The 13th Symposium on Accelerator Science and Technology, Suita, Osakn, Japan, October 2001

HIGH ENERGY ELECTRON ACCELERATIONBY HIGH PEAK POWER LASERABHideyuki Kotaki, AMasaki Kando, AShuji Kondo/Shinichi Masuda, AShuhei Kanazawa,
ATakashi Yokoyama, ATom Matoba, ABcKazuhisa Nakajima
AJapanAtomic Energy Research Institute, Kizu, Kyoto, 619-0215, Japan
8 The Graduate University for Advanced
Studies, Hayama, Kanagawa, 240-0193, JapancHigh Energy Accelerator Research Organization, Tsukuba,Ibaraki, 305-0801, JapanAbstract

2 DESIGN OFLWFA

Laser wakefield acceleration (LWFA) experimentusing lOOTW laser is planed at JAERI-Kansai. Highquality short pulse electron beams and the precise
electron-laser synchronization are necessary to accelerate
the electron beam by the laser. A microtron with aphotocathode rf-guri was developed as a high quality
electron injector. The quantum efficiency(QE) of thephotocathode was measured to be 2xl o-5. Emittance anda pulse width of the electron beams were 61t mm-mradand lOps, respectively. In order to produce a short pulse
electron beam, and to synchronize between the electronbeam and the laser pulse, the inverse free electron laser(IFEL) is planed. A gas jet was set up and a density of thegas was measured by Mach-Zehnder interferometer. Thegas densities become uniform at 1.5 mm from the nozzle
at 25 atm.

1 INTRODUCTION
The laser wakefield acceleration (LWFA)[l] has apossibility of a high energy small accelerator. The
gradient of the LWFA is 100 to 1000 times larger thanthat of the usual rf accelerators. In fact, a few hundred
MeV acceleration gain and a gradient of I 0 GeV/m wereobserved[2-5]. However, the maximum energy gain has
been limited at most to I 00 MeV with energy spread of
I 00 % because of dephasing and wavebreaking effects inhighly dense plasmas where thermal plasma electrons areaccelerated. The first high energy gain acceleration
exceeding 200 MeV has been observed with the injectionof an electron beam at an energy matched to thewakefield phase velocity in a fairly underdense
plasma[4,5]. Hence the second-generation research has
dealt with the injection of ultrashort electron bunches into
an appropriate correct acceleration phase of laser
wakefields and the optical guiding of ultraintense
ultrashort laser pulses in underdense plasmas toaccomplish high energy gains and high quality beam
acceleration with a small energy spread. Here theconceptual designs of GeV laser wakefield accelerator are
discussed from the points of view on the ultrashort pulse
and high quality electron beam injection, and the opticalguiding. The recent achievements of the laser acceleration
research at JAERI-APR are reported.

Plasma waves exited by an intense laser beaminteractions with plasma has a higher gradient than usualrf accelerator. The electron beam interacts with the laser
wakefield in the plasma. When a Gaussian driving laser
pulse with the peak power P [TW] and wavelength A0[!lm] is focused on the spot size r 0 [!lm] the maximum
axial wakefield yields
P[ TW ]A. 2 (,um]( eE )= 8.6 X 10 4

z max0,r 2 [,urn ]r [fs ]r0

L0

where 't [fs] is a pulse duration of the laseer pulse, Yo =(I+aa2/2) 112 takes account of nonlinear relativistic effects,and .au = 6.8xf.o P 112/r0 is the laser strength parameter forthe linear polarization[6]. Several effects limit the energy
gain in a single-stage of laser-plasma accelerators; laser
diffraction, electron dephasing, pump depletion and laserplasma instabilities. For a Gaussian beam propagation of
the laser pulse with the peak power P in underdense
plasma, the effective acceleration length can be limited toa diffraction length. For a properly phased electron, themaximum energy gains limited by diffraction effects isgiven by
AWdif [GeV] = 0.85 P [TW] A0 [!lm] I (y0 'tL [fs]).
The electrons will eventually outrun the acceleratingphase and move to the decelerating phase, if the phasevelocity of the plasma is constant and the electron
velocity is accelerated to the speed of light. The
maximum energy gain limited by dephasing effects is

I

0. 1

I; ······j·········"-"""''-~~""'~··~•-• ""'-t"""""'-""'""""~'•'+""''"'~'"""-""""''""~'"'""'~"""'"~'-"'~

I!
i

wltho~t Plasma.jwa.veguld~

i!

!

!

0.01 L---l...--.L-~-l..--.J........--.J40206080
100'ofl"fTI]

Fig.l Energy gain ofLWFA by 40TW, 50fs laser.

-298-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

f:J.Wct [GeV] = 0.01 P [TW] 'tL2 [fs] I r0 2 [!lm].
The laser loses its energy when its pulse excites a plasma
wave. The pump depletion length is that the length of thelaser pulse loses a half of its total energy to excite plasma
waves. The maximum energy gain limited by pumpdepletion isf:J.W ct [GeV] = 0.9lxl0-3 'tL2 [fs] Yo2 I 'Aa2 [!lm].
The e;ergy gain of LWFA by the 40 TW, 50 fs laser isshown in Fig. 1. The energy gain is 1 GeV with the 3 em
plasma waveguide at the laser spot size is 30 11m. The
electron beam line for the LWFA experiment is shown inFig. 2. An electron beam injector and a plasma source forthe LWFA are discussed in section 3 and 4.

