SE543358C2 - A method and a system for generating a pulse of radiation - Google Patents

Abstract

The present invention relates to a method (30) and a system (10) for generating a pulse of radiation. The method (30) comprises: generating (302) a sound wave in a fluid, wherein the sound wave is configured to form a spatially and temporally modulated fluid-density distribution in the fluid; ionizing (304) the fluid, thereby creating a plasma having a spatially and temporally modulated plasma-density distribution relating to the spatially and temporally modulated fluid-density distribution in the fluid; and transmitting (306) a high-intensity light pulse along an optical axis (122) through the plasma, wherein the high-intensity light pulse interacts with the plasma, thereby generating the pulse of radiation.

Description

A I\/IETHOD AND A SYSTEM FOR GENERATING A PULSE OFRADIATION Technical fieldThe present invention relates to a method and a system for generatinga pulse of radiation.

Background of the invention Plasma acceleration of electrons is an active field of research. Aplasma can sustain very strong accelerating fields, several orders ofmagnitude higher than in a conventional accelerator. ln plasma acceleration,a laser pulse of ultra-high intensity is focused in or near a fluid or a plasma,and electrons are accelerated to l\/leV or GeV energies through a processcommonly known in the field as Laser Wakefield Acceleration (LWFA). Duringthe process, the focused laser pulse propagates through the plasma andexcites a plasma wave, in which electrons are accelerated. The strength ofthe plasma wave typically depends on the intensity of the laser pulse. As theelectron are accelerated in the plasma wave, they experience strong radialforces and the accelerated electrons oscillate about a central axis of theplasma wave. This motion of the electrons results in the generation of x-rays.

Different methods to increase the energy of electrons accelerated in aplasma exists. For example, the acceleration length can be increased byguiding the laser pulse in a hollow dielectric capillary tube filled with a gas.However, for an optimal guiding, the diameter of the tube must be small.Small diameters of the tube typically lead to high laser intensities at the wallsof the tube, which can damage the tube. As a result, the maximum laserintensity for a hollow dielectric capillary tube is limited, and by extension,limiting the energy of accelerated electrons.

Therefore, there exists a need for an improved method for increasing the energy of accelerated electrons.

Summary of the invention ln view of the above, it is an object of the present invention to provide amethod and a system for generating a pulse of radiation. lt is an object to mitigate, alleviate or eliminate one or more of theabove-identified deficiencies in the art and disadvantages singly or in anycombination and solve at least the above mentioned problem.

According to a first aspect, a method for generating a pulse of high;energy electrons and/or a pulse of x-rayfs-radiatien- is provided. The methodcomprises: Transverse to an optical axis, generating a sound wave in a fluid,wherein the sound wave is configured to form a spatially and temporallymodulated fluid-density distribution in the fluid; ionizing the fluid, therebycreating a plasma having a spatially and temporally modulated plasma-density distribution relating to the spatially and temporally modulated fluid-density distribution in the fluid; and transmitting a high-intensity light pulsealong the aforententiciïedaa optical axis through the plasma, wherein thehigh-intensity light pulse interacts with the plasma, thereby generating thepulse of hidh-ezwerdv electrons and/oi' a pulse of xwavsraæfâat-ien.

The wording “fluid-density distribution” should, within the context of thisapplication be construed as a number density distribution of atoms and/ormolecules in the fluid.

The wording “plasma-density distribution” should, within the context ofthis application be construed as a number density distribution of freeelectrons in the plasma. ln other words, a plasma density depends on thefluid density and a level of ionization. For instance, in a fully ionized helium(He) plasma, the plasma density is twice as large as the fluid density, as eachHe atom comprises two electrons. As is generally known in the context ofLWFA, free electrons in the plasma may be displaced by the high-intensitylaser pulse, thereby changing a local plasma density.

