Laser induced proton acceleration systems and methods

Abstract

The present disclosure provides a laser-based proton acceleration system, comprising: a laser source configured to generate laser pulses suitable for inducing proton acceleration upon interaction with a target material; a laser focusing assembly configured to focus the laser pulses; a target including a suspended slat made of said target material and configured for liberating and accelerating protons upon interaction with said laser pulses. A slat thickness is such that electrons from a rear face of the suspended slat opposite to the front face significantly contribute to forming a normal sheath electric field. The laser source and the laser focusing assembly are configured so that laser pulses impinge on a front face of said suspended slat and a transverse cross section of said suspended slat may be fully immersed within a laser pulse focal volume.

Description

[0001] LASER INDUCED PROTON ACCELERATION SYSTEMS AND METHODS

[0002] TECHNOLOGICAL FIELD

[0003] The present invention pertains to the field of laser-induced proton acceleration systems. Specifically, it relates to a laser-based proton acceleration system designed for precise control of proton beams, and its adaptation for applications, particularly proton radiotherapy.

[0004] BACKGROUND

[0005] The general observation that the interaction of intense laser pulses with matter results in the emission of multiple forms of radiation, including x-rays, electrons, ions and positrons, has motivated the research of compact laser-based particle accelerators. The prospects of accelerating ions to MeV energies and beyond on a compact scale hold potential for many applications, including proton fast-ignition, proton radiography, neutron generation and ion-radiotherapy. The latter may result in a tremendous social impact, since the enormous size and cost of ion-radiotherapy treatment centers based on conventional accelerator technology is a bottleneck for the proliferation of this treatment method. A first requirement for laser-based technology to fit this application is to be able to accelerate protons to energies high enough to penetrate human tissue to reach any tumor, which is about 250 MeV.

[0006] Acceleration of ions out of the bulk of the target may be achieved through several mechanisms, among them the radiation-pressure acceleration, breakout afterburner, and collision-less shock acceleration. Compared to these volumetric mechanisms for which scarce experimental data exists, the target-normal sheath acceleration (TNSA) has been extensively studied in dozens of different experimental scenarios. TNSA relies on high- gradient electric fields that form between the irradiated target and the electron sheath that develops around it, to accelerate ions from contaminates at the target surface. GENERAL DESCRIPTION

[0007] The general phenomenology of TNSA is that ion energies rise with different scaling behaviors with higher laser energies and shorter pulse durations.

[0008] It has been observed for some targets that proton energies rise when the target thickness approaches the scale of the laser wavelength or smaller. Further, it has been observed for some targets that limiting the transverse size of a target enhances the accelerating gradient of the electron sheath. The inventors have found that the irradiation of single formations (e.g. slat) that are both immersed in the focal volume and have a thickness smaller than half the laser’s wavelength result in a sharp rise of the emitted ion energies, to energies higher than any other experimental method when using a comparable laser pulse energy. This effect is not a mere combination of the two aforementioned observations, but rather a manifestation of a different more efficient ability of the laser fields to transfer energy to the electrons that form the sheath. Hints for this dynamics may be obtained from few experiments that showed enhanced proton energies when the irradiated surfaces possess geometric structures with nanometric or micrometric formations.

[0009] Irradiation of truly isolated targets fully immersed in the laser focal volume is mechanically challenging and was only demonstrated using a Paul trap to levitate single micrometer-scale plastic spheres. These experiments did not result in higher proton energies than those obtained with solid foils, however the protons featured a reduced energy bandwidth. The absence of lower energy protons was identified to result from Coulombic repulsion between slower protons trailing at the back of the proton bunch and heavier, less mobile ions.

[0010] In particular, the Applicant has found that the interaction of an intense laser pulse with an object whose dimensions are transversely immersed in the focal volume and thinner than half the laser wavelength results in enhanced TNSA, with the emitted protons reaching 3 times the energy of those obtained with a conventional planar foil target. By irradiating 2 pm wide, 0.2 pm thick gold bar targets protons were accelerated to over 6 MeV using only 120 mJ of laser energy on-target. Beyond the increase in ion energies, the smaller target dimensions provide a small virtual source size and low emittance, and makes possible cascaded acceleration by irradiation of multiple targets with micrometric spacing, which could provide even higher proton energies and optical means to control their spectrum.

[0011] In accordance with the presently disclosed subject matter, there is provided a laserbased proton acceleration system, comprising: a laser source configured to generate laser pulses suitable for inducing proton acceleration upon interaction with a target material; a laser focusing assembly configured to focus the laser pulses; a target including a suspended slat made of said target material and configured for liberating and accelerating protons upon interaction with said laser pulses. A slat thickness is such that electrons from a rear face of the suspended slat opposite to the front face significantly contribute to forming a normal sheath electric field. The laser source and the laser focusing assembly are configured so that laser pulses impinge on a front face of said suspended slat and a transverse cross section of said suspended slat may be fully immersed within a laser pulse focal volume. In other words, the transverse cross-section of the slat (i.e. a slat cut) is contained within the laser pulse focal volume. The transverse cross section may be defined transverse to a slat longitudinal axis e.g. perpendicular to the longitudinal axis.

[0012] In accordance with the presently disclosed subject matter, there is also provided a laser-based proton acceleration system comprising: (a) a laser source configured to generate laser pulses suitable for inducing proton acceleration upon interaction with a target material; (b) a beam splitter configured for splitting said laser pulses into a plurality of split laser pulses; (c) a laser focusing assembly configured to focus the split laser pulses onto a target; wherein the target including a plurality of suspended slats made of said target material and configured for liberating and accelerating protons upon interaction with said split laser pulses, wherein each suspended slat has a slat thickness such that electrons from a rear face of said suspended slat opposite to a front face significantly contribute to forming a normal sheath electric field. The laser source and the laser focusing assembly are configured so that each split laser pulse impinges on the front face of a respective suspended slat and a transverse cross section of each suspended slat is contained within a respective split laser pulse focal volume.

