The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Laser Wakefield Acceleration Experiments
Masaki KANDO***, Hideyuki KOTAKI, Hideki"DEWA, Hyeyoung AHN,Fumio SAKAI, Shuji KONDOH, Toru UEDA*, Takahiro WATANABE*,
Mitsuru UESAKA *, Hiroshi NAKANISHI**, Kazuhisa NAKAJIMA**, and Atsushi OGATA**Advanced Photon Research Center, Kansai Research Establishment,
Japan Atomic Energy Research Institute, Tokai, JAPAN*Nuclear Engineering Research Laboratory, The University of Tokyo, Tokai, JAPAN**High Energy Accelerator Research Organization (KEK), Tsukuba, JAPAN***Institute for Chemical Research, Kyoto University, Uji, JAPAN

Abstract
The laser wakefield electron acceleration up to 300 MeV
has been observed in an underdense plasma driven by a 2 TW,
90 fs laser pulse synchronized with a 17 MeV RF linac at
10 Hz. The electron acceleration was enhanced at a density
higher than the resonant density due to gas ionization and selfchanneling effects. The wakefield excitation has been confirmed by measuring the electron density oscillation of theplasma wave with the frequency domain interferometer.

1 Introduction

Lac . . . , 7f Ze. where Ze is the effective Rayleigh length. For
the resonant plasma density, ne = 111freu; where re is theclassical electron radius,
LlWmax[MeV]......, 850P[TW]Ao[Jlml/ro[fs] x (ZeiZR),
where r0 = 2Vfii2uz I c is the FWHM pulse duration. Notethat the maximum energy gain is independent of a focal spot
size of laser pulses for Gaussian propagation if Ze = ZR.
Optical guiding has been proposed[5] in order to exceed thislimit. Our recent experiments using intense femtosecop.d
laser pulses report that a self-channeling has been observed
under the critical power of the relativistic self-focusing[6].

We report here demonstration of laser wakefield accel3 Experimental Setuperation (LWA)[1] of an externally injected electron beam by
a 2 TW 90 fs laser pulses in a plasma created in a as-filledchamber. A large energy gain (.-v 300 MeV) was a 'ned,
We have constructed the LWA test facility consisting ofpartly owing to self-channeling of a laser in a plasma d the T3 laser system and the electron beam injector[8]. The
partly owing to larger acceleration gradient than the predt - Ti:sapphire T3 laser system based on the chirped-pulse amplition of a linear flui~ theory. Large blue shift was observed in '~fication at Ao = 790 nm produces output pulses compressed
the laser spectrum m a dense plasma, but no trace of the Ra- by a grating to 90 fs with an energy of > 200 mJ or a peak
man scattering was found. These facts suggest a new mech- power of> 2 TW at the repetition rate of 10Hz. We usedanism of LWA us~n~ ~ laser power ~ell belo~ the critical the 2856 MHz RF linac as an electron injector to produce a
17 MeV single bunch beam with a 10 ps FWHM pulse dupower of the rela!tvtstlc s~lf-channehng[2], giVen by Pc =17(w5/w~) GW wtth wo bemg the laser frequency and wp, the ration containing . . . , 1 nC at the repetition rate of 10Hz. As
plasma frequency. A frequency domain interferometer certified the density oscillation in the laser wakefield[3].
Laser Pulse
11 MeV Electron Beam
2 TW, 90 fs

PMQ Doublet

2 Laser Wakefield Acceleration
The LWA has great potential to produce ultra-high-field
gradients of plasma waves excited by intense ultrashort laser
pulses[l, 4]. The wakefield results from the ponderomotiveforce exciting the plasma wave with the frequency Wp =J41fe 2 nelme in a plasma with an ambient electron density ne. The maximum amplitude of an axial wakefield isachieved at the plasma wavelength >.P = 1fUz, where Uz isa temporal1le half-width of the pulse. Assuming Gaussian
beam propagation of the laser pulse at the peak power P in
an underdense plasma (w 0 ~ wp), a peak amplitude of theaccelerating wakefield iseEz = {Ei__!_(Ao) (kpuz) exp(- k~u;),~ mec2 Ap2ZR4V