A refractive index, n, of the plasma depends on the plasma density, ne: rep-Ef? <1) nc where nc is a critical plasma density for the high-intensity light pulse. As isshown in Eq. 1, the refractive index of the plasma decreases with increasingplasma density. ln other words, the refractive index of the plasma depends onthe fluid density and a level of ionization. A skilled person realizes that thehigh-intensity light pulse may be guided in the plasma in case the plasmaresembles a positive lens, i.e. that the refractive index at the optical axis ishigher than at larger radial distances from the optical axis in a planetransverse to the optical axis. For example, a high-intensity light pulse havinga Gaussian intensity distribution may be guided in a plasma having aparabolic distribution of the refractive index as: w) = m, + An - <2) where r is the radial distance from the optical axis in a plane transverse to theoptical axis, 170 is the refractive index on axis, and An is the change inrefractive index at radius rm. lt is to be understood that the plasma density nemay vary over time and space.

By means of the present method, a refractive index of the plasma maybe controlled by controlling the spatially and temporally modulated plasma-density distribution. Controlling the refractive index of the plasma may allowfor an improved guiding of the high-intensity light pulse in the plasma, therebyincreasing an energy of the generated electron pulse. The improved guidingof the high-intensity light pulse in the plasma may further allow for an increase of a number of generated x-rays. -lïhe-gen-erated--eul-se--af--raeiiettan---rnay--be--a--peEse--eí--hägiverfaergy An advantage of the generated sound wave being configured to formthe spatially and temporally modulated fluid-density distribution in a directiontransverse to the optical axis is that the guiding of the high-intensity lightpulse in the plasma may be improved. The guiding of the high-intensity lightpulse in the plasma may be improved by controlling the spatially andtemporally modulated fluid-density distribution and thereby the refractiveindex of the plasma in a direction transverse to the optical axis.

The sound wave may be configured such that the spatially andtemporally modulated fluid-density distribution has a local minimum at theoptical axis.

The wording “local minimum” should, within the context of thisapplication be construed as a local minimum of the fluid density within anaccuracy of 110 °/>. Accordingly, a difference within 110 °/> of a mathematicallocal minimum is acceptable.

An advantage of the spatially and temporally modulated fluid-densitydistribution having a local minimum at the optical axis is that the spatially andtemporally modulated plasma-density distribution in the plasma may have alocal minimum at the optical axis. The plasma may thereby have a localmaximum of the refractive index at the optical axis. A local maximum of therefractive index at the optical axis in plasma may allow for an improvedguiding of the high-intensity light pulse. An improved guiding of the high-intensity light pulse may allow for an increase in acceleration length forelectrons accelerated in the plasma, thereby increasing an energy of thegenerated pulse of electrons.

A further advantage of increasing the acceleration length for thegenerated electron pulse in the plasma is that an increased number of x-raysmay be generated.

According to a second aspect, a system for generating a pulse ofradiation is provided. The system comprises: an acousto-optic elementcomprising: a fluid inlet configured to admit fluid, wherein the admitted fluid defines a fluid volume, and one or more sound generators, wherein each sound generator is configured to generate a sound wave in the fluid volumesuch that a spatially and temporally modulated fluid-density distribution withinthe fluid volume is formed; a fluid valve configured to admit fluid from a fluidreservoir through the fluid inlet of the acousto-optic element; a high-intensitylight source configured to emit high-intensity light pulses; and a controllerconfigured to: control the fluid valve to admit fluid through the fluid inlet,control each of the one or more sound generators to generate sound waves,and control the high-intensity light source to emit a high-intensity light pulse;and wherein the acousto-optic element and the high-intensity light source arearranged to align a longitudinal axis of the acousto-optic element and anoptical axis of the high-intensity light source.

The system may further comprise electrodes configured to ionize theadmitted fluid. ln use, the fluid having the spatially and temporally modulated fluid-density distribution is ionized, thereby forming a plasma having a spatially andtemporally modulated plasma-density distribution. The high-intensity lightpulse may propagate through a plasma having a spatially and temporallymodulated plasma-density distribution. The fluid may be ionized by theelectrodes. The fluid may be ionized by the high-intensity light pulse. The fluidmay be ionized by a further high-intensity light pulse.

The above mentioned features of the method, when applicable, applyto this second aspect as well. ln order to avoid undue repetition, reference ismade to the above.

The controller may be further configured to synchronize the control ofthe fluid valve, the control of each of the one or more sound generators, andthe control of the high-intensity light source, and wherein the synchronizationis configured such that the spatially and temporally modulated fluid-densitydistribution has a local minimum at the axis of the acousto-optic elementwhen the emitted high-intensity light pulse reaches the fluid volume in theacousto-optic element.