[0013] In accordance with the presently disclosed subject matter, there is also provided a laser-based proton acceleration system for cascaded proton acceleration. The system comprises: a laser source configured to generate and split laser pulses suitable for inducing proton acceleration upon interaction with a target material; a laser focusing assembly configured to individually focus the split laser pulses; a target comprising a plurality of suspended slats, each made of said target material (or a similar target material) and configured for liberating and accelerating protons upon interaction with the respective split laser pulses. The slats thickness is such that electrons from a rear face of the suspended slat opposite to the front face significantly contribute to forming a sheath electric field. The system further comprises a delay line configured to synchronize the arrival of protons liberated from one suspended slat on a rear face of an adjacent suspended slat with the formation of an accelerating sheath on said adjacent suspended slat, ensuring sequential proton acceleration across the plurality of slats. The laser source, the laser focusing assembly, and the delay line are collectively configured so that each split laser pulse impinges on the front face of a corresponding suspended slat, and a transverse cross-section of each suspended slat is fully immersed within its respective split laser pulse focal volume.

[0014] In accordance with the presently disclosed subject matter, there is also provided a method for laser-induced proton acceleration. The method includes the steps of: generating laser pulses with a laser source, said laser pulses suitable for inducing proton acceleration upon interaction with a target material; providing a target, wherein said target includes at least one suspended slat made of said target material, and said target being configured for liberating and accelerating protons upon interaction with said laser pulses, said target having a slat thickness smaller than a laser wavelength; focusing the laser pulses with a laser focusing assembly for immersing the suspended slat within a laser pulse focal volume along a sagittal extension of said suspended slat; optionally, receiving the accelerated protons with a beam shaping assembly and forming a focused proton beam. A corresponding method for sequential proton acceleration is also provided by the present disclosure. In addition to the above features, the laser-based proton acceleration systems, the laser-based proton acceleration system for cascaded proton acceleration and the corresponding methods according to the presently disclosed subject matter, can optionally comprise one or more of features (i) to (xviii) below, in any technically possible combination or permutation:

[0015] (i) the slat thickness is smaller than half of the laser wavelength.

[0016] (ii) the slat width is smaller than 6 microns, preferably smaller than 3 microns, more preferably smaller than 1 micron.

[0017] (iii) a slat width is greater than 0.5 micron.

[0018] (iv) the laser source is configured so that an electric field of the laser pulses includes a component along a transverse axis parallel to the width of the slat.

[0019] (v) the laser source is configured to produce laser pulses such that the pulse rise time may be so that its peak power do not exceed 100 kW at 10 picosecond before its maximal value.

[0020] (vi) the laser source is configured for generating laser pulses having laser pulse duration in the range between 10 to 800 femtoseconds.

[0021] (vii) the laser focusing assembly is configured such that the laser pulse focal volume includes a cylindrical volume having a longitudinal extension of about a Rayleigh length and a radius of about a laser beam waist.

[0022] (viii) the radius of said cylinder in the range of 1.5 to 2.5 microns.

[0023] (ix) the target includes another suspended slat; the laser source and laser focusing assembly are further configured so that laser pulses impinge on a front face of said other suspended slat with a controllable time delay with respect to laser pulses impinging on the suspended slat, wherein the system is further configured so that protons liberated from the suspended slat are capable of reaching a rear face of the other suspended slat; and the time delay is controllable so that said protons liberated from the suspended slat within a selected energy range selectively reach the rear face of the other slat when an accelerating sheath is formed on the other suspended slat.

[0024] (x) the laser focusing assembly comprises at least one beam splitter configured for splitting a laser pulse emitted by the laser source into a first attenuated pulse and a second attenuated pulse;

[0025] (xi) The laser focusing assembly comprises at least a first focusing assembly for focusing the first attenuated pulse on the suspended slat; a controllable delay leg configured for receiving the second attenuated pulse and forming a delayed attenuated pulse; at least a second focusing assembly configured for focusing the delayed attenuated pulse on the other suspended slat.

[0026] (xii) the target material is gold.

[0027] (xiii) The system comprises a target positioning assembly configured to adjust a position of the target.

[0028] (xiv) The system includes a vacuum chamber configured to at least surround the target.

[0029] (xv) The system includes a proton beam shaping assembly configured to form a focused proton beam from the protons accelerated from the target.

[0030] (xvi) The system comprises a control system including a spectrometer for analyzing an energy spectrum of protons accelerated from the target.

[0031] (xvii) The system comprises a proton radiotherapy control system adapted to deliver the protons accelerated from the target to a patient for medical treatment, wherein the proton radiotherapy control system is configured to adjust a proton beam energy for targeting a tumor while minimizing damage to healthy tissue.

[0032] (xviii) the proton radiotherapy control system is configured to adjust the proton beam energy by controlling the time delay so that protons liberated from one suspended slat (e.g. the first suspended slat) reach a rear face of an adjacent suspended slat (e.g. the second suspended slat) when an accelerating sheath forms on said adjacent suspended slat. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0034] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0035] Fig. 1 illustrates generally a laser-based proton acceleration system in some embodiments of the present disclosure.

[0036] Fig. 2 illustrates an intense laser irradiating a target and protons liberation in some embodiments of the present disclosure.

[0037] Fig. 3 illustrates a Scanning Electron Microscope image of a target in some embodiments of the present disclosure.

[0038] Fig. 4 illustrates experimental results for laser-based proton acceleration system according to some embodiments of the present disclosure compared to prior art.

[0039] Figs. 5A-5B illustrate experimental results for laser-based proton acceleration system according to some embodiments of the present disclosure compared to prior art.

[0040] Figs. 6A-6G illustrate simulation results for laser-based proton acceleration system according to some embodiments of the present disclosure compared to prior art.