where >. 0 is the laser wavelength, kv = 27fI Ap and Z R =>.0 is the vacuum Rayleigh length, Ro is the spot radius at the focus. The maximum energy gain is given byLlW = eEzLac with the acceleration length Lac·In a homogeneous plasma, however, diffraction of thelaser beam limits the laser-plasma interaction distance to1rRJI50 ema MagnetFig. 1 Experimental setup around the interaction chamber.
shown in Fig. 1, focusing optics and the injection electron
beamline were installed in the vacuum chamber filled with Hegas. Laser pulses were focused with f/10 off~axis parabolic
mirror with a focal length of 480 mm. The measured focal
spot radius was l3Jlm. An electron beam from the injector isbrought to a focus in the chamber with the FWHM beam sizeof 0.8 mm through a beamline consisting of a triple focusing magnet and a permanent quadrupole triplet. The RF linac
and the beamline are separated with a 50Jlm thick titanium
window from the interaction chamber to maintain ultrahigh-513-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

vacuum in the electron injector. Since the multiple scattering of electrons at thi~ window causes emittance blow-up, acollimator slit is installed at the downstream of the window toreduce the beam emittance. The beam collimation reduced anelectron charge to"" 100 pC per pulse. Only the electrons injected to the diameter less than a half laser spot size would betrapped and accelerated by the wakefield. An electron pulse
was synchronized to laser pulses with the phase locked control of the mode-locked oscillator. The phase locked loop
maintains synchronization of the oscillator repetition period(79.33 MHz) with every 36th RF period of the linac (2856
MHz). A streak camera with a time resolution of 200 fs measured a timing jitter between the laser pulse and Cherenkov
radiation from the electron bearp. Synchronization between
the two pulses was achieved within the rms jitter of 3.7 ps.
The energy of accelerated electrons was measured withthe magnetic spectrometer consisting of a dipole magnet andan array of 32 scintillation detectors. Injected electrons undergoing no acceleration were swept out of the detectors by thespectrometer magnet following a permanent quadrupole doublet. The spectrometer covered the energy range of 10"" 300
MeV. The energy calibration of the spectrometer was madeby varying the magnetic field to measure a 17 MeV electron
beam from the RF linac.
The timing between laser and electron pulses was adjusted by changing a phase delay of the reference RF to thephase locked loop so that streak images of two pulses wereoverlapped. After a He gas was filled in the accelerationchamber, fine adjustment of overlapping two spots of laserand electron beams was carried out within 50 J.Lm. Two setsof pulse height data of the scintillator array were taken withpump laser pulses and without them as a background. A net
pulse height proportional to the number of electrons accelerated was obtained from subtracting the data without the pumppulses from the data with them. The number of electrons wasestimated to be ranged from 2 to 4 per ADC count for all detectors.

Fig. 2 Energy gain spectra of accelerated electrons for a)
3.4 Torr, P=0.9 TW, b) 20 Torr, P=0.9 TW, c) 2 Torr,
P=l.8 TW, and d) 20 Torr, P=l.8 TW.
dient was obtained from the measured maximum energy gain
and the FWHM length of the scattered light lineout equal to2Z J. Figure 4 shows the peak accelerating gradient for the4 Experimental Results
In the acceleration e'xperiment the gas pressure of He wasscanned from 1 Torr to 300 Torr. Figure 2 shows the pressure dependence of energy gain spectra of electrons accelerated by the laser peak power of 1.8 TW. The maximum energy gain up to 300 MeV was obtained. When the electron
pulse preceded the pump laser pulse, no acceleration of electrons was observed. Accelerated electrons visibly appearedas the electron pulse was delayed. No accelerated electron
was observed at a delay of 1 ns. We have tried to observe
the self-trapping of plasma eleCtrons due to wakefield, without the electron beam injection, increasing the gas pressure
up to 760 Torr. No electrons with energies higher than ,...., 1
MeV were however observed. It implies that the ultrashortlaser pulses of 100 fs do not excite the stimulated Raman instability and resultant large amplitude wakefields. The side
scattered laser light from the plasma region were imaged ontoa CCD camera through a 10 nm FWHM interferential filter tomea~ure the laser intensity distribution. Figure 3 shows anintensity profile of the scattered light projected onto the propagation axis at various gas pressures. The lineout width of thescattered light proportional to the laser intensity gives a good
estimate of the acceleration length. The acceleration field gra--20-100102030Propagation Axis (mm)Fig. 3 The example of the CCD image of the scattered
light (Upper). The lineouts of the side scatteredlaser light at various He gas pressures for P=2.5 TW(Lower). The dashed line indicates the position ofvacuum best focus.
laser peak powers of 0.9 TW and 1.8 TW as a function of thegas pressure of He. The data indicate a good agreement withtheoretical prediction based on the linear fluid model below
10 Torr, assuming the focal spot radius of 13 J.Lm. However,they show that the acceleration occurs even at much higherpressures than a plasma wake resonance. Recent particle-incell simulations elucidate that a large amplitude wakefield canbe excited by self-modulation at the front of a laser pulse in
rapidly ionizing plasmas[2]. In Fig. 4 the acceleration gradient jumps to"" 15 GeV/m above 20 Torr for 1.8 TW. Above-514-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

this pressure of 20 Torr, it was also observed that the length ofthe lineouted laser light turned out constant, ....., 1 em. Thesephenomena suggest that self-channeling accompanied by theelectron density depletion may take place above 20 Torr.

