PROVIDING ULTRAFAST HIGH-ENERGY LASER PULSES

Abstract

A method for providing an ensemble of beamlets effectively acting as a high-energy laser pulse is disclosed. According to the method, a beamlet pattern with a plurality of spatially distributed laser beamlets is provided. The beamlets are spread in time by introducing a different temporal delay to each of the beamlets. The beamlets are spectrally broadened. The beamlets are incoherently combined in space and time to provide the ensemble of beamlets. Also disclosed is a method for accelerating charged particles. Further disclosed is an optical arrangement for providing an ensemble of beamlets effectively acting as a high-energy laser pulse. The optical arrangement comprises a beamlet generating device providing a beamlet pattern of spatially distributed laser beamlets, a step optic for spreading the spatially distributed laser beamlets in time, a spectral broadening device, and a combining device for incoherently combining the spectrally broadened beamlets in space and time to provide the ensemble of beamlets. Additionally disclosed is a laser-plasma accelerator comprising the optical arrangement.

Claims

1-15. (canceled)

16. A method for providing an ensemble of pulsed laser beamlets having a defined envelope, comprising the steps of: (a) providing a beamlet pattern comprising a plurality of spatially distributed laser beamlets, (b) separating the beamlets in time by introducing different temporal delays to each of the beamlets, (c) spectrally broadening the beamlets, and (d) incoherently combining the beamlets in space and time to provide the ensemble of beamlets.

17. The method according to claim 16, wherein the steps are executed in order from (a) to (d).

18. The method according to claim 16, wherein the step of providing the beamlet pattern comprises splitting a single-aperture laser beam into the beamlets.

19. The method according to claim 16, wherein providing the beamlet pattern comprises generating the beamlet pattern by a spatial arrangement of single laser beams, or (ii) from an arrangement of multiple fibers, or (iii) from a multicore fiber.

20. The method according to claim 16, further comprising the step of fine-tuning spatio-temporal properties of at least one of the beamlet pattern and individual beamlets.

21. The method according to claim 16, further comprising the step of modifying optical properties of the beamlets prior to the step of spectrally broadening.

22. The method according to claim 16, further comprising the step of directly or indirectly controlling the step of incoherently combining the beamlets.

23. The method according to claim 22, wherein the step of directly or indirectly controlling comprises controlling a length of an optical path attributed to a respective beamlet.

24. The method according to claim 22, wherein a figure of merit of an application being driven by the ensemble of beamlets is used as control input for controlling the incoherent combination.

25. A method for accelerating charged particles, comprising the steps of (a) providing an ensemble of laser beamlets by the method according to claim 16, (b) driving a plasma wave in a plasma target with the provided ensemble of laser beamlets, (c) injecting particles into a wake field of the plasma wave to provide injected particles, (d) accelerating the injected particles by the wake field to provide accelerated particles, and (e) extracting the accelerated particles from the plasma.

26. An optical arrangement, comprising: a beamlet generating device providing a beamlet pattern of spatially distributed laser beamlets, a first optical device for spreading spatially distributed laser beamlets in time by introducing a different temporal delay to each of the beamlets, a spectral broadening device, and a combining device for incoherently combining the beamlets in space and time to provide an ensemble of beamlets having a defined envelope.

27. The optical arrangement according to claim 26, wherein the spectral broadening device comprises at least one multi-pass cell.

28. The optical arrangement according to claim 27, wherein the at least one multi-pass cell is a Herriott Cell.

29. The optical arrangement according to claim 26, wherein the spectral broadening device is configured for amplitude filtering the beamlets by non-linear effects in a medium inside the spectral broadening device.

30. The optical arrangement according to claim 26, wherein the combining device comprises a second optical device for temporally combining the beamlets.

31. The optical arrangement according to claim 30, wherein at least one of the first optical device and the second optical device is a step optic.

32. The optical arrangement according to claim 26, wherein the combining device comprises at least one of at least one diffractive, refractive and reflective optical element for spatially combining the beamlets.

33. The optical arrangement according to claim 32, wherein the optical element comprises a lens array or a phase plate.

34. The optical arrangement according to claim 26, wherein the beamlet generating device comprises a spatial beam splitter for splitting a single-aperture laser beam into the beamlet pattern.

35. The optical arrangement according to claim 34, wherein the single-aperture laser beam is provided by a thin-disk, slab or fiber laser.