[0041] Fig. 7 illustrates experimental results for laser-based proton acceleration system according to some embodiments of the present disclosure compared to prior art.

[0042] Fig. 8 illustrates generally a laser-based proton acceleration system in some embodiments of the present disclosure.

[0043] Fig. 9 illustrates simulation results for laser-cased proton acceleration system according to some embodiments of the present disclosure.

[0044] Fig. 10 illustrates simulation results showing the effect of the p-bar width for laserbased proton acceleration system according to some embodiments of the present disclosure. Fig. 11A-11C illustrate simulation results showing the effect of the p-bar width for laser-based proton acceleration system according to some embodiments of the present disclosure.

[0045] Fig. 12 illustrates simulation results showing the effect of the p-bar width for laserbased proton acceleration system according to some embodiments of the present disclosure.

[0046] Fig. 13 illustrates simulation results showing the effect of the p-bar width for laserbased proton acceleration system according to some embodiments of the present disclosure

[0047] Fig. 14A-14C illustrate simulation results showing the effect of the p-bar thickness for laser-based proton acceleration system according to some embodiments of the present disclosure.

[0048] Fig. 15 illustrates simulation results showing the effect of the p-bar thickness for laser-based proton acceleration system according to some embodiments of the present disclosure

[0049] DETAILED DESCRIPTION OF EMBODIMENTS

[0050] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding. However, it will be understood by those skilled in the art that the presently disclosed subject matter may be practiced without these specific details. In other instances, well-known methods and features have not been described in detail so as not to obscure the presently disclosed subject matter.

[0051] Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "processing", "computing", "comparing", or the like, refer to the action(s) and / or process(es) of a computer that manipulate and / or transform data into other data, said data represented as physical, such as electronic, quantities and / or said data representing the physical objects. The term “computer” should be expansively construed to cover any kind of hardware -based electronic device with data processing capabilities.

[0052] The various illustrative logical blocks, modules, and steps described in connection with the examples disclosed herein can be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. The described functionality can be implemented in varying ways for each particular application, but such implementation decisions should not be interpreted as causing any departure from the scope of the disclosure.

[0053] It will also be understood that a system according to the present disclosure may be, at least partly, implemented on a suitably programmed computer. Likewise, the present disclosure contemplates a computer program being readable by a computer for executing the methods of the present disclosure. The present disclosure further contemplates a non- transitory computer-readable memory tangibly embodying a program of instructions executable by the computer for executing the method of the present disclosure.

[0054] Fig. 1 illustrates generally a laser-based proton acceleration system 1 in some embodiments of the present disclosure. The system 1 may be configured to generate and control proton beams for various applications, including scientific research and / or medical proton radiotherapy.

[0055] The system 1 includes a laser source 10 for generating laser pulses 15 suitable for inducing proton acceleration upon interaction with a target material, a laser focusing assembly 20 for focusing the laser pulses to provide a focused laser pulse 25 onto a suspended slat of a target 30 made of the target material. The suspended slat may be standalone i.e. its main body may be free-standing while only its ends may be held to a surrounding structure. The suspended slat may be elongated and define a longitudinal (lengthwise) axis direction. The suspended slat may have a major front face (i.e. a face having a largest area) extending along the longitudinal axis and defining a width direction. The suspended slat may include a minor side face (i.e. a face having a smallest area) extending in a direction transverse to the major front face (e.g. perpendicular thereto) and defining a thickness direction. In some embodiments, the suspended slat may have an elongated rectangular parallelepiped shape. The slat may also be hereinbelow referred to as a “bar”. The focused laser pulse 25 may be such that a transverse cross section (i.e. a cut) of said suspended slat may fully be immersed within a laser pulse focal volume. In some embodiments, the transverse cross section may be defined perpendicular to the longitudinal axis of the suspended slat. In some embodiments, when the laser source impinges the slat with an incidence angle, the transverse cross section may be defined transversely to the longitudinal axis. For example as a cross section of the slat along an plane perpendicular to the incidence plane and containing the laser beam (also referred to as sagittal plane). The focused laser pulse 25 induces acceleration of protons 35 for example according to the Target Normal Sheath Acceleration (TNSA) mechanism. The accelerated protons 35 may be received by a proton beam shaping assembly 40 to form a focused proton beam 45.

[0056] Target Normal Sheath Acceleration (TNSA) is a principal mechanism for laser- driven proton acceleration. In TNSA, an intense laser pulse striking the target material surface rapidly ionizes electrons, creating a plasma primarily at the surface. This interaction generates strong electric fields that accelerate electrons away from the target, leading to a charge separation. The resulting electrostatic field, known as the sheath electric field, forms near the rear surface opposite the front surface of the target. This field accelerates the positively charged ions, including protons, away from the target rear surface in a direction normal to it. The protons gain kinetic energy as they are propelled through this field. Generally, the characteristics of the resulting proton beam, such as its energy and direction, can be tailored for specific applications by adjusting the laser pulse parameters and the target properties. In the following, an additional method for tailoring the resulting proton beam is provided in the embodiment in which the target includes a plurality of suspended slats.

[0057] The laser source 10 may generate laser pulses 15 with specific characteristics: the pulse rise time may be so that its peak power do not exceed 100 kW at 10 picosecond before its maximal value, the pulse duration may range from about 10 to 800 femtoseconds. For example, the laser source may be a high intensity Neodymium-doped glass laser or Titanium-doped sapphire laser. These laser pulses 15 may be directed towards the laser focusing assembly 20, which may focus the pulses forming the laser pulse focal volume, resulting in focused laser pulses 25. The pulse focal volume may be a cylindrical volume having a longitudinal extension of about a Rayleigh length and a radius of about a laser beam waist. The pulse focal volume may be configured so as to contain a significant portion of the pulse energy (at least 70%). The focal volume of these pulses 27 may be controlled to be contained within a cylinder of a diameter of about 3 to 5 microns and a length of about the Rayleigh length. The laser source may be configured so that an electric field of the laser pulses includes (optionally only) a component along a transverse axis parallel to the width of the slat. The laser source may be linearly polarized. The linear polarization of the electric field may be parallel to the width direction.