P=l.STW IlI'}t\I~)>-0.40 L......_~.._l_~~.L......~...--L~~..L....~.......J.~...........J
1.4
1.3
1.2
1.1
1.0
0.9
0.8

Time (ps)Pressure (Torr)

Fig. 4 The peak accelerating field gradient deduced fromthe maximum energy gain and the acceleration
length. The solid curve shows theoretical expectation for the spot radius Ro = l3Jlm.
The onset of self-channeling is also inferred from thespectra of the forward scattered laser light shown in Fig. 5.
At the low pressure of 1 Torr, only a small blue-shift wasobserved as the laser intensity increases. When the pressure
turned out to be as high as 20 Torr, however, a drastic blueshifting of the laser light was induced above 0.5 TW. As thepeak power increased to more than 2 TW, a whole spectrum of
the incident laser pulse at 790 nm was shifted to 750 nm. It has
been known that a large amount of the self-phase modulation
results from a long distant self-channeling[?]. In our experiment we have not observed red-shifting nor forward Ramanscattering.

Fig. 6 Measurement of the plasma density oscillation excited by a 1 TW pump power in a He gas of 2 Torr.
The solid curve shows a fit of the plasma wave withoscillation period of 360 fs.
while the plasma frequency is 2.1 x 1013 radls assuming afully ionized plasma at 2 Torr. A frequency decrease of theplasma wave results from nonlinear wakefield excitation. The
density perturbation was 6n/ne ....., 10% corresponding to thelongitudinal wakefield of ....., 3 GeV /m. This measured amplitude is in good agreement with the accelerating wakefield
theoretically expected.

5 Conclusion
We have carried out electron acceleration by wakefields
excited by intense ultrashort laser pulses delivered from a 2TW, 90 fs T 3 laser system synchronized with the 17 MeV RFlinac electron beam injector at the repetition rate of 10Hz.
We have observed high energy electrons accelerated over 100
MeV up to 300 MeV by the wakefield of....., 15 GeV/m excited over a few em long underdense plasma. Acceleration
enhancement at higher pressures of He gas than the resonantpressure may be elucidated from the pulse self-modulation
due to ionization and self-channeling of the laser pulse in aplasma. A direct measurement of the electron density oscillation by the frequency domain interferometry verified wakefield excitation consistent with the results of acceleration experiments.

References
50mJ
40mJ
740

760

780

800

Wavelength (nm)Fig. 5 Forward scattered spectra measured in a He gas ofa) 1 Torr and b) 22 Torr.
In order to make confirmation of wakefield excitation, wemeasured the plasma oscillation with the frequency-domaininterferometer[3]. Figure 6 shows the electron plasma wave
measured at 2 Torr at the pump peak power of 1 TW. The frequency of the density oscillation is deduced to be 1. 7 x 1013
rad/s from the measured period of the density oscillation,[1] T. Tajima and J. M. Dawson, Phys. Rev. Lett., 43, 267(1979); P. Sprangle et al., Phys. Rev. Lett. 53, 2146
(1988).
[2] S. Kato, Y. Kishimoto, and J. K. Koga, to be publishedin Proc. Int. Conf. on Plasma Physics (ICPP 96), Nagoya(1996).
[3] J. R. Marques et al., Phys. Rev. Lett., 76, 3566 (1996);
C. W. Siders et al., Phys. Rev. Lett., 76, 3570 (1996).
[4] K. Nakajima et al., Phys. Rev. Lett., 74, 4428 (1995);
A. Modena et al., Nature 377, 606 (1995).
[5] E. Esarey et al., Phys. Fluids B, 5, 2690 (1993).
[6] M. Kando et al., to be published in AlP Conf. Proc ..
[7] A. Braun et al., Opt. Lett., 20, 73 (1995).
[8] K. Nakajima, Phys. Plasmas, 3, 2169 (1996).

-515-