36. The optical arrangement according to claim 34, wherein the spatial beam splitter comprises a diffractive, refractive or reflective optical device.

37. The optical arrangement according to claim 34, wherein the optical device comprises a lens array, a mask, a grating or a phase plate.

38. The optical arrangement according to claim 26, further comprising an imaging system having one or more of at least one of refractive and reflective optical elements configured for modifying optical properties of the beamlets when imaging the beamlet pattern into the spectral broadening device.

39. The optical arrangement according to claim 26, further comprising adaptive optics for aberration and wavefront control of at least one of the beamlet pattern and individual beamlets.

40. The optical arrangement according to claim 26, wherein at least one of the first and second optical device for introducing or removing a temporal delay between beamlets comprises a reflective step optic, or a transmittive step optic.

41. The optical arrangement according to claim 40, wherein at least one of the first and second optical device comprises the reflective step optic, wherein a thin-film polarizer and a quarter-wave plate are associated with the reflective step optic for modifying the temporal delay under normal or close-to-normal incidence.

42. The optical arrangement according to claim 26, further comprising actuated mirror elements for fine-tuning spatio-temporal properties of the beamlets.

43. The optical arrangement according to claim 42, wherein the spatio-temporal properties to be tuned comprise a temporal delay between the beamlets.

44. A laser-plasma accelerator comprising the optical arrangement according to claim 26 for providing an ensemble of beamlets effectively acting as a laser pulse driving the laser-plasma accelerator.

45. The laser-plasma accelerator according to claim 44, comprising (a) a plasma target including an injector for injecting particles into a plasma wave driven by the ensemble of beamlets, (b) an accelerator for accelerating the injected particles in a wake field of the plasma wave, and (c) an extractor for extracting the accelerated particles from the plasma wave.

46. The laser-plasma accelerator according to claim 45, further comprising at least one of at least one laser sensor and at least one electron diagnostic sensor for outputting a control signal controlling the incoherent beamlet combination in the optical arrangement.

Description

[0058] in FIG. 1: a schematic representation of an optical arrangement for providing an ultrafast high-energy laser pulse,

[0059] in FIG. 2: a schematic representation of a laser-plasma accelerator, and

[0060] in FIG. 3: a schematic representation of an incoherent beamlet combination.

DETAILED DESCRIPTION

[0061] The optical arrangement 100 depicted in FIG. 1 comprises a beamlet generating device 2 providing a beamlet pattern 4 of spatially distributed laser beamlets 5. A vast variety of beamlet patterns can be provided. Optionally, the beamlet generating device 2 comprises a spatial beam splitter 6 for splitting a pulsed single-aperture laser beam 3 into the beamlet pattern 4. The single-aperture laser beam 3 can be provided by a laser 1. The laser 1 can be a thin-disk, slab or fiber laser. In this particular example, the spatial beam splitter 6 comprises an optical device, which device can be a diffractive, refractive or reflective optical device. For example, the optical device can comprise a lens array, a mask, a grating or a phase plate. Alternatively, the beamlet pattern 4 can directly be generated by an arrangement of fibre lasers, in particular in/by a multicore fibre of a multicore fibre laser 7.

[0062] The optical arrangement 100 further comprises an optical arrangement for spreading the spatially distributed laser beamlets 5 in time. The optical arrangement comprises a first optical device 8 for spreading spatially distributed pulsed laser beamlets 5 in time by introducing a different temporal delay d to each of the beamlets 5. In this particular example, the first optical device 8 is a reflective step optic. Optionally, the optical arrangement comprises a thin-film polarizer 9 and a quarter-wave plate 10 associated with the reflective step optic for introducing the temporal delay d under normal incidence or close to normal incidence. Alternatively, the optical device 8 can be a transmittive step optic.

[0063] Still further, the optical arrangement comprises a spectral broadening device 11. The spectrally broadening device 11 spectrally broadens the beamlets 5. In this particular example, the spectral broadening device 11 comprises at least one multi-pass cell 12, particularly a Herriott Cell. The Herriott Cell 12 is made up of a plurality of opposing (curved) mirrors 13. In this particular example, a hole is machined into one of the mirrors 13 to allow the input laser beamlets 5 to enter the cavity. The beamlets 5 pass a medium 14 multiple times, while being reflected back and forth between the mirrors 13. In this particular example, the beamlets 5 may exit through the same hole or another hole in one of the mirrors 13. Mere optionally, the spectral broadening device 11 is configured for amplitude filtering the beamlets 5 by non-linear effects in the medium 14 inside the spectral broadening device 11.

