane of the YAG screen 206 (e.g., a Ce: YAG screen) positioned at the focus point of the pulsed laser beam 204, a coarse spatial overlap between the pulsed electron beam 202 and the pulsed laser beam 204 can be achieved using the array of holes 224 on the YAG screen 206.

[0059] In some embodiments, the array of holes 224 serves as apertures for centering the pulsed electron beam 202 and the pulsed laser beam 204. The spacing between holes is predetermined (e.g., 1.0 mm center-to-center or edge-to-edge). In particular, the array of holes 224 can be considered entirely as a single aperture. The size of the single aperture can be adjusted by translating the YAG screen 206 based on the predetermined spacing (e.g., 1.0 mm) and the combined radii of corresponding holes (e.g., the current hole and its adjacent holes). Stated another way, this approach allows the array of holes 224 to function collectively as a single aperture to maximize the precision of coarse tuning of spatial overlap. In some embodiments, during the coarse tuning, the positions of YAG screen 206 and / or the pulsed laser beam 204 are adjusted, while the position the pulsed laser beam 204 remains fixed. In some embodiments, during the coarse tuning, all three positions of YAG screen 206, the pulsed laser beam 204, and the pulsed laser beam 204 can be adjusted as needed.

[0060] In some embodiments, the coarse tuning of spatial overlap starts with aligning both pulsed electron beam 202 and pulsed laser beam 204 with the largest hole (e.g., 1.0 mm hole) of the array of holes 224. The YAG screen 206 is moved to a position, such that the largest hole is centered around the pulsed electron beam 202. At this stage, the pulsed laser beam 204 is blocked (e.g., using an optical shutter), and any detected fluorescence signal (e.g., two-photon fluorescence signal 306) around the edge of the largest hole originates solely from the pulsed electron beam 202. Stated another way, the pulsed electron beam 202 is centered when the fluorescence signal is symmetric around the edge of the largest hole. Then, the pulsed electron beam 202 is turned off and the pulsed laser beam 204 is unblocked. The pulsed laser beam 204 is centered in a similar manner as the pulsed electron beam 202. In this circumstance, the position of the YAG screen 206 remains fixed and the position of the pulsed laser beam 204 is adjusted by translating a lens upstream. Once centered, the pulsed laser beam 204 is blocked, and the pulsed electron beam 202 is turned on. In some embodiments, the alignment process discussed above is repeated iteratively with progressively smaller holes (e.g., from 1.0 mm hole to 0.01 mm hole). For larger holes, the spot of the pulsed laser beam 204 may be too small to generate a strong two-photon fluorescence signal at the edges of these larger holes. In this circumstance, the pulsed laser beam 204 can be centered by scanning the beam (e.g., using lens’ translation stage) around outside of the larger holes until the two-photon fluorescence signal aligns with either vertical or horizontal axis. In particular, the corresponding translation stage control values are recorded, and their superposition is used to determine the center of the beam.

[0061] Coarse Tuning: Temporal

[0062] In some embodiments, the spatiotemporal overlap diagnostics includes coarse tuning of temporal overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and a pulsed optical beam (e.g., pulsed laser beam 204) using the instrumental setup 200. In some embodiments, the coarse tuning of temporal overlap is performed after the coarse tuning of spatial overlap (e.g., when the pulsed electron beam 202 and the pulsed laser beam 204 are coarsely spatially overlapped in space).

[0063] In some embodiments, since an initial relative temporal position of each pulse of the pulsed laser beam 204 can occur anywhere within in the 1 kHz cycle (e.g., the repetition rate of the accelerator), the coarse tuning of temporal overlap is needed to determine relative arrival times of the pulsed electron beam 202 and the pulsed laser beam 204 with sub-nanosecond precision. In particular, the coarse tuning of temporal overlap synchronizes arrivals of pulses closely enough, such that their relative timing can be detected through transient absorption induced in the YAG screen 206 with few-picosecond precision, which is within the timing window of a mechanical delay stage (e.g., optical stage 220).

[0064] FIG. 4 illustrates an example coarse tuning of temporal overlap 400 based on a SMA cable test, in accordance with some embodiments. In some embodiments, the SMA cable test is a compact diagnostic method that utilizes a bare SMA cable to measure coarse timing the pulsed electron beam 202 and the pulsed laser beam 204. As discussed above (e.g., in reference to FIG. 2C), during the SMA cable test, a stripped end of the SMA cable 208 is aligned with the calibration hole 226 (e.g., a 1.0 mm hole), and another end of the SMA cable 208 is connected to an oscilloscope. A current signal is induced when the pulsed electron beam 202 passes through or near the calibration hole 226 and strikes the SMA cable 208. Another current signal is induced when the pulsed laser beam 204 passes through or near the same hole and strikes the SMA cable 208. Each current signal is detected and displayed on an oscilloscope as voltage spikes (e.g., oscillatory signals), which correspond to the timing of the respective beam to an external trigger of the oscilloscope.