[0058] The focused laser pulses 25 may interact with the target 30, which may be made of a specific target material suitable for liberating and accelerating protons upon interaction with said laser pulses. The target material may for example be gold. In some embodiments, the suspended slat may have a width smaller than 6 microns, preferably smaller than 3 microns, more preferably smaller than 1 micron. The Applicant has discovered that reducing the slat width enhances proton acceleration effectiveness up to a certain point. Specifically, the enhancement holds for widths down to approximately 0.5 micron. Below this threshold, the reduction in width no longer yields an increase in proton acceleration. Therefore, the slat width may preferably be larger than 0.5 micron.

[0059] The suspended slat may be suspended over a gap in a silicon wafer. The suspended slat may be fabricated using an etching process. The two ends of the suspended slat may form two enlarged pads on the silicon wafer. The suspended slat may be positioned so that the laser pulse hit a front face of the suspended slat. For example, the suspended slat may be positioned perpendicular to the laser propagation axis by a target positioning assembly (not shown).

[0060] The slat thickness may be such that electrons from a rear face of the suspended slat opposite the front face significantly contribute (together with electrons from the front face) to forming the sheath electric field. In order to yield this effect, the Applicant has found that the slat thickness may preferably be less than half of the laser wavelength. Thus, the suspended slat positioning and dimensioning, and the laser pulse focusing may facilitate the liberation and acceleration of protons 35 from the slat target 30 when impacted by the focused laser pulses 25. The liberated protons 35 may then be collected and directed by the (optional) proton beam shaping assembly 40, which may form a focused proton beam 45. This proton beam may be analyzed by a spectrometer within a control system to determine its energy spectrum. Additionally, the system may be incorporated into a proton radiotherapy control system for medical applications, allowing the proton beam energy and / or penetration depth to be adjusted for the precise targeting of tumors while minimizing damage to surrounding healthy tissue. The entire process, especially the interaction with the slat target 30, may be conducted within a vacuum chamber to ensure optimal conditions for proton acceleration and beam formation. In other words, at least the target, and preferably also parts of the laser source, the focusing assembly and optionally the other components of the system may be positioned in the vacuum chamber.

[0061] The target may include a sequence of suspended slats positioned next to each other. The sequence of suspended slats may be configured so that protons liberated from one suspended slat are capable of reaching a rear face of an adjacent suspended slat. The laser source and laser focusing assembly may further be configured so that pulses impinge on a front face of said suspended slats. The system may further be configured so as to allow for a controllable a time delay between pulses impinging on adjacent slats. The time delay may be controllable so that protons within a selected energy range liberated from a given suspended slat selectively reach the rear face of the adjacent slat when an accelerating sheath is formed on said adjacent suspended slat. The proton radiotherapy control system may further configured to adjust the proton beam energy by controlling the time delay(s) between successive slats.

[0062] Such a laser-based proton acceleration system 2 including a plurality of (standalone) suspended slats is generally described with reference to Fig. 8.

[0063] The system 2 is configured for cascaded proton acceleration. The system 2 may comprise a laser source 100 and a beam splitter 160 configured to generate laser pulses 150 and split laser pulses 150 into split laser pulses 161, 162. The split laser pulses may be suitable for inducing proton acceleration upon interaction with a target material. The beam splitter may divide the laser pulses 150 into split laser pulses of similar characteristics. The system 2 may further include a laser focusing assembly including a first focusing assembly 201 and a second focusing assembly 202 configured to individually focus the split laser pulses into focused laser pulses 251, 252. The system may include a target comprising a plurality of suspended slats 301, 302. Each suspended slat 301, 302 may be made of said target material (or different suitable target material) and configured for liberating and accelerating protons upon interaction with the respective focused laser pulses. The thickness of each slat may be such that electrons from a rear face of the suspended slat opposite to the front face significantly contribute to forming a normal sheath electric field. As explained above, the Applicant has shown that such a mechanism is observed for thicknesses below half a wavelength of the laser source. The system 2 may further include a controllable delay line 50 configured to synchronize the arrival of protons liberated from one slat on the rear face of an adjacent suspended slat with the formation of an accelerating sheath on said adjacent suspended slat. This ensures sequential proton acceleration across the plurality of slats. The laser source, the laser focusing assembly, and the delay line may be collectively configured so that each split laser pulse impinges on the front face of a corresponding suspended slat, and a transverse cross-section of each suspended slat is fully immersed within its respective laser pulse focal volume.