An advantage of synchronizing the fluid valve, each of the one or moresound generators, and the high-intensity light source such that the spatiallyand temporally modulated fluid-density distribution has a local minimum at theaxis of the acousto-optic element when the emitted high-intensity light pulsereaches the fluid volume in the acousto-optic element is that the formedplasma may have a local minimum of the plasma density at the axis of theacousto-optic element. A local minimum of the plasma density at the axis ofthe acousto-optic element may improve the guiding of the high-intensity lightpulse through the plasma, thereby increasing an energy of the generatedpulse of electrons.

The acousto-optic element may further comprise: a fluid cellcomprising: an entrance on a first side of the fluid cell, and an exit on asecond side of the fluid cell; and wherein the fluid cell is arranged to acceptthe admitted fluid.

An advantage of the acousto-optic element further comprising a fluidcell arranged to accept the admitted fluid is that a more well-defined fluidvolume may be allowed. A more well-defined fluid volume may result in a fluidvolume having a more homogenous fluid-density distribution prior to thegeneration of a sound wave in the fluid volume. A more homogenous fluid-density distribution may allow for an improved stability of the generated pulseof electrons or x-rays.

A first sound generator in the acousto-optic element may be arrangedto generate a sound wave in a first direction, and a second sound generatorin the acousto-optic element may be arranged to generate a sound wave in asecond direction, and wherein the first direction and the second directionintersect at an angle.

An advantage of arranging a first and a second sound generator togenerate sound waves in a first and a second direction, and wherein the firstand second directions intersect at an angle is that a size and or a position ofan overlap of the sound waves generated by the first and second sound generators may be adjusted by adjusting the angle.

An advantage of the first direction and the second direction intersectingat an angle is that the sound waves generated by the first and second soundgenerators may interfere in the overlap of the sound waves. The interferingsound waves may increase a fluid-density modulation in the spatially andtemporally modulated fluid-density distribution in the fluid, thereby improvingthe guiding of the high-intensity light pulse in the plasma.

The acousto-optic element may further comprise two oppositelyarranged sound generators configured to generate sound waves inantiparallel directions.

An advantage of two oppositely arranged sound generators configuredto generate sound waves in antiparallel directions is that the sound wavesmay interfere in an overlap of the sound waves. The interfering sound wavesmay increase a fluid-density modulation in the spatially and temporallymodulated fluid-density distribution in the fluid, thereby improving the guidingof the high-intensity light pulse in the plasma.

The acousto-optic element may further comprise a sound generatoroppositely arranged to a sound reflector.

An advantage of a sound generator being oppositely arranged to asound reflector is that a sound wave generated by the sound generator maybe reflected by the sound reflector. The generated sound wave and thereflected sound wave may interfere in an overlap of the generated soundwave and the reflected sound wave. The interference of the generated soundwave and the reflected sound wave may increase a fluid-density modulationin the spatially and temporally modulated fluid-density distribution in the fluid,thereby improving the guiding of the high-intensity light pulse in the plasma.

The acousto-optic element may further comprise a plurality of soundgenerators arranged along an extension of the fluid cell between the entranceand the exit.

An advantage of the acousto-optic element further comprising aplurality of sound generators arranged along the extension of the fluid cell between the entrance and the exit is that the spatially and temporally modulated fluid-density distribution may be further modulated along theextension of the fluid cell. ln other words, the spatially and temporallymodulated fluid-density distribution may be further modulated along theoptical axis.

A further advantage of the acousto-optic element further comprising aplurality of sound generators arranged along the extension of the fluid cellbetween the entrance and the exit is that a length along the extension of thespatially and temporally modulated fluid-density distribution may beincreased.

Two adjacent sound generators of the plurality of sound generators inthe acousto-optic element may be separated by a distance along theextension of the fluid cell.

The acousto-optic element may further comprise a first structurearranged at the first side of the fluid cell configured to scatter and/or absorbsound waves emitted by the one or more sound generators.

The acousto-optic element may further comprise a second structurearranged at the second side of the fluid cell configured to scatter and/orabsorb sound waves emitted by the one or more sound generators.