[0065] As shown in FIG. 4, an oscilloscope reading 402 is recorded during the SMA cable test. The oscilloscope reading 402 includes an electron beam reference rending 410, a first laser beam reading 412 with a negative 100 mm delay (e.g., -100 mm as the negative extreme), a second laser beam reading 414 with a 0 mm delay (e.g., 0 mm as the center), and a third laser beam reading 416 with a positive 100 mm delay (e.g., +100 mm as the positive extreme). In some embodiments, the pulsed electron beam 202 is first turned on. The electron beam referencerending 410 is detected when the pulsed electron beam 202 is directed to strike the SMA cable 208 through the calibration hole 226, generating an RF pickup on the SMA cable 208 due to the transient Coulomb field of the electron bunch. The electron beam reference rending 410 is recorded on the oscilloscope as a reference, and the pulsed electron beam 202 is turned off. In particular, the electron beam reference rending 410 includes a negative spike 420, which signifies arrival time of the pulsed electron beam 202. Then, the pulsed laser beam 204 is directed to pass through the same hole and strike the SMA cable 208, thereby producing a transient signal (e.g., voltage spike) based on ionization at the tip of the SMA cable 208. In particular, the transient voltage spike (e.g., first to third laser beam readings 412, 414, and 416) is compared with the timing of the electron beam reference rending 410 to determine temporal delays between the pulsed electron beam 202 and the pulsed laser beam 204. In some embodiments, the delay of the pulsed laser beam 204 is adjusted by a mechanical delay stage (e.g., optical stage 220).

[0066] In some embodiments, the temporal resolution of the SMA cable test may be limited by the bandwidth of the oscilloscope (e.g., 25 picoseconds). In some embodiments, the SMA cable test for the coarse tuning of temporal overlap is performed prior to the fine tuning of temporal. In some embodiments, the pulsed laser beam 204 is seeded from the same 72 MHz laser oscillator (e.g., at a repetition rate of 13.8 ns) that is used for the photoinjector to produce the pulsed electron beam 202. Accordingly, the pulsed laser beam 204 is intrinsically locked to nanosecond precision (e.g., 1 x 13.8 ns, 2x 13.8 ns, 3x 13.8 ns, etc.). In this circumstance, a specific 13.8 ns bucket can be selected during the coarse tuning of temporal overlap.

[0067] Fine Tuning: Spatial and Temporal

[0068] In some embodiments, the spatiotemporal overlap diagnostics includes fine tuning of spatiotemporal overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and a pulsed optical beam (e.g., pulsed laser beam 204) using the instrumental setup 200. It should be noted that the spatial and temporal fine tuning processes and apparatus described herein are independent of each other, as well as any processes used to generate coarse spatiotemporal overlap, including those processes and apparatus described above. In addition, although the transmission of a YAG is used as an example of an optical property of a material that changes as a result of interaction with a charged particle beam, any material having a measurable optical property that changes as a result of interaction with a charged particle may be used, including optical properties that are measured in reflection or in some other way.

[0069] In some embodiments, the fine tuning of spatiotemporal overlap is performed after the coarse tuning of spatiotemporal overlap (e.g., when the pulsed electron beam 202 and the pulsed laser beam 204 are coarsely spatiotemporally overlapped). In particular, the fine tuning of spatiotemporal overlap leverages transient absorption induced in the YAG screen 206. In some embodiments, the fine tuning of spatiotemporal overlap starts with the fine tunning of spatial overlap. Stated another way, spatial overlap is fine-tuned before temporal overlap is finely adjusted.

[0070] In some embodiments, optimizing spatial overlap is achieved by scanning the pulsed laser beam 204 across the fluorescence on the YAG screen 206, which is excited by the pulsed electron beam 202. In particular, a section of the YAG screen 206 with no holes is moved to the intersection of the pulsed electron beam 202 and the pulsed laser beam 204. The camera 216 is configured to detect the transmitted pulsed laser beam 204 through the YAG screen 206 (e.g., FIGS 2A and 2B).When the pulsed electron beam 202 strikes the YAG screen 206, electrons inelastically scatter off carriers in the YAG, generating a population of free carriers. These free carriers absorb incoming infrared radiation from the pulsed laser beam 204, leading to a reduction in the transmission of the YAG screen 206. Accordingly, both spatial overlap and the temporal overlap can be measured separately by monitoring the transmitted intensity of the pulsed laser beam 204.