[0064] We demonstrate numerically the potential of a cascaded proton accelerator composed of micrometric targets in Fig. 9. Two d = 0.2 pm p-bars are positioned parallel to their w = 1 pm sides, with a separation of 6 pm between them. The targets are irradiated at a 45 ° angle of incidence with two p-polarized 800-nm wavelength laser pulses having a 30 fs (FWHM) wide Gaussian temporal profile and 120 mJ of energy. The laser pulses were focused to a spot size of 3.5 pm (FWHM), yielding a normalized laser intensity of aO = 4.6. The second pulse is delayed by At = 160 fs with respect to the first, so an electron sheath forms around the second p-bar at the time of arrival of ~6 MeV protons emitted from the first target. The position of the second p-bar is chosen at the minimal separation in which the intensity of the first pulse would not induce a sheath around it prematurely taking advantage of the increased divergence of the proton beam discussed above. Fig. 9 illustrates four snapshots of the simulations at t = 0 fs, t = 90 fs, t = 120 fs and t = 180 fs for a laser-based proton acceleration system including two (standalone) suspended slats as described above. The transverse component of the laser field (in the slat width direction) is shown in a red-to-blue color scale, with the electron density superimposed in a green color scale. The proton energy is shown in a temperature color scale. The final snapshot demonstrates how the sheath around the second target further accelerates those protons to energies of up to 14 MeV (frame in the right bottom corner). The effect of the relative delay between the two pulses is shown in the 5 snapshots illustrated on the right side of Fig. 9. The right side of Fig. 9 illustrates snapshots at t=l 80 fs of the area marked with a frame (right bottom corner) in Fig. 9, from multiple simulations under the same conditions but different delays between the two pulses in the range of At=140-180 fs. When the second pulse arrives too early (t < 160 fs) or too late (t >160 fs), the maximal proton energy falls below the optimum. In comparison to double foil target experiments, we may consider for example a beam of protons emitted from the first target with energies close to 5 MeV, and a 1 pm thick sheath prevailing for 50 fs of effective acceleration time around the second target. A 6-pm separation distance will correspond to effective acceleration of 5 ± 0.64 MeV protons, while a 1-mm separation distance will only accelerate protons arriving within 5 ± 0.004 MeV.

[0065] Experimental results and numerical simulations

[0066] Fig. 2 illustrates an irradiation geometry. Laser pulses impinge a bar target. For example, 27-fs long laser pulses of central wave length 800 nm, with 120 mJ energy (on target) and pulse contrast better than 1011before t=-60 ps, that are polarized along the width of the bar, are focused using an f / 2.5 off-axis parabolic mirror unto a target including a suspended slat made of gold that is d = 0.2 pm thick. A width of the bar may be between 1-10 pm, or 1-6 pmor 2-10 pm . 70% of the laser energy was measured to be contained within a circle of 3.5 pm diameter, corresponding to a normalized laser intensity of a0= 4.6. The laser pointing stability was measured to be 0.43 pm (RMS). Spatial overlap between the target and the focused laser was achieved by imaging the target with xlOO magnification under vacuum, while introducing the laser beam in low-power.

[0067] Fig. 3 shows a scanning electron microscope image of an example bar target. The free-standing Au bars are suspended over rectangular openings in a 250 pm thick Si wafer support. A fabrication process of the suspended slat may start with a Si wafer pre-coated on its front with a 200-nm thick layer of high-stress SisN4. The back side of the wafer is spin-coated with layers of resist (MicroChem SF9) and photoresist (MicroChem AZ- 1518), on which 3.0 mm x 0.4 mm rectangular gaps are photolithographed. The Si is then etched in a 30% KOH solution at 90°C. The process spontaneously stops when the inner surface of the front side SisN4 is exposed. Next, the SisN4 side of the wafer is spin-coated with layers of the same resist and photoresist. 0.5-6 m wide rectangular openings, which would form the micro-bars, are photolithographed over the gaps. The wafer is coated with a 10- nm thick Ti adhesion layer and a 190-nm thick layer of Au. The SisN4 around the bars is removed by reactive ion etching and immersion in Acetone. Finally, the remaining SisN4 layer below the Au bars is removed by dry-etching.

[0068] Fig. 4 illustrate the resulting differential proton spectra (Y axis is in Protons / MeV / Sr, X axis in MeV) for flat foils (curves crossing the X axis at about 2 MeV) and p-bars (w in a temperature color scale, curves crossing the X axis at around 6 MeV corresponding to w of 1.8 p). The energy spectra of protons emitted at the laser propagation direction were measured using a Thomson parabola type ion spectrometer operating with an electrode voltage difference of 2 kV. A charge coupled device imaged a CsI(Tl) scintillator positioned at the back of the spectrometer. Absolute proton energy calibration was obtained by taking shots with parts of the scintillator covered by foil filters of known thickness and composition. The conversion of the scintillation signal to an absolute proton dose was calibrated by recording spectrometer traces using image plates, which were crosscalibrated using a beta emitter calibration source. On a separate set of laser shots, the angular distribution of the ions were measured by placing a (0.4 mm X 25 mm) slit made of 2 mm thick Al plates, 25 mm downstream to the target. Particles passing through this aperture traversed a 0.5 T magnetic field for a distance of 25 mm, and then propagated 275 mm to a CsI(Tl) scintillating screen.

[0069] Each curve represents the result of a single laser shot. The irradiation of narrower p-bars targets results in higher proton cut-off energies, reaching beyond 6 MeV for w = 1.8 pm. Other spectral features which are observed for these narrower targets include (1) a drop in the total proton number, (2) flattening and modulation of the low energy spectra, and (3) an emergence of a slightly increasing low energy cutoff (see inset). These three observations would serve as indicators for validating the results of numerical simulations to identify the origin of the increasing proton energies.

[0070] Figs. 5A and 5B illustrate two raw spectrograms respectively for an irradiated reference 0.2 pm thick Au foil and for a d = 0.2 pm thick, w = 1.8 pm wide p-bar.

[0071] The underlying dynamics was analyzed through 2D particle -in-cell (PIC) simulations using the EPOCH code. In these simulations, d = 0.2 pm thick p-bars targets of various widths were irradiated with p-polarized 30-fs long laser pulses of 120 mJ energy, which were focused to a spot size of 3.5 pm (FWHM). Figs. 6A-6C respectively show snapshots of the simulations at t = 40 fs and t = 90 fs, for the cases of a slat having width of w = 10 pm (Fig. 6A), 2 pm (Fig. 6B), and 1 pm (Fig. 6C).