An advantage of the acousto-optic element further comprisingstructures, configured to scatter and/or absorb sound waves emitted by theone or more sound generators, arranged at the first and/or second sides ofthe fluid cell is that reflections of sound waves at the first and/or second sidesof the fluid cell may be reduced. Reducing reflections on the first side of thefluid cell may improve a guiding of the high-intensity light pulse, sincereflected sound waves may not contribute to the formation of the spatially andtemporally modulated fluid-density distribution in the fluid.

The one or more sound generators in the acousto-optic element maybe further configured to generate focused sound waves.

The wording “focused sound wave” should, within the context of this application, be construed as a sound wave converging in one or two dimensions. ln other words, the sound wave may converge in one dimensiontowards a line in space, or in two dimensions towards a point in space.

An advantage of the one or more sound generators in the acousto-optic element being further configured to generate focused sound waves isthat a size of an overlap of the generated focused sound waves may besmaller. A smaller size of the overlap of the generated focused sound wavesmay allow for a more symmetric spatially and temporally modulated fluid-density distribution in the fluid in a plane defined by the generated focusedsound waves.

The one or more sound generators in the acousto-optic element maybe configured to generate ultrasound.

The wording “ultrasound” should, within the context of this application,be construed as sound having a frequency higher than 20 kHz.

According to a third aspect, use of the present system, therebygenerating a pulse of radiation, is provided.

The above mentioned features of the method and the system, whenapplicable, apply to this third aspect as well. ln order to avoid unduerepetition, reference is made to the above.

A further scope of applicability of the present disclosure will becomeapparent from the detailed description given below. However, it should beunderstood that the detailed description and specific examples, whileindicating preferred variants of the present inventive concept, are given byway of illustration only, since various changes and modifications within thescope of the inventive concept will become apparent to those skilled in the artfrom this detailed description.

Hence, it is to be understood that this inventive concept is not limited tothe particular steps of the methods described or component parts of thesystems described as such method and system may vary. lt is also to beunderstood that the terminology used herein is for purpose of describingparticular embodiments only, and is not intended to be limiting. lt must be noted that, as used in the specification and the appended claim, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more ofthe elements unless the context clearly dictates otherwise. Thus, for example,reference to “a unit” or “the unit” may include several devices, and the like.Furthermore, the words “comprising”, “including”, “containing” and similar wordings do not exclude other elements or steps.

Brief description of the drawings The above and other aspects of the present invention will now bedescribed in more detail, with reference to appended drawings showingembodiments of the invention. The figures should not be considered limitingthe invention to the specific embodiment; instead they are used for explainingand understanding the invention.

As illustrated in the figures, the sizes of layers and regions areexaggerated for illustrative purposes and, thus, are provided to illustrate thegeneral structures of embodiments of the present invention. Like referencenumerals refer to like elements throughout.

Figure 1A illustrates a system for generating a pulse of radiation.

Figure 1 B illustrates a top view of an acousto-optic element.

Figure 1C illustrates a cross-sectional view of the acousto-opticelement in Fig. 1B.

Figure 2A illustrates a top view of an acousto-optic element comprisinga fluid cell.

Figure 2B illustrates a cross-sectional side view of the acousto-opticelement in Fig. 2A.

Figure 3 is a block scheme of a method for generating a pulse ofradiation.

Figure 4 illustrates a spatially and temporally modulated fluid-densitydistribution in a plane transverse to a longitudinal axis of an acousto-optic element. 11 Detailed descriptionThe present inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which currentlypreferred variants of the inventive concept are shown. This inventive conceptmay, however, be implemented in many different forms and should not beconstrued as limited to the variants set forth herein; rather, these variants areprovided for thoroughness and completeness, and fully convey the scope ofthe present inventive concept to the skilled person.

A system 10 for generating a pulse of radiation will now be describedwith reference to Fig. 1A-Fig. 2B.

Figure 1A illustrates a system 10 for generating a pulse of radiation.The system 10 comprises an acousto-optic element 100. The acousto-opticelement 100 comprises a fluid inlet 104 configured to admit fluid, wherein theadmitted fluid defines a fluid volume 106, and one or more sound generators140, 240, 242, 244, 250, 252, 260, wherein each sound generator 140, 240,242, 244, 250, 252, 260 is configured to generate a sound wave in the fluidvolume 106 such that a spatially and temporally modulated fluid-densitydistribution within the fluid volume 106 is formed.