[0071] FIG. 5A-5B illustrate an example fine tuning of spatial overlap 500, in accordance with some embodiments. FIG. 5A illustrates a beam profile 502, which is one of a series of images taken by the camera 216. In particular, the beam profile 502 includes a first curve 510 and a second curve 512 that represent vertical and horizontal sums with the pulsed laser beam 204 at the beginning and middle of a spatial overlap scan, respectively. FIG. 5B illustrates a transmitted optical intensity plot 504 that shows a series of data points 520 representing total optical intensity of the transmitted pulsed laser beam 204 as a function of the position of a focusing lens (e.g., first lens 212-1) along the vertical (y) axis. The decrease in the middle of the data points 520 signifies a spatial overlap between the pulsed electron beam 202 and the pulsed laser beam 204. The data points 520 are fitted to a Gaussian curve 522 (e.g., an inverse Gaussian distribution) to determine the spatial overlap. The Gaussian curve 522 for fitting is in the form of(x - x0)2g(x = A • exp + BThe center of the fitted Gaussian curve 522 corresponds to the position of the spatial overlap. In some embodiments, the spatial overlap scan is performed along both vertical (y) and horizontal (x) axes, such that the pulsed laser beam 204 can be easily relocated to the optimal position.

[0072] FIG. 6A-6B illustrate an example fine tuning of temporal overlap 600, in accordance with some embodiments. In particular, the fine tuning of temporal overlap 600 is performed using a mechanical delay stage (e.g., optical stage 220) to scan the pulsed laser beam 204 in real-time. In some embodiments, a start and end point for the temporal overlap scan is selected that passes over the estimated overlap time of overlap (e.g., tO) as determined via the SMA cable test. FIG. 6A illustrates a beam profile 602, which is one of a series of images taken by the camera 216. In particular, the beam profile 602 includes a first curve 610 and a second curve 612 that represent vertical and horizontal sums with the pulsed laser beam 204 arriving before and after tO. FIG. 6B illustrates a transmitted optical intensity plot 604 that shows a series of data points 620 representing total optical intensity of the transmitted pulsed laser beam 204 as a function of the temporal delay. In particular, the pulsed laser beam 204 arrives earlier relative to the pulsed electron beam 202 and is therefore unattenuated. As pulses of the pulsed laser beam 204 passes tO, the transmitted optical intensity starts to decrease due to excitation from the pulsed electron beam 202. The data points 620 are fitted to an error function 622 to determine the temporal overlap. The error function 622 for fitting is in the form of: / t — t\ (t) = A ■ erf ( - ) + B\ <J / A derivative 624 of the fitted error function 622 is taken to find the extremum (e.g., minimum) for determining the temporal overlap. In particular, the derivative 624 follows a Gaussian distribution centered around to. In other circumstances when the pulsed laser beam 204 arrives relative to the pulsed electron beam 202, the transmitted optical intensity rises as it passes to. In this situation, the extremum in the derivative of the error function corresponds to a maximum.

[0073] In some embodiments, the fine tuning of spatiotemporal overlap is performed after the coarse tuning of spatiotemporal overlap. In some embodiments, the fine tuning of temporal overlap is performed either before or after the fine tuning of spatial overlap. In some circumstances, the order can depend on the specific combination of components (e.g., pulsed charged particle beam, pulsed optical beam, YAG screen, optical components, etc.) used in the instrumental setup 200. In some embodiments, alternating spatial and temporal overlap scans are repeated multiple times to determine and optimize spatiotemporal overlap between a chargedparticle beam and a pulsed optical beam. For example, each spatial overlap scan may slightly alter tO, and vice versa.