[0072] The w = 10 pm targets are much wider than the laser spot size and so represent planar foil targets, t = 0 here represents the instant in which the peak of the laser field impinged on the p-bar. The longitudinal component of the laser field (in the slat thickness direction) is shown in a red- to-blue color scale, with the electron density superimposed in a green color scale. In Fig. 6B and Fig. 6C the p-bars are completely immersed in the focal volume. The field amplitudes are highly augmented due to diffraction near the target edges, and two trains of attosecond duration electron bunches are observed to emerge in the laser direction. Inspection of the t = 90 fs snapshots reveals that the electron sheath around the target becomes thicker for narrower targets, due to the increased field ionization and the restricted target surface over which the electrons may expand. Such thicker sheaths imply higher field gradients, which indeed result in higher acceleration gradients for ion contaminants, as is evident from the space average energy of protons at their front, which is superimposed in a temperature color scale.

[0073] Fig. 6D presents the resulting energy spectra from simulated irradiation of p-bars targets of w = 1, 2, and 4 pm, and for flat foil targets of protons going into forward angles as solid curves. These spectra feature the flattening of the spectrum and the modulations at low-energy which are observed in the experiment. The source for these modulations is identified when inspecting the differences between the curve of a w = 1 pm target (solid red), and an otherwise identical simulation in which only proton contaminants are included (dashed red). The difference between these two cases indicate that the heavier ion contaminants have a role in the acceleration of lighter ions (protons) to higher energies. This role may be elucidated by the proton phase-space plot in Fig. 6E. At an early time (40 fs), the phase-space of protons emitted from the back of the w = 1 p-bar is not very different whether the simulation contain protons only (green) in the contamination layer or whether it include also heavier ions (red). At later times (90 fs) however, a difference between the two cases develops (marked with a double-headed arrow). The low energy protons that in the absence of heavier ions trail at the back, are pushed to higher energies by the coulomb repulsion of the of these heavy, less mobile ions.

[0074] So far we discussed the role of the target’s lateral dimension and the role of the heavier contaminates in increasing the proton energies. The effect of the target’s dimension along the laser propagation direction (denoted d for the case of p-bars) is demonstrated in Fig. 6F. Fig. 6F illustrates the proton cut-off energy for w = 1 pm foils of different thicknesses. A sharp increase in the proton cut-off energy is observed for when the thickness of p-bars fully immersed in the laser’s focal volume (w = 1 pm) is smaller than half a wavelength.

[0075] An underlying reason may be identified in Fig. 6G, which shows only the density distributions of electrons originating from the pre-plasma layer in p-bar’ s front face and back face (red and blue respectively). Fig. 6G illustrates the density distributions of electrons forming the pre-plasma on the front surface (red) and back surface (blue) of a w =1 pm p-bar for a 0.5 thick bar and a 0.2 micron thick bar. After 60 fs, the plasma sheath for a d = 0.5 pm p-bar (top) consists entirely of electrons originating from the front face, while for d = 0.2 pm p-bar (bottom) the sheath features a mixture of electrons from the front face and the rear face of the bar. In other words, it can be observed that for the case of a target thicker than half of the laser wavelength (d = 1 pm), the sheath consists only of electrons from the front-face. For the case of a p-bar thinner than half-the laser’s wavelength, the sheath is a mixture of front and back electrons. The addition of back electrons to the sheath enhances the field gradients, which are in turn responsible for the observed higher ion energies.

[0076] From the early days of TNSA, it was realized that the proton cut-off energies roughly scale with the square root of the laser intensity. The emergence of richer experimental data-sets of a wide range of laser and target parameters prompted the refinement of empirical scaling laws with the various experimental parameters. Fig. 7 presents a compilation of the cut-off energies in TNSA experiments, as a function of the laser pulse energy. Also shown are the results of this study, for the irradiation of p-bar targets where w is indicated with the same color scale as in Fig. 4.

[0077] Fig. 10 illustrates other simulated differential proton energy spectra for the same experimental parameters as in Fig. 4 overlaid with snapshots of the transverse electric field (blue-to-red), electron density (green) and space averaged electron and proton energy (color scale).

[0078] The underlying dynamics were revealed by 3D particlein-cell (PIC) simulations using the EPOCH code. In these simulations, d = 0.2 pm thick p-bars of various widths were irradiated with p-polarized 800-nm wavelength laser pulses having a 30 fs (FWHM) wide Gaussian temporal profile and 120 mJ of energy. The laser pulses were focused to a spot size of 3.5 pm (FWHM), yielding a normalized laser intensity of aO = 4.6. The 3D simulation space was defined as a (32 pm)x X (20 pm)y X (24 pm)z box divided into (1000)x X (1000)y X (150)z computational mesh cells. We conducted one computationally heavy simulation with a high resolution of (3000)x X (3000)y X (300)z cells to verify the consistency of the results. The bulk of the targets was representative of Au4+ ions

[0035] and electrons with densities of 30 and 4 X 30 times that of the critical plasma density respectively. The targets were surrounded on all sides by an exponential density gradient with a scale length of 2 / 60. An external contaminate layer composed of H+, C4+, and 04+ ions in equal parts was set with a uniform density 30 times that of the critical plasma density over a thickness of 0.1 pm. The distribution of the composition of the target along the long dimension of the p-bar was uniform over the range of Izl < 5.5 pm.

[0079] The simulations results are presented in Fig. 10. The differential proton energy spectra for irradiated p-bar targets in the same parameter range as previously described are shown with the same color scale. Overlaid are snapshots taken at t = 0, 60, and 450 fs, for the cases of w = 10 pm and 2 pm wide p-bars. t = 0 represents the instant in which the peak of the laser field impinges on the p-bar. The transverse component of the laser field (Ey) is shown in a red-to-blue color scale, with the electron density superimposed in a green color scale. The w = 2 pm p-bar is narrower than the laser focus and therefore is transversely immersed in the focal volume. Two trains of attosecond duration electron bunches are observed to emerge with a small opening angle around the laser propagation direction.

