A class of planar microstructure is proposed which provide high accelerating gradients when excited by an infrared laser pulse. These structures consist of parallel dielectric slabs separated by a vacuum gap; the dielectric or the outer surface coating are spatially modulated at the laser wavelength along the beam direction so as to support a standing wave accelerating field. We have developed numerical and analytic models of the accelerating mode fields in the structure. We show an optimized coupling scheme such that this mode is excited resonantly with a large quality factor. The status of planned experiments on fabricating and measuring these planar structures will be described

Rosenzweig, J B

Schoessow, P

Rosenzweig, J. B.

Schoessow, P. V.

UNT Digital Library

“1 ANL-HEP-CP-99-65iSLAB SYMMETRIC DIELECTRIC MICRON SCALE STRucTu~S FORHIGH GRADIENT ELECTRON ACCELERATION*The submitted manuscript has been created,.by the University of Chicago as operator ofArgonne National Laboratory (“Argonne”)under Contract No. W-31-109-ENG-38 withthe U.S. Department of Energy. The U.S.Government retains for itself, and others act-ing on its behalf, a paid-up, nonexclusive,irrevocable worldwide license in said aflicleto reproduce, prepare derivative works, clis-tribute copies to the public, and perform pub.Iicly and display publicly, by or on behalf ofthe Government.P. V. Schoessow,Argonne National Laboratory, Argonne, IL 60439J. B. Rosenzweig,UCLA Department of Physics and Astronomy,405 Hilgard Ave., Los Angeles, CA 90095AbstractA class of planar microstructure is proposed whichprovide high accelerating gradients when excited by aninfrared laser pulse. These structures consist of paralleldielectric slabs separated by a vacuum gap; the dielectricor the outer surface coating are spatially modulated at thelaser wavelength along the beam direction so as tosupport a standing wave accelerating field. We havedeveloped numerical and analytic models of theaccelerating mode fields in the structure. We show anoptimized coupling scheme such that this mode is excitedresonantly with a large quality factor, The status ofplanned experiments on fabricating and measuring theseplanar stntctures wiU be described.1 INTRODUCTIONAdvances in the technology of lasers have led toincreased interest in their potential applications foraccelerating particles. Plasma beat wave[ 1] and laserwakefie1d[2] acceleration methods use laser radiation todrive a plasma wave with Iuminal phase velocity. Thelongitudinal electric fields in this plasma wave are in turnused to accelerate an electron beam. InverseCherenkov[3] and inverse free electron laser [4]acceleration techniques use light optics or magnetic fieldsrespectively to achieve an electric field componentparallel to the beam direction from the transverse electricfield in the laser pulse. We have proposed a class ofresonant dielectric loaded planar structures[5] capable ofproducing GeV/m accelerating gradients which are drivenby laser radiation much as a conventional rf cavity isdriven by microwave power from a klystron.The basic idea is the use of a dielectric microstructure,analogous to a Fabry-Perot resonator, consisting of a twoparallel dielectric planes separated by a vacuum gap andwith a partially transmissive coating on the exterior (seeFigure 1). Some parameters of the structure are assumedto vary periodically in the beam direction at thewavelength of the illuminating laser. A standing wavewith an appropriate phase velocity forward component isinduced in this periodic structure by the laser pulse, whichin turn accelerates the beam. The laser pulse is sweptalong the surface of the structure such that the cavity isfilled only in the neighborhood of the beam. TheSubmitted to the proceedings of the 1999New York, New York, March 29 to April 2,relatively low Q of these devices ( Ia&%dcorrespondingly short fill times (-0.5 ps) allows gradientsof 1 GeV/m to be obtained before breakdown becomesproblematic[9]. The beam aspect ratio is highly asym-metric; the ribbon beam-planar structure configuration isadvantageous in that dipole deflecting modes are sup-pressed[6], analogous to the vanishing of the