[0064] In this particular example, the optical arrangement 100 further comprises an optional imaging system 15 having one or more refractive and/or reflective optical elements 16 configured for modifying optical properties of the beamlets 5 (e.g. beam divergence, spot size, etc.) when imaging the beamlet pattern 4 into the spectral broadening device 11.

[0065] Mere optionally, the optical arrangement 100 comprises adaptive optics 17 for aberration and wavefront control of the beamlet pattern 4 and/or individual beamlets 5.

[0066] In this particular example, the optical arrangement 100 additionally comprises actuated mirror elements 21 for fine-tuning spatio-temporal properties of the beamlet pattern 4 (e.g. a temporal delay between beamlets) or of individual beamlets 5 (e.g. for controlling the pulse envelope). The actuated mirror elements 21 are positioned in the beam path depending on the properties to be fine-tuned.

[0067] The optical arrangement 100 also comprises a combining device 18 for incoherently combining the beamlets 5 in space and time to provide the ensemble of beamlets having a defined envelope; the ensemble of beamlets effectively acting as a single laser pulse 3.

[0068] In this particular example, the combining device 18 comprises a second optical device 19 for temporally combining the beamlets 5 (compressing the envelope of the ensemble of superimposed beamlets). The second optical device 19 is a second step optic. The second step optic can be a transmittive or reflective step optic. The combining device can further comprise a thin-film polarizer and a quarter-wave plate associated with the reflective step optic. The second optical device 19 can be actuated to controlling characteristics of the beamlets.

[0069] Optionally or alternatively, the combining device 18 also comprises at least one diffractive, refractive and/or reflective optical element 20 for spatially combining the beamlets 5 into an ensemble of laser beamlets having a defined envelope, the ensemble of beamlets effectively acting as a laser beam 3. For example, the optical element 20 can comprise a lens array or a phase plate.

[0070] It should be noted, though, that the actuated mirror elements 21 may optionally be integrated into or part of any of the first optical device 8, second optical device 19 and/or combining device 18.

[0071] As depicted, the optical arrangement 100 can also comprise an application 22 being driven by the combined ensemble of beamlets. From a key performance parameter of this application, a control signal 23 is derived. The control signal 23 is used to determine the spatio-temporal shape of the ensemble of beamlets, in particular by controlling the actuated mirror elements 21 and/or other actuated optical elements for altering e.g. the path length of the beamlets.

[0072] FIG. 2 depicts a simplified schematic laser-plasma accelerator 24 as an example application 22 being part of and driven by high-energy laser pulses provided by the already described optical arrangement 100.

[0073] The laser-plasma accelerator 24 comprises an optical laser beam transport 25. The (laser) beam transport 25 comprises optical elements to deliver the laser pulse 3 to the application 22 where it interacts with another element, here: with the plasma target 26 of the laser-plasma accelerator 24. The beam transport 25 includes at least a focusing element to focus the laser beam 3 to the desired spot size and/or intensity, to drive the application 22. The beam transport 25 also includes elements to diagnose the laser 3, in particular its alignment (position and direction) with respect to the interaction point, as well as other properties, such as the wavefront or beam profile. The laser transport 25 can include elements to correct the alignment of the laser beam with respect to the interaction point, or other properties of the laser beam 3, such as the wavefront.

[0074] The laser-plasma accelerator 24 further comprises of a plasma target 26. The plasma target 26 is typically located in a vacuum. The plasma can be provided by a gas jet or a gas-filled volume of at least one gas species. Typical gas species are hydrogen or helium. The gas is ignited, for example by a discharge, another laser pulse, or the pre-pulse of the drive laser 3, to form a plasma 35. Within the plasma target 26, the drive laser pulse 3 drives a plasma wave. First, particles (e.g. electrons) are injected into the plasma wave by an injector 27, and second, accelerated in an accelerator 28 by the plasma wave. Finally, both laser 3 and accelerated particle beam 34 (e.g. an electron beam) are extracted from the plasma 35. The plasma target 26 typically provides a very specific plasma density profile, where regions of different plasma density can be tailored to the specific task, e.g injection, acceleration, extraction.

[0075] After extraction in an extractor 29, the laser pulse 3 and the particle beam 34 are separated in a separator 30, e.g. by a mirror featuring a hole on axis to allow the particle beam 34 to pass, or a magnet that deflects the particle beam 34.