[0074] Resolution and Sensitivity

[0075] In some embodiments, temporal resolution of the fine tuning of temporal is determined by a few factors including (1) duration re of the pulsed electron beam 202, (2) duration T1 of the pulsed laser beam 204, (3) jitter rj of between the pulsed electron beam 202 and the pulsed laser beam 204, (4) response time TYAG of the YAG screen 206, and (5) effect rd of the finite thickness of the YAG screen 206, as the beams counterpropogate. In particular, the overall temporal response function is a convolution of all response functions associated with the factors discussed above. Assuming that all response functions are approximately Gaussian, the total temporal resolution rtot (e.g., response temporal width) is the quadrature sum of the individual factors in the form of:For example, as shown in FIGS. 6A and 6B, the duration n of the pulsed laser beam 204 is measured to be 1.5 picosecond (ps) at FWHM, the duration reof the pulsed electron beam 202 is approximately 100 femtosecond (fs) at FWHM, the jitter Tj is approximately 282 fs at FWHM, and the effect ra of the finite thickness of the YAG screen 206 is approximately 830 fs. In some circumstances, the response time TYAG of the YAG screen 206 is negligible on either the fs or ps scale. Accordingly, the total temporal resolution rtot is determined to be approximately 1.7 ps.

[0076] In some embodiments, the spatiotemporal overlap diagnostics accommodates both small and large electron beams and ensures high sensitivity. For example, a 25 MeV electron beam with 20 pC of charge can produce a 40% change in YAG transmission when focused to a 10 z / m RMS spot. This corresponds to a peak fluorescence signal in the YAG of approximately F = 80 J / cm2. The fluorescence signal may saturate close to this level. On the other hand, the noise floor for a single scan (e.g., 100 shots per time point) is approximately 103J / cm2. As a result, even an electron pulse with a bunch charge of 0.25 pC can achieve a signal- to-noise ratio of 5 dB, which is sufficient for the fine tuning of temporal overlap. In another example, a large electron beam with a 90 z / m RMS spot and 20 pC of bunch charge may be detectable.

[0077] Feedback Correction Mechanism

[0078] In some embodiments, the spatiotemporal overlap diagnostics includes a feedback correction mechanism to correct the alignment including spatial and temporal overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and a pulsed optical beam (e.g., pulsed laser beam 204) using the instrumental setup 200. In particular, the spatiotemporal overlap diagnostics is capable of tracking long-term drift in spatiotemporal overlap. In some circumstances, temporal drift of the pulsed electron beam 202 and the pulsed laser beam 204 result from different factors including thermal variations in amplifier cavities, thermal expansion of beamlines and optical tables, and drifts in the RF phase relative to laser timing. In some embodiments, the temporal drift is measured by repeated spatial and temporal overlap scans at a predetermined interval (e.g., periodically throughout the day). This allows for tuning positions of the pulsed electron beam 202 and / or the pulsed laser beam 204 back to optimal values.

[0079] FIG. 7A-7E illustrate an example feedback correction mechanism 700 over a period, in accordance with some embodiments. Specifically, the feedback correction mechanism 700 operates over a 136-minute period of beam operation, during which the spatiotemporal overlap between the pulsed electron beam 202 and the pulsed laser beam 204 is periodically measured. FIGS. 7 A and 7B illustrate vertical and horizontal drifts in the spatial overlap as a function of time, respectively. FIGS. 7C illustrates the magnitude of the spatial overlap drift relative to an RMS size 702 (e.g., 1 / e2radius) of the pulsed electron beam 202 at various time points throughout the period (e.g., 0 min, 22 min, 61 min, 68 min, 85 min, 114, min). FIGS. 7D illustrates a long-term drift of the temporal overlap away from the optimal temporal position. FIGS. 7E illustrates a series of temporal overlap scans (e.g., error function fittings as discussed above in FIG. 6B) including a reference temporal overlap scan 704 at a 40-minute time point. As shown in FIG. 7A-7E, uncorrected spatiotemporal drift can cause the pulsed electron beam 202 and the pulsed laser beam 204 to fall out of the optimal spatiotemporal overlap within a few minutes timescale.

[0080] Figures 8A-8C collectively provide a flow chart of a method 800 for tuning spatiotemporal overlap between a pulsed charged particle beam (e.g., pulsed electron beam 202) and a pulsed optical beam (e.g., pulsed laser beam 204), in accordance with some embodiments of the present disclosure.

[0081] Block 802 in FIG. 8A. Method 800 includes directing pulses of the pulsed charged particle beam to impinge an area of a screen (e.g., YAG screen 206).

[0082] Block 804 in FIG. 8A. A property (e.g., optical property such as transmission, reflection, absorption, etc.) of the screen changes as a result of interaction with the pulsed charged particle beam.

[0083] Block 806 in FIG. 8A. Method 800 includes temporally scanning a delay (e.g., temporal delay Z>t) between the pulsed optical beam and the pulsed charged particle beam.