The fluid inlet 104 may be a fluid nozzle. The fluid nozzle may beconfigured to admit a supersonic jet of fluid. The fluid may be a gas. The gasmay be one or more of hydrogen, helium, argon, and/or nitrogen. The fluidmay be ionized, thereby forming a plasma. The plasma may be partly or fullionized. The acousto-optic element 100 may comprise electrodes. Theelectrodes may be configured to ionize the fluid. The formed plasma mayhave a spatially and temporally modulated plasma-density distribution relatingto the spatially and temporally modulated fluid-density distribution in the fluid.The generated sound wave may be a pulsed sound wave. The generatedsound wave may be a continuous sound wave.

The system 10 further comprises a fluid valve 110 configured to admitfluid from a fluid reservoir 112 through the fluid inlet 104 of the acousto-optic element 100. 12 The system 10 further comprises a high-intensity light source 120configured to emit high-intensity light pulses.

The high-intensity light source 120 may be a high-intensity lasersystem. The high-intensity light pulse may be a high-intensity laser pulse. Thehigh-intensity laser system may be configured to emit the high-intensity laserpulse. A temporal duration of the high-intensity light pulse may be in a rangefrom 1 fs to 500 fs. The high-intensity light pulse may carry an energy in arange from 1 mJ to 10 kJ.

The system 10 further comprises a controller 130 configured to: controlthe fluid valve 110 to admit fluid through the fluid inlet 104, control each of theone or more sound generators 140, 240, 242, 244, 250, 252, 260 to generatesound waves, and control the high-intensity light source 120 to emit a high-intensity light pulse.

The acousto-optic element 100 and the high-intensity light source 120are arranged to align a longitudinal axis 102 of the acousto-optic element 100and an optical axis 122 of the high-intensity light source 120.

The high-intensity light source 120 may further comprise optics 124,126, as exemplified in Fig. 1A. The optics 124, 126 may comprise focusingoptics 126. The focusing optics 126 may be arranged to have a focal plane inthe admitted fluid. The focusing optics 126 may be reflective optics. The high-intensity light pulse may have a spot size in a range 1 um to 100 um in thefocal plane. The high-intensity light pulse may have an intensity higher than1015 W/cm2 in the focal plane. The high-intensity light pulse may ionize thefluid. A further high-intensity light pulse may ionize the fluid. The high-intensitylight pulse may propagate through the plasma. ln use, a pulse of radiationmay be generated when the high-intensity light pulse interacts with theplasma. The pulse of radiation may be generated by laser wakefieldacceleration.

The controller 130 may be further configured to synchronize the controlof the fluid valve 110, the control of each of the one or more sound generators140, 240, 242, 244, 250, 252, 260, and the control of the high-intensity light 13 source 120. The synchronization may be configured such that the spatiallyand temporally modulated fluid-density distribution has a local minimum at theIongitudinal axis 102 of the acousto-optic element 100 when the emitted laserpulse reaches the fluid volume 106 in the acousto-optic element 100.

The spatially and temporally modulated fluid-density distribution mayincrease at larger radial distances from the Iongitudinal axis 102 of theacousto-optic element 100. The spatially and temporally modulated fluid-density distribution may have a plurality of local maxima 410, 412, 414 andlocal minima 420, 422, 424, 426 along a radial direction in a plane transverseto the Iongitudinal axis 102 of the acousto-optic element 100, as exemplifiedin Fig. 4. For a duration of a laser pulse, the temporal modulation of thespatially and temporally modulated fluid-density distribution may be verysmall. ln Fig. 4, a spatially and temporally fluid-density distribution isexemplified. A vertical axis 404 in Fig. 4 represents a normalized fluid-density,and a horizontal axis 402 represent the radial direction in a plane transverseto the Iongitudinal axis 102 of the acousto-optic element 100. The spatiallyand temporally modulated fluid-density distribution may be modulated aboutan unperturbed fluid density 404A, as is exemplified in Fig. 4. The fluiddensity may increase from a first local minimum 420 to a first local maximum410 with increasing radial distance from the Iongitudinal axis 102. The fluiddensity may decrease from the first local maximum 410 to a second localminimum 422. ln other words, the spatially and temporally modulate fluid-density distribution may comprise a plurality of local maxima 410, 412, 414and local minima 420, 422, 424, 426 in the radial direction in a planetransverse to the Iongitudinal axis 102 of the acousto-optic element 100.

