High-quality electron beams with up to 1 GeV energy have been generated by a  laser-driven plasma-based accelerator by guiding a 40 TW peak power laser pulse in a 3.3 cm long  gas-filled capillary discharge waveguide

Esarey, E.

Geddes, C.G.R.

Gonsalves, A.J.

Hooker, S.M.

Leemans, W.P.

Nagler, B.

Nakamura, K.

Schroeder, C.B.

Toth, Cs.

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Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleGeV electron beams from a laser-plasma acceleratorPermalinkhttps://escholarship.org/uc/item/8bz4j1drAuthorsSchroeder, C.B.Toth, Cs.Nagler, B.et al.Publication Date2008-05-20eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaGeV electron beams from a laser-plasma acceleratorC. B. Schroeder∗, Cs. Tóth∗, B. Nagler∗, A. J. Gonsalves†, K. Nakamura∗,C. G. R. Geddes∗, E. Esarey∗, S. M. Hooker† and W. P. Leemans∗∗Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA†Department of Physics, University of Oxford, Clarendon Laboratory, Oxford, UKAbstract— High-quality electron beams with up to 1 GeVenergy have been generated by a laser-driven plasma-basedaccelerator by guiding a 40 TW peak power laser pulse in a3.3 cm long gas-filled capillary discharge waveguide [1].I. INTRODUCTIONGeV electron accelerators are used in synchrotron radiationfacilities and free electron lasers, and as modules for high-energy particle physics. Conventional radio-frequency (RF)accelerators are limited to relatively low accelerating fields(10–50 MV/m) due to material breakdown, requiring tens tohundreds of meters to reach the multi-GeV beam energiesneeded to drive light sources, and many kilometers to generateparticle energies of interest to high-energy physics.Plasma waves can produce accelerating fields on the orderof E ∼ mecωp/e, where ωp = (4πnee2/me)1/2 is theplasma frequency with ne the electron number density. Forplasma densities 1018–1019 cm−3 the fields can be of theorder ∼100 GV/m, several orders of magnitude greater thanconventional RF accelerators. Such plasma waves can be pon-deromotively driven by relativistically intense (>1018 W/cm2),short pulse (tens of fs, such that the pulse duration ∼ ω−1p )lasers, enabling extremely compact accelerators [2], [3].One challenge for laser-driven plasma-based accelerators ismaintaining the laser intensity that drives the plasma wave.Without guiding (e.g., via self-focusing or using preformedplasma density channels) the laser-plasma interaction length islimited to the order of the Rayleigh range, ZR, (e.g., a few mmfor rs = 25 µm, where rs is the laser spot size). Relativisticself-guiding [3], [4] can extend the propagation distance ofhigh-power pulses due to self consistent modification of theplasma refractive index, but is limited by nonlinear effectssuch as the erosion of the leading edge of the laser pulse.In past experiments, the required laser intensity was notmaintained over the distance needed to reach GeV energies,and hence acceleration was limited to the 100 MeV scale [5]–[7]. In these previous experiments [5]–[7], an ultra-intense∼1019 W/cm2 laser pulse focused on a gas jet, with typicallength of a few millimeters. Obtaining GeV energies withouta plasma channel to guide therefore requires large laser spotsizes that increases ZR, but also increases the required laserpower to PW levels.A more efficient approach relies on using a plasma channel(a positive plasma density gradient away from the axis ofpropagation) to guide laser beams with smaller spot sizesover cm-scale distances. Theory and simulation indicate thatsuch channel guided accelerators could produce GeV electronbeams with 10–50 TW of laser power [3], [4], [6].Demonstration of high-quality electron beams with up to1 GeV energy, generated by a laser-driven plasma-basedaccelerator via guiding a 40 TW peak power laser pulse in a3.3 cm long gas-filled capillary discharge waveguide (plasmachannel), has recently been achieved at LBNL [1].II. CHANNEL-GUIDED LASER WAKEFIELD