transversedeflection for TMOn modes in conventional structures.ILam propgatbn directionMh-ror trmsminkm‘:”*S kture invarknt in<= Metal ❑ DiektrtcIJ- rBeam propagation ●nd l-rpolarization direction in zFigure 1. Geometry of slab-symmetric, laser-excited,dielectric-loaded resonant accelerator structure.A number of different methods have been investigatedto introduce the necessary periodic variation in thestructure geometry. The original paper in this area[5]suggested the use of a longitudinally modulatedpermittivity in the dielectric. While this was attractivefrom the point of view of construction and analysis (onecould concievably generate the modulation by “writing”an interference pattern in a photorefractive dielectric witha second laser) it proved to be difficult to obtain goodcoupling of the laser energy to the accelerating field.More recent analyses have concentrated on using aspatially unvarying dielectric medium and modulating thetransmittivity of the coating [7]. This approach was foundto improve the laser coupling to the structure significantlybut was found to yield uncomfortably large surface fieldson the structure.We discuss here some recent progress in theunderstanding of slab structure design. We haveinvestigated the use of finite thickness conductivecladding on the structure, with a single coupling slot perperiod. With this geometry good coupling of laser energyto the accelerating fields can be obtained, while at thesame time reducing the surface field/accelerating fieldratio significimtly over our previous results. The shape ofthe coupling slot is also seen to be important, in that theparticle Accelerator Conference (PAC99)1999.DISCLAIMERThis repofl was prepared as an account of work sponsoredbyanagency of the United States Government. Neither theUnited States Government nor any agency thereof, nor anyof their employees, make any warranty, express or implied,or assumes any legal liability or responsibility for theaccuracy, completeness, or usefulness of any information,apparatus, product, or process disclosed, or represents thatits use would not infringe privately owned rights. Referenceherein to any specific commercial product, process, orservice by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the UnitedStates Government or any agency thereof. The views andopinions of authors expressed herein do not necessarilystate or refiect those of the United States Government orany agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic imageproduced from thedocument.products. Images arebest available original4 introduction of a taper at the slot opening providesimproved coupling over the case of a rectangular slotprofile.2 NUMERICAL RESULTSIn the study of these structures we have found it useful torely on numerical solution of the Maxwell equations inslab geometry. We use a custom finite difference timedomain code to inject and propagate the laser fields intothe structure geomerty under study. The programimplements periodic boundary conditions in z (the beampropagation direction) and absorbing boundary condhionsin y (the laser propagation direction) to handle anyreflected laser energy from the structure. The structure isassumed to be of infinite extent in the x-view. For a givenstructure geometry, the integration is continued until asteady state condition is reached. The structure is “tuned”numerically by adjusting one of its parameters (typicallythe dielectric constant) until the asymptotic stored energyis maximized. (In the laboratory one would tune thestructure by changing a geometrical parameter like thevacuum gap size.)We have analyzed a structure with dielectric thickness(b-a) equal to the vacuum gap @ =1.6 ~m), period of10.6 ~m (corresponding to a commone C02 laser line),and dielectric constant &s 3.7. The conductive claddingthickness in the simulation is 0.3 pm. The laser couplingand field strengths in the structure were studied as afunction of the shape of the coupling slot. While notexhaustive, these calculations indicate a promisingapproach to the problem of coupling optimization. Thefollowing results are normalized to a peak laser electricIn reference[7], the fields of a similar structure