An acousto-optic element 100 comprising a fluid cell 200 will now bedescribed with reference to Fig. 2A and Fig. 2B.

The acousto-optic element 100 may further comprise a fluid cell 200,The fluid cell 200 comprises an entrance 204 on a first side of the fluid cell200, and an exit 208 on a second side of the fluid cell 200. The fluid cell 200 may be arranged to accept the admitted fluid. 14 A length of the acousto-optic element 100 may be the length of thef|uid cell 200. A plurality of sound generators 240, 242, 244, 250, 252, 260may be placed after each other in a direction from the first side and thesecond side of the f|uid cell 200. The high-intensity light pulse may enter thef|uid cell 200 through the entrance 204. The entrance 204 of the f|uid cell 200may be a high-intensity light pulse entrance. The high-intensity light pulseentering the entrance may exit the f|uid cell 200 through the exit 208. The exit208 of the f|uid cell 200 may be a high-intensity light pulse exit. Thelongitudinal axis 102 of the acousto-optic element 100 may coincide with theentrance 204 and the exit 208 of the f|uid cell 200. The generated pulse ofradiation may exit the f|uid cell 200 through the exit 208. The one or moresound generators 240, 242, 244, 250, 252, 260 may be arranged in an innerspace of the f|uid cell 200 as exemplified by sound generators 240, 242, 244,250, 260 in Fig. 2B. Alternatively or additionally, the one or more soundgenerators 240, 242, 244, 250 252, 260 may be arranged on an outer surfaceof the f|uid cell 200 as exemplified by the sound generator 252 in Fig. 2B. Thespatially and temporally modulated plasma-density distribution in the plasmamay act as a waveguide for the high-intensity light pulse entering theentrance 204. The spatially and temporally modulated plasma-densitydistribution in the plasma may act as a focusing element for the high-intensitylight pulse entering 204 the entrance.

A first sound generator 240 in the acousto-optic element 100 may bearranged to generate a sound wave in a first direction. A second soundgenerator 244 in the acousto-optic element 100 may be arranged to generatea sound wave in a second direction. The first direction and the seconddirection may intersect at an angle. The angle between the first and seconddirections may be in a range from 10°to 90 °. A first sound wave generated inthe first direction and a second sound wave generated in the second direction may interfere at the intersection.

The acousto-optic element 100 may further comprise two oppositely240, 242, 250, 252 arranged sound generators configured to generate soundwaves in antiparallel directions.

Sound waves generated by the two oppositely arranged soundgenerators 240, 242, 250, 252 may interfere. Sound waves generated by thetwo oppositely arranged sound generators 240, 242, 250, 252 may form astanding wave.

The acousto-optic element 100 may further comprise a soundgenerator 260 oppositely arranged to a sound reflector 268.

A sound wave generated by the sound generator 260 may be reflectedby the sound reflector 268. The direction of the reflected sound wave may beantiparallel to the sound wave generated by the sound generator 260. Thegenerated sound wave and the reflected sound wave may interfere. Thegenerated sound wave and the reflected sound wave may form a standingwave.

The acousto-optic element 100 may further comprise a plurality ofsound generators 240, 242, 244, 250, 252, 260 arranged along an extensionof the fluid cell 200 between the entrance 204 and the exit 208.

Two adjacent sound generators of the plurality of sound generators240, 242, 244, 250, 252, 260 in the acousto-optic element 100 are separatedby a distance along the extension of the fluid cell 200.

The distance separating two adjacent sound generators of the pluralityof sound generators 240, 242, 244, 250, 252, 260 may be less than a wavelength of sound waves generated by the two adjacent sound generators.

The acousto-optic element 100 may further comprise a first structure214 arranged at the first side of the fluid cell 200 configured to scatter and/orabsorb sound waves emitted by the one or more sound generators 240, 242,244, 250, 252, 260.