ACCELERATORLaser-plasma-based accelerators have previously demon-strated generation of low energy spread, ∼100 MeV electronbeams [5]–[7] using an ultra-intense laser pulse focused on agas jet with typical length of a few millimeters. The limitationsof gas jet experiments can be overcome by employing agas-filled capillary discharge waveguide [8], [9] to guiderelativistically intense laser pulses in cm-scale, lower densityplasma channels.The experiments preformed at LBNL used a 10 Hz repeti-tion rate Ti:sapphire laser system (λ = 810 nm) delivering asshort as 38 fs (FWHM) pulses with up to 40 TW peak power.These pulses were focused by a 2 m focal length off-axisparabola (f/25) to rs = 25 µm at the capillary entrance. Thecapillaries [8] were laser machined into 33 mm long sapphireblocks with diameters ranging from 190 µm to 310 µm.Hydrogen gas, introduced through holes near the capillaryends, was ionized by striking a discharge between electrodes atthe capillary ends. Measurements [8] and modelling [10], [11]showed that a fully-ionized, approximately parabolic (withdensity minimum on axis) plasma channel is formed.Intense laser pulses guided in the plasma channel exciteplasma waves (wakefields) with phase velocities approxi-mately equal to the group velocity of the drive laser pulse.These plasma waves can trap background plasma electronsand accelerate them to relativistic energies. A fundamen-tal limitation to the energy gain is due to phase slippagebetween the trapped electrons and the plasma wave. Thelinear dephasing length, Ld = λ3p/λ2 ∝ n−3/2e , over whichelectrons outrun the wake and slip into a decelerating phase,limits the distance over which acceleration occurs. Here λpis the plasma wavelength and λ the laser wavelength. Forlaser intensities I ≤ 1018 W/cm2, a rough estimate of theelectron energy gain over a distance Ld in a channel-guidedlaser wakefield accelerator [3], [4] can be obtained fromW [GeV] ∼ 0.4I[W/cm2]/ne[cm−3] ∼ 0.9(kprs)−2P [TW];the energy gain in the laser-plasma-based accelerator scalesapproximately linearly with laser power P .Fig. 1. Measured single-shot electron beam spectra from a capillary-guided laser-plasma accelerator [1]. (a) 0.50 GeV electron beam produced using 12 TWinput laser power and plasma density of 3.5 × 1018 cm−3. (b) 1.0 GeV electron beam produced using 40 TW input laser power and plasma density of4.3× 1018 cm−3. The horizontal axis is the beam energy and the vertical axis is the beam size (in the plane undeflected by the magnetic spectrometer). Thecolor scale denotes the bunch charge in pC/GeV/sr.In recent laser-plasma accelerator experiments at LBNLusing plasma-channel-guiding in a cm-scale capillary, highquality electron beams with GeV energies were generated [1].Figure 1 shows single-shot electron beam spectra measured bya 1.2 T magnetic spectrometer that deflected the electrons ontoa phosphor screen. Figure 1(a) shows an electron beam withenergy of 0.50+0.02−0.015 GeV (5% rms projected energy spread,2.0 mrad rms divergence, and 50 pC of charge) generated ina 3.3 cm capillary (225 µm diameter) with a plasma densityof 3.5 × 1018 cm−3 and input laser power of 12 TW (72 fsduration). Figure 1(b) shows an electron beam with energy1.0+0.08−0.05 GeV (2.5% rms energy spread, 1.6 mrad divergencerms, and 30 pC of charge) generated in a 3.3 cm capillary(310 µm diameter) with plasma density of 4.3 × 1018 cm−3and input laser power of 40 TW (38 fs duration). In Fig. 1(b)a second beam at 0.8 GeV is also generated. Experimentalresults using 2 J of laser energy have produced 1 nC electronbeams at 0.5 GeV.III. CONCLUSIONThese laser-plasma accelerator experimental results [1] openthe possibility of a new class of compact accelerators. Sucha laser-plasma accelerator could be used to drive a high-peakflux, x-ray free-electron laser in which the conventional RFaccelerator (10–100m length) is replaced by a GeV-class laser-plasma accelerator (several cm length), greatly reducing