[0080] Figs. 11A-11C illustrate the values in the proton cutoff energy, peak-values of the electric field in the sheath and the cone angles of protons emitted with E> 0.5 MeV for various width of the p-bars. Rerunning the simulations with initial ion charge states up to Au8+, and with a pre-plasma scale length in the range of 2 / 80 - 2 / 40, resulted in an overall shift of the proton energies by a factor of 0.88- 1.09, but the dependence on w remained unchanged. Fig. 11B shows how the increased proton cutoff energies for narrower p-bars are correlated with the sheath field amplitude (snapshots taken at t = 60 fs). Fig. 11C shows the cone angles of E > 0.5 MeV proton beams emitted from d = 0.2 pm p-bar targets plotted as a function of w. Both the experiment and the simulation feature a sharp increase in the divergence of the proton beam for the narrowest (w = 2 pm) targets. This geometric effect occurs when both d and w are smaller than the sheath scale length, which is on the order of microns, so the sheath no longer maintains the target’s aspect ratio. Fig. 12 shows, for the case of a d = 0.2 pm, w = 2 pm p-bar, energy resolved “virtual” source distributions of the proton beam that were obtained from the simulation results by projecting the proton angle at the end of the acceleration phase back to the target plane. Fig. 13 shows the normalized RMS values of the proton beam emittance. These values are evaluated as where crr(jr, are the RMS values of the source beam width and divergence angle.

[0081] Fig. 14A-14-C and Fig. 15 illustrate the effect of the p-bar thickness. Fig. 14A illustrates proton cutoff energies (lower curve) and the maximal amplitude of the sheath field (sampled at t = 60 fs, upper curve). Fig. 14B illustrates the density and average energy of electrons in the sheath sampled at t = 60 fs for d = 0.2 pm and w = 2- 10 pm p-bars. Fig. 14C illustrates the density and average energy of electrons in the sheath sampled at t = 60 fs for w = 2 pm and d = 0.2- 1.0 pm p-bars. Fig. 15 illustrates density distributions of the electron sheath forming at t = 60 fs around w = 2 pm, d = 0.2 pm (top) and 0.5 pm (bottom) p-bars. Electrons originating from the back side or the front side of the target are shown on the left and right respectively.

[0082] The effect of the target thickness d on the proton maximal energy is demonstrated in Fig. 14A. A sharp increase of up to 15% in the proton cutoff energy (lower curve) emerge for d < 2 / 2 targets. This increase in energy is correlated with an increase in the peak value of the sheath electric field (upper curve). Some properties of TNSA may be obtained using a simple self-similar isothermal fluid model in which the proton cutoff energy is given Here Teis the temperature of the hot electron population, taccis an effective acceleration time, and a)pi~ 'onplasmafrequency. Fig. 14B illustrates the strong dependence ofecutoffonTe where the sheath density and average electron energy were sampled at t = 60 fs, 1-pm behind the rear side of the target. For d = 0.2 pm p-bars, smaller values of a> result in higher electron energies that can account for the higher energy cutoff. However, when reducing d for fixed w = 2 pm p-bars (see Fig. 14C), the electron temperatures do not increase and the source of the increased proton cutoff energies is found to be the rising sheath density. To identify the origin of this thicker sheath, we separated the electron population according to the surface from which they originated. Fig. 15 presents sheath density distributions forming around w = 2 pm p-bars at t = 60 fs. Electrons that initially covered the plasma gradient at the front (x < 0) of the target are shown on the left, and those that originated from the back side (x~d > 0) are shown on the right. For a target thinner than half the laser wavelength (top, d = 0.2 pm), the sheath is a mixture of front and back electrons. However, when the p-bar is thicker than half the laser wavelength (bottom, d = 0.5 pm), the sheath is observed to consist of front-side electrons only.

[0083] An additional advantage of using p-bar targets for TNSA is the small ‘virtual’ source size of the protons. This property sets a limit on the spatial resolution when performing proton radiography, a method used for a wide range of basic research [1] and medical applications. In TNSA off planar foils the virtual source size is of the order of 10 pm, while for a p-bar it is found to be smaller than 1 pm for the high energy part of the proton beam (see Fig. 12). This advantage is further highlighted by the low transverse emittance of the proton beam plotted in Fig. 13, which drops well below the values typical to planar foil targets of about 10-3mm mrad.

[0084] Those skilled in the art to which the present disclosure pertains, can appreciate that while the present disclosure has been described in terms of preferred examples, the concept upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, systems and processes for carrying out the several purposes of the present disclosure. For example, the laser-based proton acceleration systems and methods according to the presently disclosed subject matter may be used in applications requiring high-energy ion beams including but not limited to: high-energy proton-induced reactions, medical therapy, radiography, material science.

[0085] Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. It should be noted that the words “comprising”, "including" and "having" as used throughout the appended claims are to be interpreted to mean “including but not limited to”. The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases, and disjunctively present in other cases. The term “each” may not be exclusively understood as referring to each and every, and when technically relevant may also refer to “at least some”.

[0086] All patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present disclosure.

[0087] It is important, therefore, that the scope of the present disclosure is not construed as being limited by the illustrative examples set forth herein. Other variations are possible within the scope of the present disclosure as defined in the appended claims. Other combinations and sub-combinations of features, functions, elements and / or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to different combinations or directed to the same combinations, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the present description.

Claims

CLAIMS:

1. A laser-based proton acceleration system, comprising:(a) a laser source configured to generate laser pulses suitable for inducing proton acceleration upon interaction with a target material;(b) a laser focusing assembly configured to focus the laser pulses;(c) a target including a suspended slat made of said target material and configured for liberating and accelerating protons upon interaction with said laser pulses, a slat thickness being such that electrons from a rear face of the suspended slat opposite to a front face significantly contribute to forming a normal sheath electric field; wherein the laser source and the laser focusing assembly are configured so that laser pulses impinge on the front face of said suspended slat and a transverse cross section of said suspended slat is contained within a laser pulse focal volume.

2. The laser-based proton acceleration system of claim 1 , wherein the slat thickness is smaller than half of the laser wavelength.