having aninfinitesimally thick, sinusoidally modulated transmjt.tivity outer cladding were computed. While reasonablecoupling was achieved, it was also pointed out that thesurface electric field on the mirror-dielectric interface wasundesirably large, roughly twice the maximum field in thevacuum gap. The case of a f7nite thickness mirror with arectangular aperture was studied for this paper. For a 2~m slot width the corresponding peak surface field is 2.0Statvolt/cm for a gap field amplitude of 1.06 Statvolt/cm.The largest surface fields in this device occurred at theexterior comers of the slot; this might be further improvedby rounding the corners. (The present version of thesimulation code cannot handle curved boundaries exceptvia a stairstep approximation.) An increase in thecoupling slot width to 3 pm produced slightly worseresults with a surface fieki/gap field ratio of 1.9/0.87.The effect of tapering the aperture of the coupling slotis shown in Figure 2. The surface field/gap field ratio isrelatively small, 1.56/1.23, while the gap field strength isalso a maximum for all similar device geometries. Storedenergy and gap field strengths vs time are shown inFigure 3. From the energy history we can read off a filltime of 0.57 ps, corresponding to Q=102. As we havediscussed previous1y[7], the fields in the vacuum gap area superposition of Fabry-Perot-like (zero phase shift perperiod, providing no net acceleration) and accelerating(forward wave compcmet resonant with an ultrarelativisticparticle) modes. The Fabry-Perot mode amplitude hasbeen diminished to 20% of the accelerating modeamplitude in the present case, solely through through theperiodicity of the coupling. This proportion seems to beabout the most favorable we have been able to obtain.field of 0~25 Statvolt/cm = 75 kV/m. -IyIMknnlla mrr’m)Figure 2. Pseudocolor contour map of E, in the comput-ational area for the tapered aperture structure. Laser radia-tion is incident from the top of the figure (y=(l). The areadepicted in solid black is the conductive cladding, and thethin black line indicates the dielectric-vacuum boundary....%3--=------.~-a o//“ ❑0,; n,’ ❑!0,/:,, n,,,In,- _.. —-i’0 /=----’[;-’ –+awmFigure 3. Fill curves for the tapered aperture structure.The fill time is 0.56 ps, corre~ponding tostored energy is normalized to its maximumthe accelerating and Fabry-Perot modesrelative to the accelerating field amplitude..Q=102. Thevalue, whileare shown3 CONCLUSIONSWe have studied a class of dielectric loaded structures forlaser acceleration which can potentially produce GeV/mgradients. We have given some consideration to questionsof structure breakdown and preservation of beam quality.Resonant planar structures showing good coupling oflaser radiation to the desired accelerating mode have beendemonstrated numerically, with reasonable surfacefield/vacuum field strengths and quality factorscommensurate with the requirements of a practicalaccelerator. These properties imply that the structure is agood candidate for further development as an accelerator,as it can be coupled well (it can be fully impedancematched upon filling, just as a standing wave Iinaccavity). This development will probably proceed at 10.6micron design wavelength. Choice of this wavelength isbased both on availability (e.g. at the UCLA Neptunelaboratory [9]), and on mitigation of the experimentalchallenges one faces on moving orders of magnitudedown in accelerator wavelength.4 REFERENCES[1] C. Clayton et al., Phys. Rev.Lett.7037 (1993)[2] P. Sprangle, E. Esarey, A. Ting and G. Joyce, Appl. HIYS. L-w S3,2146 (1988).~3]W.D.-&mr& et aL, Phys. Rev. LAS.. 74, (1995).[4] Y. Liu, et al, Phys. Rev. Lew. 80,4418 (1998)[51 J. Rosenzweis?, A. Mumkh C. Pellemini, Phys. Rev. Utf. 742467;1995).~~$~Tremaine, J. Rosenzweig, P. Schoessow, Phys Rev. E 567204\- ...,.[71J. Rmenzweig.P. Schoessow.~~, (in press)[8] D. Du et al., AppL Phys. Lets. 643073 (1994)[9] C. Clayton, et al., ., JVUCLInstr. Meth. A 410, 235 (1998), J.B.Rosenzweig, et al., NUCLJnstr. Meth. A 410,437 (1998).“Work supported by US DoE Contracts W-31- 109-ENG-38, DE-FG03-92ER40693 and DE-FG03-98ER45693.‘ Email: ~osenzwei9@DhYs cs,ucla.edi u