thesize and cost of such light sources [12]. The electron bunchesemerging from a laser-plasma accelerator have naturally shortdurations on the order of the plasma period (tens of fs) [13],and are intrinsically synchronized to the short-pulse laserdriver, making such a source ideal for ultra-fast pump-probeapplications. Future staging of GeV laser-plasma acceleratormodules may open the possibility of achieving energies ofinterest to high-energy physics experiments.ACKNOWLEDGMENTThis work was supported by the U.S. Department of Energyunder Contract No. DE-AC02-05CH11231 and the Engineer-ing and Physical Sciences Research Council, UK.REFERENCES[1] W. P. Leemans, B. Nagler, A. J. Gonsalves, C. Tóth, K. Nakamura,C. G. R. Geddes, E. Esarey, C. B. Schroeder, and S. M. Hooker, “Gevelectron beams from a centimetre-scale accelerator,” Nature Phys., vol. 2,pp. 696–699, October 2006.[2] T. Tajima and J. M. Dawson, “Laser electron accelerator,” Phys. Rev.Lett., vol. 43, no. 4, pp. 267–270, 1979.[3] E. Esarey, P. Sprangle, J. Krall, and A. Ting, “Overview of plasma-based accelerator concepts,” IEEE Trans. Plasma Sci., vol. 24, no. 2,pp. 252–288, 1996.[4] W. P. Leemans, C. W. Siders, E. Esarey, N. E. Andreev, G. Shvets,and W. B. Mori, “Plasma guiding and wakefield generation for second-generation experiments,” IEEE Trans. Plasma Sci., vol. 24, pp. 331–342,1996.[5] S. P. D. Mangles, C. D. Murphy, Z. Najmudin, A. G. R. Thomas,J. L. Collier, A. E. Dangor, E. J. Divall, P. S. Foster, J. G. Gallacher,C. J. Hooker, D. A. Jaroszynski, A. J. Langley, W. B. Mori, P. A.Norreys, F. S. Tsung, R. Viskup, B. R. Walton, and K. Krushelnick,“Monoenergetic beams of relativistic electrons from intense laser-plasmainteractions,” Nature, vol. 431, no. 7008, pp. 535–538, September 2004.[6] C. G. R. Geddes, C. Toth, J. van Tilborg, E. Esarey, C. B. Schroeder,D. Bruhwiler, C. Nieter, J. Cary, and W. P. Leemans, “High quality elec-tron beams from a plasma channel guided laser wakefield accelerator,”Nature, vol. 431, no. 7008, pp. 538–541, September 2004.[7] J. Faure, Y. Glinec, A. Pukhov, S. Kiselev, S. Gordienko, E. Lefebvre,J.-P. Rousseau, F. Burgy, and V. Malka, “A laser plasma acceleratorproducing monoenergetic electron beams,” Nature, vol. 431, no. 7008,pp. 541–544, September 2004.[8] D. J. Spence and S. M. Hooker, “Investigation of a hydrogen plasmawaveguide,” Phys. Rev. E, vol. 63, no. 1, p. 015401, January 2001.[9] A. Butler, D. J. Spence, and S. M. Hooker, “Guiding of high-intensitylaser pulses with a hydrogen-filled capillary discharge waveguide,” Phys.Rev. Lett., vol. 89, no. 18, p. 185003, October 2002.[10] N. A. Bobrova, A. A. Esaulov, J. Sakai, P. V. Sasorov, D. J. Spence,A. Butler, S. M. Hooker, and S. V. Bulanov, “Simulations of a hydrogen-filled capillary discharge waveguide,” Phys. Rev. E, vol. 65, no. 1, p.016407, January 2002.[11] B. H. P. Broks, K. Garloff, and J. J. A. M. van der Mullen, “Nonlocal-thermal-equilibrium model of a pulsed capillary discharge waveguide,”Phys. Rev. E, vol. 71, no. 1, p. 016401, January 2005.[12] D. A. Jaroszynski, R. Bingham, E. Brunetti, B. Ersfeld, J. Gallacher,B. van der Geer, R. Issac, S. P. Jamison, D. Jones, M. de Loos,A. Lyachev, V. Pavlov, A. Reitsma, Y. Saveliev, G. Vieux, and S. M.Wiggins, “Radiation sources based on laser–plasma interactions,” Philos.Trans. R. Soc. London, Ser. A, vol. 364, pp. 689—710, January 2006.[13] J. van Tilborg, C. B. Schroeder, C. V. Filip, C. Tóth, C. G. R. Geddes,G. Fubiani, R. Huber, R. A. Kaindl, E. Esarey, and W. P. Leemans, “Tem-poral characterization of femtosecond laser-plasma-accelerated electronbunches using terahertz radiation,” Phys. Rev. Lett., vol. 96, p. 014801,January 2006.

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GeV electron beams from a laser-plasma accelerator

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GeV electron beams from a laser-plasma accelerator

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