3. The laser-based proton acceleration system of any one of the preceding claims, wherein the slat width is smaller than 6 microns, preferably smaller than 3 microns, more preferably smaller than 1 micron.

4. The laser-based proton acceleration system of any one of the preceding claims, wherein a slat width is greater than 0.5 micron.

5. The laser-based proton acceleration system of any one of the preceding claims, wherein the laser source is configured so that an electric field of the laser pulses includes a component along a transverse axis parallel to the width of the slat.

6. The laser-based proton acceleration system of any one of the preceding claims, wherein the laser source is configured to produce laser pulses such that the pulse rise time is so that its peak power does not exceed 100 kW at 10 picosecond before its maximal value.

7. The laser-based proton acceleration system of any one of the preceding claims, wherein the laser source is configured for generating laser pulses having laser pulse duration in the range between 10 to 800 femtoseconds.

8. The laser-based proton acceleration system of any one of the preceding claims, wherein the laser focusing assembly is configured such that the laser pulse focal volume includes a cylindrical volume having a longitudinal extension of about a Rayleigh length and a radius of about a laser beam waist.

9. The laser-based proton acceleration system of claim 8, wherein the radius of said cylinder in the range of 1.5 to 2.5 microns.

10. The laser-based proton acceleration system of any one of the preceding claims, wherein:(a) the target includes another suspended slat;(b) the laser source and laser focusing assembly are further configured so that laser pulses impinge on a front face of said other suspended slat with a controllable time delay with respect to laser pulses impinging on the suspended slat, wherein the system is further configured so that protons liberated from the suspended slat are capable of reaching a rear face of the other suspended slat; and the time delay is controllable so that said protons liberated from the suspended slat within a selected energy range selectively reach the rear face of the other slat when an accelerating sheath is formed on the other suspended slat.

11. The laser-based proton acceleration system of claim 10, wherein the laser focusing assembly comprises:(a) a beam splitter configured for splitting a laser pulse emitted by the laser source into a first attenuated pulse and a second attenuated pulse;(b) a first focusing assembly for focusing the first attenuated pulse on the suspended slat;(c) a controllable delay leg configured for receiving the second attenuated pulse and forming a delayed attenuated pulse;(d) a second focusing assembly configured for focusing the delayed attenuated pulse on the other suspended slat.

12. The laser-based proton acceleration system of any one of the preceding claims, wherein the target material is gold.

13. The laser-based proton acceleration system of any one of the preceding claims, further comprising a target positioning assembly configured to adjust a position of the target.

14. The laser-based proton acceleration system of any one of the preceding claims, further comprising a vacuum chamber configured to surround the target.

15. The laser-based proton acceleration system of any one of the preceding claims, further comprising a proton beam shaping assembly configured to form a focused proton beam from the protons accelerated from the target.

16. The laser-based proton acceleration system of any one of the preceding claims, further comprising a control system including a spectrometer for analyzing an energy spectrum of protons accelerated from the target.

17. The laser-based proton acceleration system of any one of the preceding claims, further comprising a proton radiotherapy control system adapted to deliver the protons accelerated from the target to a patient for medical treatment, wherein the proton radiotherapy control system is configured to adjust a proton beam energy for targeting a tumor while minimizing damage to healthy tissue.

18. The laser-based proton acceleration system of claim 17 as dependent on claim 10, wherein the proton radiotherapy control system is configured to adjust the proton beam energy by controlling the time delay.

19. A laser-based proton acceleration system comprising:(a) a laser source configured to generate laser pulses suitable for inducing proton acceleration upon interaction with a target material;(b) a beam splitter configured for splitting said laser pulses into a plurality of split laser pulses;(c) a laser focusing assembly configured to focus the split laser pulses onto a target; wherein the target including a plurality of suspended slats made of said target material and configured for liberating and accelerating protons upon interaction with said split laser pulses, wherein each suspended slat has a slat thickness such that electrons from a rear face of said suspended slat opposite to a front face significantly contribute to forming a normal sheath electric field; wherein the laser source and the laser focusing assembly are configured so that each split laser pulse impinges on the front face of a respective suspended slat and a transverse cross section of each suspended slat is contained within a respective split laser pulse focal volume.

20. A method for laser-induced proton acceleration, comprising the steps of:(a) generating laser pulses with a laser source, said laser pulses suitable for inducing proton acceleration upon interaction with a target material;(b) providing a target, wherein said target includes at least one suspended slat made of said target material, and said target being configured for liberating andaccelerating protons upon interaction with said laser pulses, said target having a slat thickness smaller than a laser wavelength;(c) focusing the laser pulses with a laser focusing assembly for immersing the suspended slat within a laser pulse focal volume along a transverse extension of said suspended slat.

21. The method of claim 20, further comprising the step of receiving the accelerated protons with a beam shaping assembly and forming a focused proton beam.

22. A laser-based proton acceleration system for cascaded proton acceleration, comprising:(a) a laser source configured to generate and split laser pulses suitable for inducing proton acceleration upon interaction with a target material;(b) a laser focusing assembly configured to individually focus the split laser pulses;(c) a target comprising a plurality of suspended slats, each made of said target material and configured for liberating and accelerating protons upon interaction with the respective split laser pulses, wherein each slat thickness is such that electrons from a rear face of the suspended slat opposite to the front face significantly contribute to forming a sheath electric field;(d) a delay line configured to synchronize the arrival of protons liberated from one suspended slat on a rear face of an adjacent suspended slat with the formation of an accelerating sheath on said adjacent suspended slat, ensuring sequential proton acceleration across the plurality of slats; wherein the laser source, the laser focusing assembly, and the delay line are collectively configured so that each split laser pulse impinges on the front face of a corresponding suspended slat, and a transverse cross-section of each suspended slat is fully immersed within its respective laser pulse focal volume.

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