The acousto-optic element 100 may further comprise a secondstructure 218 arranged at the second side of the fluid cell 200 configured to 16 scatter and/or absorb sound waves emitted by the one or more soundgenerators 240, 242, 244, 250, 252, 260.

The first and/or the second structure 214, 218 may comprise anirregularly shaped surface. Sound waves may be scattered on an irregularlyshaped surface. The first and/or the second structure 214, 218 may comprisea sound-absorbing material. The first and/or the second structure 214, 218may be further arranged in the inner space of the f|uid cell 200. The firstand/or second structure 214, 218 may face the inner space of the f|uid cell200.

The one or more sound generators 140, 240, 242, 244, 250, 252, 260in the acousto-optic element 100 may be further configured to generatefocused sound waves.

The focused sound waves may be focused at the longitudinal axis 102of the acousto-optic element 100. The focused sound wave may begenerated by curved sound generators. Generated sound waves may befocused by acoustic lenses and/or by curved acoustic mirrors.

The one or more sound generators 140, 240, 242, 244, 250, 252, 260in the acousto-optic element 100 may be configured to generate ultrasound.

Figure 3 is a block scheme of a method 30 for generating a pulse ofradiation. The method 30 comprises generating 302 a sound wave in a f|uid,wherein the sound wave is configured to form a spatially and temporallymodulated f|uid-density distribution in the f|uid.

The method 30 further comprises ionizing 304 the f|uid, therebycreating a plasma having a spatially and temporally modulated plasma-density distribution relating to the spatially and temporally modulated f|uid-density distribution in the f|uid.

The method 30 further comprises transmitting 306 a high-intensity lightpulse along an optical axis 122 through the plasma, wherein the high-intensitylight pulse interacts with the plasma, thereby generating the pulse ofradiation. 17 The fluid may be ionized by an electrode (not shown in the figures).The fluid may be ionized by the high-intensity light pulse. The fluid may beionized by a further high-intensity light pulse. The high-intensity light pulsemay be focused in the plasma.

The generated pulse of radiation may be a pulse of high-energyelectrons and/or a pulse of x-rays.

The pulse of high-energy electrons and/or the pulse of x-rays may begenerated through an interaction between the high-intensity light pulse andthe plasma. The pulse of high-energy electrons and/or the pulse of x-raysmay be generated through a process commonly known in the field as “laserwakefield acceleration”.

The generated sound wave may be configured to form the spatially andtemporally modulated fluid-density distribution in a direction transverse to theoptical axis 122.

The high-intensity light pulse may propagate through the plasma alongthe optical axis 122.

The sound wave may be configured such that the spatially andtemporally modulated fluid-density distribution has a local minimum at theoptical axis 122.

The light pulse may be guided or focused by refraction in the spatiallyand temporally modulated plasma.

The person skilled in the art realizes that the present inventive conceptby no means is limited to the preferred variants described above. On thecontrary, many modifications and variations are possible within the scope ofthe appended claims.

For example, the acousto-optic element illustrated in Fig. 1B andFig. 1C may comprise additional sound generators as exemplified in relationto Fig. 2A and Fig. 2B. Further, the fluid cell may have other shapes of itscross section. For instance, the cross section may be rectangular instead ofelliptical as exemplified in Fig. 2A. 18 Additionally, Variations to the disclosed variants can be understood andeffected by the skilled person in practicing the claimed invention, from a studyof the drawings, the disclosure, and the appended claims.

Claims (15)

19 CLAl|\/IS

1. A method (30) for generating a pulse of radiation, the method (30)comprising: generating (302) a sound wave in a fluid, wherein the sound wave isconfigured to form a spatially and temporally modulated fluid-densitydistribution in a direction transverse to the optical axis (122).; ionizing (304) the fluid, thereby creating a plasma having a spatiallyand temporally modulated plasma-density distribution relating to the spatiallyand temporally modulated fluid-density distribution in the fluid; and transmitting (306) a high-intensity light pulse along an optical axis (122)through the plasma, wherein the high-intensity light pulse interacts with theplasma, thereby generating the pulse of radiation of high-energy electrons and/or a pulse of x-rays.

2. The method (30) according to any claim 1, wherein the sound wave isconfigured such that the spatially and temporally modulated fluid-densitydistribution has a local minimum at the optical axis (122).