[0084] Block 808 in FIG. 8A. Method 800 includes while temporally scanning the delay, repeatedly measuring the property of the screen, e.g., at a predetermined time interval that allows the camera 216 to capture and average multiple optical signals for enhanced precision and signal fidelity.

[0085] Block 810 in FIG. 8 A. Method 800 includes determining, based on the repeated measurements of the property of the screen, a temporal relationship between the pulsed charge particle beam and the pulsed optical beam, e.g., using a pump-probe measurement technique.

[0086] Block 812 in FIG. 8 A. Method 800 includes tuning (e.g., minimizing), based on the temporal relationship, the delay between the pulsed optical beam and the pulsed charged particle beam.

[0087] Block 814 in FIG. 8B. Method 800 includes spatially scanning the pulsed optical beam across the area of the screen to obtain a spatial overlap, e.g., adjusting a spatial position of the pulsed optical beam using an optical stage (e.g., translation stage, rotation stage, piezoelectric stage, lens mount, etc.).

[0088] Block 816 in FIG. 8B. Method 800 includes while spatially scanning the spatial overlap, repeatedly measuring the property of the screen, e.g., at a predetermined space interval that allows the camera 216 to capture a series of optical signals as a function of spatial positions.

[0089] Block 818 in FIG. 8B. Method 800 includes determining, based on the repeated measurements of the property of the screen, a spatial relationship between the pulsed charge particle beam and the pulsed optical beam. For example, the instrumental setup 200 includes a computer system for performing signal / image processing for collected measurement data points (e.g., images detected by the camera 216, voltage spikes detected by an oscilloscope)

[0090] Block 820 in FIG. 8B. Method 800 includes tuning (e.g., optimizing), based on the spatial relationship, the spatial overlap between the pulsed optical beam and the pulsed charged particle beam.

[0091] In some embodiments, temporally scanning the delay between the pulsed optical beam and the pulsed charged particle beam is performed after the spatial overlap is tuned.

[0092] In some embodiments, tuning the delay between the pulsed optical beam and the pulsed charged particle beam includes: in accordance with a determination that the temporal relationship does not meet temporal criteria, tuning, based on the temporal relationship, the delay between the pulsed optical beam and the pulsed charged particle beam to minimize the delay. For example, the spatiotemporal overlap diagnostics includes a feedback correction mechanism (e.g., in reference to FIGS. 7A-7E) to correct drift of the temporal overlap over time.

[0093] In some embodiments, tuning the spatial overlap between the pulsed optical beam and the pulsed charged particle beam includes: in accordance with a determination that the spatial relationship does not meet spatial criteria, tuning, based on the spatial relationship, the spatial overlap between the pulsed optical beam and the pulsed charged particle beam to optimize the spatial overlap. For example, the spatiotemporal overlap diagnostics includes a feedback correction mechanism (e.g., in reference to FIGS. 7A-7E) to correct drift of the spatial overlap over time.

[0094] In some embodiments, the temporal criteria include a predetermined temporal threshold in a scale of femtosecond (e.g., a few fs).

[0095] In some embodiments, the spatial criteria include a predetermined positional threshold defined by root mean square (RMS) size of the pulsed charged particle beam (e.g., a few / / m).

[0096] In some embodiments, the screen is at least partially transparent to the pulsed optical beam and the property is measured in transmission of the pulsed optical beam. For example, the fine tuning of spatiotemporal overlap leverages transient absorption induced in the YAG screen 206.

[0097] In some embodiments, the screen includes yttrium aluminum garnet (YAG) (e.g., Ce:YAG).

[0098] In some embodiments, Method 800 includes generating x-ray pulses through an interaction of the pulsed charged particle beam and the pulsed optical beam (e.g., in references to FIGS. 1A-1C).

[0099] In some embodiments, the x-ray pulses are generated through inverse Compton scatter (ICS) (e.g., in references to FIGS. 1A-1C).

[0100] In some embodiments, the pulsed charged particle beam includes a pulsed electron beam (e.g., a UV laser with a final kinetic energy after acceleration being 31.8 MeV).

[0101] In some embodiments, the pulsed optical beam is a pulsed laser beam (e.g., a 1030 nm near-IR laser).

[0102] Block 822 in FIG. 8C. Method 800 includes providing a screen (e.g., YAG screen 206).

[0103] Block 824 in FIG. 8C. A property (e.g., optical property such as transmission, reflection, absorption, etc.) of the screen changes as a result of interaction with a pulsed charged partial beam.