[0076] After the plasma target, properties of the laser 3 and the particle beam 34 are diagnosed by at least one sensor for laser diagnostics 31 and/or particle diagnostics 32, for example, the remaining laser pulse energy, or the energy of the generated particle beam. The particle beam 34 can then be delivered to an (particle) application 33, e.g. an electron application.

[0077] The result of said laser diagnostics 31, particle beam diagnostics 32, or application 33, e.g. properties of the generated particle beam 34 (its energy, charge, or shot-to-shot stability) or properties of the laser beam 3, which are typically reduced in pulse energy where the reduction serves as a measure for how efficiently the application process is driven, can be used to generate a control signal 23 that directly or indirectly controls the incoherent combination of the beamlets.

[0078] Some exemplary applications especially suitable for using laser driven particle acceleration include, with “high charge” referring to a range from few pC to several nC. [0079] sterilization of samples with electron beams (energies: MeV scale; charge: High) [0080] cross-linking of materials by means of electron beams (energies: scale MeV; charge: High) [0081] generation of X-rays using bremsstrahlung (energies: scale MeV; charge: High) [0082] all applications using bremsstrahlung, esp. sterilization by gammas [0083] generation of radioisotopes by photonuclear reaction. Examples are all nuclides which are also used medically, e.g., Mo99, Cu64, F18, C11, N13, O15 and others (electron energies in the range of a few MeV up to few hundreds of MeV are of particular interest, charge: as high as possible, average electron beam powers of kW are desirable) [0084] radiotherapy applications: FLASH Therapy and VHEE (energies in the range of a few 10 MeV up to a few 100 MeV, charge: high) [0085] electrons as the basis of a Compton backscattering source (electron energies in the range of a few 10 MeV to a few 100 MeV, charge as high as possible, from a few pC up to nC) [0086] a free-electron laser driven by a laser-plasma accelerated electron beam (MeV to GeV scale electron beam energies and high charge)

[0087] In FIG. 3, a graphical representation for describing the incoherent combination of laser beamlets 37 into a beamlet ensemble 38 is shown.

[0088] Here, a number of individual beamlets 37 form an ensemble 38, which can be described by a pulse envelope 36, forming a distinct 3D pulse shape. The pulse shape (envelope 36) is determined by the delay and transverse offset between individual beamlets 37. The beamlets 37 typically are of similar size or smaller, compared to the desired pulse envelope. 36. It is important to note, however, that for an incoherent combination of beamlets 37, the phase difference Δφi between the i beamlets 37 can be and usually is random, since the method disclosed does not necessarily provide measuring the phase difference between individual beamlets 37.

[0089] Instead, the method may use the performance of the application 22, which is driven by the incoherently combined beamlets 37, as a figure of merit to control and optimize the shape of the pulse envelope 36.

[0090] For example, for applications 22 that depend on the intensity of the drive laser pulse 3, such as a laser-plasma accelerator 24, the resulting energy of the generated particle beam 34 can be used to control the shape of the envelope 36 and the distribution of beamlet 37 within the envelope 36.

REFERENCE SIGNS

[0091] 100 optical arrangement [0092] 1 laser [0093] 2 beamlet generating device [0094] 3 single-aperture laser beam [0095] 4 beamlet pattern [0096] 5 laser beamlets [0097] 6 spatial beam splitter [0098] 7 multicore fibre or multicore fibre laser [0099] 8 first optical device (e.g. a step optic) [0100] 9 thin-film polarizer [0101] 10 quarter-wave plate [0102] 11 spectral broadening device [0103] 12 multi-pass cell [0104] 13 mirror [0105] 14 medium [0106] 15 imaging system [0107] 16 optical element for modifying optical properties of the beamlets [0108] 17 adaptive optics [0109] 18 combining device [0110] 19 second optical device (e.g. a step optic) [0111] 20 optical element for spatially recombining the beamlets [0112] 21 actuated mirror element [0113] 22 application [0114] 23 control signal [0115] 24 laser-plasma accelerator [0116] 25 laser transport incl. diagnostic and focusing [0117] 26 plasma target [0118] 27 injector [0119] 28 accelerator [0120] 29 extractor [0121] 30 separator [0122] 31 laser diagnostics [0123] 32 particle diagnostics [0124] 33 particle application [0125] 34 particle beam [0126] 35 plasma [0127] 36 pulse envelope [0128] 37 beamlet [0129] 38 beamlet ensemble