SLAB symmetric dielectric micron scale structures for high gradient electron acceleration.

CERN Document Server

SLAB SYMMETRIC DIELECTRIC MICRON SCALE STRUCTURES FORHIGH GRADIENT ELECTRON ACCELERATION*            P. V. Schoessow,Argonne National Laboratory, Argonne, IL 60439J. B. Rosenzweig,UCLA Department of Physics and Astronomy,405 Hilgard Ave., Los Angeles, CA 90095AbstractA class of planar microstructures is proposed whichprovide high accelerating gradients when excited by aninfrared laser pulse. These structures consist of paralleldielectric slabs separated by a vacuum gap; the dielectricor the outer surface coating are spatially modulated at thelaser wavelength along the beam direction so as tosupport a standing wave accelerating field. We havedeveloped numerical and analytic models of theaccelerating mode fields in the structure. We show anoptimized coupling scheme such that this mode is excitedresonantly with a large quality factor. The status ofplanned experiments on fabricating and measuring theseplanar structures will be described.1  INTRODUCTIONAdvances in the technology of lasers have led toincreased interest in their potential applications foraccelerating particles. Plasma beat wave[1] and laserwakefield[2] acceleration methods use laser radiation todrive a plasma wave with luminal phase velocity.  Thelongitudinal electric fields in this plasma wave are in turnused to accelerate an electron beam. InverseCherenkov[3] and inverse free electron laser [4]acceleration techniques use light optics or magnetic fieldsrespectively to achieve an electric field componentparallel to the beam direction from the transverse electricfield in the laser pulse. We have proposed a class ofresonant dielectric loaded planar structures[5] capable ofproducing GeV/m accelerating gradients which are drivenby laser radiation much as a conventional rf cavity isdriven by microwave power from a klystron.     The basic idea is the use of a dielectric microstructure,analogous to a Fabry-Perot resonator, consisting of a twoparallel dielectric planes separated by a vacuum gap andwith a partially transmissive coating on the exterior (seeFigure 1).  Some parameters of the structure are assumedto vary periodically in the beam direction at thewavelength of the illuminating laser. A standing wavewith an appropriate phase velocity forward component isinduced in this periodic structure by the laser pulse, whichin turn accelerates the beam. The laser pulse is sweptalong the surface of the structure such that the cavity isfilled only in the neighborhood of the beam. Therelatively low Q  of these devices (100-1000) andcorrespondingly short fill times (~0.5 ps) allows gradientsof 1 GeV/m to be obtained before breakdown becomesproblematic[9]. The beam aspect ratio is highly asym-metric; the ribbon beam-planar structure configuration isadvantageous in that dipole deflecting modes are sup-pressed[6], analogous to the vanishing of the transversedeflection for TM0n modes in conventional structures.abyxzBeam propagation and laser polarization direction in zMetal Dielectricy=0Structure invariant in xMirror transmissionmodulated in zLaser propagation directionFigure 1. Geometry of slab-symmetric, laser-excited,dielectric-loaded resonant accelerator structure.     A number of different methods have been investigatedto introduce the necessary periodic variation in thestructure geometry. The original paper in this area[5]suggested the use of a longitudinally modulatedpermittivity in the dielectric. While this was attractivefrom the point of view of construction and analysis (onecould concievably generate the modulation by “writing”an interference pattern in a photorefractive dielectric witha second laser) it proved to be difficult to obtain goodcoupling of the laser energy to the accelerating field.More recent analyses have concentrated on using aspatially unvarying dielectric medium and modulating thetransmittivity of the coating [7].  This approach was foundto improve the laser coupling to the structure significantlybut was found to yield uncomfortably large surface fieldson the structure.     We discuss here some recent progress in theunderstanding of slab structure design. We haveinvestigated the use of finite thickness conductivecladding on the structure, with a single coupling slot perperiod. With this geometry good coupling of laser energyto the accelerating fields can be obtained, while at thesame time reducing the surface field/accelerating fieldratio significantly over our previous results. The shape ofthe coupling slot is also seen to be important, in that the0-7803-5573-3/99/$10.00@1999 IEEE. 3624Proceedings of the 1999 Particle Accelerator Conference, New York, 1999introduction of a taper at the slot opening providesimproved coupling over the case of a rectangular slotprofile.2  NUMERICAL RESULTSIn the study of these structures we have found it useful torely on numerical solution of the Maxwell equations inslab geometry. We use a custom finite difference timedomain code to inject and propagate the laser fields intothe structure geomerty under study. The programimplements periodic boundary conditions in z (the beampropagation direction) and absorbing boundary conditionsin y (the laser propagation direction) to handle anyreflected laser energy from the structure. The structure isassumed to be of infinite extent in the x-view. For a givenstructure geometry, the integration is continued until asteady state condition is reached. The structure is “tuned”numerically by adjusting one of its parameters (typicallythe dielectric constant) until the asymptotic stored energyis maximized.  (In the laboratory one would tune thestructure by changing a geometrical parameter like thevacuum gap size.)     We have analyzed a structure with  dielectric thickness(b-a) equal to the vacuum gap (a =1.6 m m),  period of10.6 m m (corresponding to a commone CO2 laser line),and dielectric constant ε ≅ 3 7. . The conductive claddingthickness in the simulation is 0.3 m m. The laser couplingand field strengths in the structure were studied as afunction of the shape of the coupling slot. While notexhaustive, these calculations indicate a promisingapproach to the problem of coupling optimization. Thefollowing results are normalized to a peak laser electricfield of 0.25 Statvolt/cm = 75 kV/m.Figure 2. Pseudocolor contour map of Ez in the comput-ational area for the tapered aperture structure. Laser radia-tion is incident from the top of the figure (y=0). The areadepicted in solid black is the conductive cladding, and thethin black line indicates the dielectric-vacuum boundary.In reference[7], the fields of a similar structure having aninfinitesimally thick, sinusoidally modulated transmit-tivity outer cladding were computed.  While reasonablecoupling was achieved, it was also pointed out that thesurface electric field on the mirror-dielectric interface wasundesirably large, roughly twice the maximum field in thevacuum gap. The case of a finite thickness mirror with arectangular aperture was studied for this paper. For a 2m m slot width the corresponding peak surface field is 2.0Statvolt/cm for a gap field amplitude of 1.06 Statvolt/cm.The largest surface fields in this device occurred at theexterior corners of the slot; this might be further improvedby rounding the corners. (The present version of thesimulation code cannot handle curved boundaries exceptvia a stairstep approximation.) An increase in thecoupling slot width to 3 m m produced slightly worseresults with a surface field/gap field ratio of 1.9/0.87.     The effect of tapering the aperture of the coupling slotis shown in Figure 2. The surface field/gap field ratio isrelatively small, 1.56/1.23, while the gap field strength isalso a maximum for all similar device geometries.  Storedenergy and gap field strengths vs time are shown inFigure 3. From the energy history we can read off a filltime of 0.57 ps, corresponding to Q=102.  As we havediscussed previously[7], the fields in the vacuum gap area superposition of Fabry-Perot-like (zero phase shift perperiod, providing no net acceleration) and accelerating(forward wave componet resonant with an ultrarelativisticparticle) modes. The Fabry-Perot mode amplitude hasbeen  diminished to 20% of the accelerating modeamplitude in the present case, solely through through theperiodicity of the coupling. This proportion seems to beabout the most favorable we have been able to obtain.Figure 3. Fill curves for the tapered aperture structure.The fill time is 0.56 ps, corresponding to Q=102. Thestored energy is normalized to its maximum value, whilethe accelerating and Fabry-Perot modes are shownrelative to the accelerating field amplitude.3625Proceedings of the 1999 Particle Accelerator Conference, New York, 19993 CONCLUSIONSWe have studied a class of dielectric loaded structures forlaser acceleration which can potentially produce GeV/mgradients. We have given some consideration to questionsof structure breakdown and preservation of beam quality.Resonant planar structures showing good coupling oflaser radiation to the desired accelerating mode have beendemonstrated numerically, with reasonable surfacefield/vacuum field strengths and quality factorscommensurate with the requirements of a practicalaccelerator. These properties imply that the structure is agood candidate for further development as an accelerator,as it can be coupled well (it can be fully impedancematched upon filling, just as a standing wave linaccavity).   This development will probably proceed at 10.6micron design wavelength. Choice of this wavelengthe isbased both on availability (e.g. at the UCLA Neptunelaboratory[9]), and on mitigation of the experimentalchallenges one faces on moving orders of magnitudedown in accelerator wavelength.4 REFERENCES[1] C. Clayton et al., Phys. Rev. Lett. 70 37 (1993)[2] P. Sprangle, E. Esarey, A. Ting and G. Joyce, Appl. Phys. Lett.  53,2146 (1988).[3] W.D. Kimura, et al.,  Phys. Rev. Lett.. 74,   (1995).[4] Y. Liu, et al., Phys. Rev. Lett. 80, 4418 (1998)[5] J. Rosenzweig, A. Murokh, C. Pellegrini, Phys. Rev. Lett. 74 2467(1995).[6] A. Tremaine, J. Rosenzweig, P. Schoessow, Phys. Rev. E 56 7204(1997).[7] J. Rosenzweig, P. Schoessow,   Proc. 1998 Advanced AcceleratorConcepts   , (in press)[8] D. Du et al., Appl. Phys. Lett. 64 3073 (1994)[9] C. Clayton, et al., ., Nucl. Instr. Meth. A 410, 235 (1998), J.B.Rosenzweig, et al., Nucl. Instr. Meth. A  410, 437 (1998)._____________________*Work supported by US DoE Contracts W-31-109-ENG-38, DE-FG03-92ER40693  and DE-FG03-98ER45693.# Email:   rosenzweig@physics.ucla.edu  3626Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Slab symmetric dielectric micron scale structures for high gradient electron acceleration

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Slab symmetric dielectric micron scale structures for high gradient electron acceleration

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