[0104] Block 826 in FIG. 8C. Method 800 includes providing a camera (e.g., camera 216) configured to produce an image (e.g., with two-photon fluorescence signals) of an area of the screen where the pulsed charged particle beam impinges and a light beam interacts.

[0105] Block 828 in FIG. 8C. Method 800 includes providing a computer system (e.g., as part of the instrumental setup 200) including one or more processors and memory storing instructions for determining, based on the image of the screen, a temporal relationship and / or a spatial relationship between the pulsed optical beam and the pulsed charged particle beam using any of the above methods.

[0106] Block 830 in FIG. 8C. Method 800 includes providing an optical stage (e.g., a mechanical delay stage such as the optical stage 220) configured to scan a delay (e.g., temporal delay Z>t) between the pulsed optical beam and the pulsed charged particle beam.

[0107] Block 832 in FIG. 8C. The delay is used to determine the temporal relationship between the pulsed optical beam and the pulsed charged particle beam.

Claims

CLAIMSWhat is claimed is:

1. A method for tuning temporal overlap between a pulsed charged particle beam and a pulsed optical beam, comprising: directing pulses of the pulsed charged particle beam to impinge an area of a screen, wherein a property of the screen changes as a result of interaction with the pulsed charged particle beam; temporally scanning a delay between the pulsed optical beam and the pulsed charged particle beam; while temporally scanning the delay, repeatedly measuring the property of the screen; determining, based on the repeated measurements of the property of the screen, a temporal relationship between the pulsed charge particle beam and the pulsed optical beam; and tuning, based on the temporal relationship, the delay between the pulsed optical beam and the pulsed charged particle beam.

2. The method of claim 1, further comprising: spatially scanning the pulsed optical beam across the area of the screen to obtain a spatial overlap; while spatially scanning the spatial overlap, repeatedly measuring the property of the screen; determining, based on the repeated measurements of the property of the screen, a spatial relationship between the pulsed charge particle beam and the pulsed optical beam; and tuning, based on the spatial relationship, the spatial overlap between the pulsed optical beam and the pulsed charged particle beam.

3. The method of claim 2, wherein temporally scanning the delay between the pulsed optical beam and the pulsed charged particle beam is performed after the spatial overlap is tuned.

4. The method of claim 1, wherein tuning the delay between the pulsed optical beam and the pulsed charged particle beam includes: in accordance with a determination that the temporal relationship does not meet temporal criteria, tuning, based on the temporal relationship, the delay between the pulsed optical beam and the pulsed charged particle beam to minimize the delay.

5. The method of claim 2, wherein tuning the spatial overlap between the pulsed optical beam and the pulsed charged particle beam includes: in accordance with a determination that the spatial relationship does not meet spatial criteria, tuning, based on the spatial relationship, the spatial overlap between the pulsed optical beam and the pulsed charged particle beam to optimize the spatial overlap.

6. The method of claim 4, wherein the temporal criteria include a predetermined temporal threshold in a scale of femtosecond.

7. The method of claim 5, wherein the spatial criteria include a predetermined positional threshold defined by root mean square (RMS) size of the pulsed charged particle beam.

8. The method of claim 1, wherein the screen is at least partially transparent to the pulsed optical beam and the property is measured in transmission of the pulsed optical beam.

9. The method of claim 1, wherein the screen includes yttrium aluminum garnet (YAG).

10. The method of claim 1, further comprising: generating x-ray pulses through an interaction of the pulsed charged particle beam and the pulsed optical beam.

11. The method of claim 10, wherein the x-ray pulses are generated through inverse Compton scatter (ICS).

12. The method of claim 1, wherein the pulsed charged particle beam includes a pulsed electron beam.

13. The method of claim 1, wherein the pulsed optical beam is a pulsed laser beam.

14. A device for tuning temporal and spatial overlap between a pulsed charged particle beam and a pulsed optical beam, comprising: a screen, wherein a property of the screen changes as a result of interaction with a pulsed charged partial beam; a camera configured to produce an image of an area of the screen where the pulsed charged particle beam impinges and a light beam interacts; anda computer system including one or more processors and memory storing instructions for determining, based on the image of the screen, a temporal relationship and / or a spatial relationship between the pulsed optical beam and the pulsed charged particle beam using the method of claim 1.

15. The device of claim 14, further comprising: an optical stage configured to scan a delay between the pulsed optical beam and the pulsed charged particle beam, wherein the delay is used to determine the temporal relationship between the pulsed optical beam and the pulsed charged particle beam.

Images