3. A system (10) for generating a pulse of radiation, the system (10)comprising: an acousto-optic element (100) comprising: a fluid inlet (104) configured to admit fluid, wherein the admittedfluid defines a fluid volume (106), andone or more sound generators (140, 240, 242, 244, 250, 252, 260), wherein each sound generator (140, 240, 242, 244, 250, 252, 260) is configured to generate a sound wave in the fluid volume (106) such that a spatially and temporally modulated fluid-density distribution within the fluid volume (106) is formed; a fluid valve (110) configured to admit fluid from a fluid reservoir (112)through the fluid inlet (104) of the acousto-optic element (100); a high-intensity light source (120) configured to emit high-intensity lightpulses; anda controller (130) configured to:control the fluid valve (110) to admit fluid through the fluid inlet(104),control each of the one or more sound generators (140, 240,242, 244, 250, 252, 260) to generate sound waves, andcontrol the high-intensity light source (120) to emit a high-intensity light pulse; andwherein the acousto-optic element (100) and the high-intensity lightsource (120) are arranged to align a longitudinal axis (102) of the acousto-optic element (100) and an optical axis (122) of the high-intensity light source(120).

4. The system (10) according to claim 3, wherein the controller (130) isfurther configured to synchronize the control of the fluid valve (110), thecontrol of each of the one or more sound generators (140, 240, 242, 244,250, 252, 260), and the control of the high-intensity light source (120), andwherein the synchronization is configured such that the spatially andtemporally modulated fluid-density distribution has a local minimum at thelongitudinal axis (102) of the acousto-optic element (100) when the emittedhigh-intensity light pulse reaches the fluid volume (106) in the acousto-opticelement (100).

5. The system (10) according to claim 3 or 4, wherein the acousto-opticelement (100) further comprises:a fluid cell (200) comprising:an entrance (204) on a first side of the fluid cell (200), andan exit (208) on a second side of the fluid cell (200); andwherein the fluid cell (200) is arranged to accept the admitted fluid. 21

6. The system (10) according to any one of claims 3-5, wherein a firstsound generator (240) in the acousto-optic element (100) is arranged to generate a sound wave in a first direction, and a second sound generator(244) in the acousto-optic element (100) is arranged to generate a soundwave in a second direction, and wherein the first direction and the second direction intersect at an angle.

7. The system (10) according to any one of claims 3-7, wherein theacousto-optic element (100) further comprises two oppositely arranged soundgenerators (240, 242, 250, 252) configured to generate sound waves in antiparallel directions.

8. The system (10) according to any one of claims 3-7, wherein theacousto-optic element (100) further comprises a sound generator (260)oppositely arranged to a sound reflector (268).

9. The system (10) according to any one of claims 5-8, wherein theacousto-optic element (100) further comprises a plurality of sound generators(240, 242, 244, 250, 252, 260) arranged along an extension of the fluid cell(200) between the entrance (204) and the exit (208).

10. The system (10) according to claim 9, wherein two adjacent soundgenerators of the plurality of sound generators (240, 242, 244, 250, 252, 260)in the acousto-optic element (100) are separated by a distance along theextension of the fluid cell (200).

11. The system (10) according to any one of claims 5-10, wherein theacousto-optic element (100) further comprises a first structure (214) arrangedat the first side of the fluid cell (200) configured to scatter and/or absorbsound waves emitted by the one or more sound generators (240, 242, 244,250, 252, 260). 22

12. The system (10) according to any one of claims 5-11, wherein theacousto-optic element (100) further comprises a second structure (218)arranged at the second side of the fluid cell (200) configured to scatter and/orabsorb sound waves emitted by the one or more sound generators (240, 242,244, 250, 252, 260).

13. The system (10) according to any one of claims 3-12, wherein the oneor more sound generators (140, 240, 242, 244, 250, 252, 260) in the acousto- optic element (100) are further configured to generate focused sound waves.

14. The system (10) according to any one of claims 3-13, wherein the oneor more sound generators (140, 240, 242, 244, 250, 252, 260) in the acousto- optic element (100) are configured to generate ultrasound.

15. Use of the system (10) according to any one of claims 3-14, thereby generating a pulse of radiation.