Plasma Wavt!guid<!

Fig.2 Electron beam line for LWF A.
Mt~gnetie shulJ

Untb.latoY.(f:;::~3 HIGH QUALITY ELECTRON BEAMINJECTORIt is required that femtosecond electron bunches

should be injected with the energy higher than trapping
threshold and femtosecond synchronization with respect
to a wakefield accelerating phase space which is typicallyless than 100 fs in a longitudinal scale and 10 11m in atransverse size. For the purpose, we have developed anelectron injection system consisting of a photocathode RFgun and a compact race-track microtron.

3.1 Photocathode RF gun
The photocathode RF gun[7 ,8] is prepared as a highqulity electron source. The S-band RF gun has beendeveloped by the collaboration with BNL, KEK and
Sumitomo Heavy Industries, Ltd (SHI). A copper cathodeis illuminated by an UV light of 263 nm with an incident
angle of 68°. This laser can generate the output pulse
energy of 200 111 at 263 nm with fluctuation of 0.5 % andthe pulse width of approximately 6 ps FWHM. We have
performed tests of the photocathode RF gun and thedriving laser. The quantum efficiency of 2x10-5 and theenergy of 3.5 MeV has been observed.

3.2 Microtron
We installed the photocathode RF gun as an electron
source of the race-track microtron (RTM) instead of thethermionic gun. The 3.5 MeV electron beams from thephotocathode RF gun enter the RTM. The electron beamis accelerated 25 times circulation and the energy of theelectron beam becomes 150 MeV. The emittance of themicrtron is 61t mm-mrad at 50 pC/pulse. The emittance isenough to focus as small as 50 11m of the spot size. Thel 0 ps pulse width of the electron beam is measured by astreak camera.

3.3 Bunch slicing and synchronization
We apply the energy modulation technique toproduction of an ultrashort slice of a few 10 femtoseconds~ "'~i!tJ.mlum.p..... .

Fig.3 Setup of the bunch slice.
duration from a electron bunch of a few picoseconds
duration delivered by the RTM. The mechanism ofenergy modulation is based on the inverse free electron
laser (IFEL)[9,10], which generates the efficient energy
exchange between electrons and laser fields in an
undulator when the laser wavelength 'AL satisfies theresonance condition of free electron lasers, given by 'AL='Au (1 +K 2/2)/(2y), where 'Au is the undulator period, Y isthe Lorentz factor, and K = eB 0 A/(21t mec) =0.934'Au[cm]B 0 [T] is the deflection parameter of theundulator with the peak magnetic field of B0 . In addition
to the resonance condition, when the transverse mode andthe spectral bandwidth matching between the laser andthe undulator spontaneous radiation can be satisfied, anamplitude of energy modulation is given by[9](M/ = 4na4.LEL

K 2 12min(M. I ML,1)
1 + K 2 12
where AL is the laser pulse energy, a is the fine structure
constant, EL is the photon energy, Mu is the number ofundulator periods, and ML is the laser pulse length in
optical cycles. We consider the energy modulation for themicrotron beam atE= 150 MeV with an expected energy
spread of 0.1 % using the undulator with 'Au= 3.3 em, Mu= 61.5 and the maximum K value of 1.8. The resonancecondition is satisfied for K = 1.5, provided by adjusting
the gap. Assuming a fe.motosecond laser pulse with 'AL =400 nm, the second harmonics of a 20 fs Ti:sapphire laser
pulse splitted from a high peak power pump pulse, ~epulse energy required to produce an energy modulatiOnf:J.E can be estimated as AL [!11] f:J.E 2 [MeV]. The energy
modulation f:J.E = 15 MeV can be produced by a laser
pulse with AL = 243 111. The setup of the bunch slice isshown in Fig. 3. The laser pulse is separated by a beamsplitter. One is for the IFEL and the other is for the-299-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

L WFA. The electron beam interacts with the laser pulsein the undulator, and the energy of electron beam ismodulated. The modulated electron beams are separated
in the chicane magnet. A high energy part of the electron
beam is guided to the laser acceleration chamber, The
expected pulse width of the modulated electron beam willbe 20 fs, that is the same as that of the laser pulse. In
addition to this, the perfect synchronization between theelectron beam and the laser pulse.

B e<m Spli11er

4. PLASMA SOURCE FOR LWFA

Fig.4 Schematic ofMach-Zehnder interferometer.

The gas is needed for LWFA as a plasma source invacuum. We chose two type of gas source. One is a gas
jet discussed in this section, and the other is a plasma
waveguide for optical guiding[l1,12] discussed in anotherproceeding of this meeting by Dr. S. Masuda.

L6--r~:;;-___,.,::::;:;:~--:;:::::::::::::::;:---:;::::::::=----;ool

1.4

e 1.2
.§

>. 1.04.1 Gas Jet

0.8

The gas jet was set up in a vacuum chamber to test it.
The orifice of the gas jet nozzle is 0.8 mm in diameter.
The gas density was measured by a interferometer with632.8nm He-Ne laser as shown in Fig. 4. The gas wasNitrogen, the backing pressure of the gas jet is 25 atm andthe pulse duration of the gas was 2ms. The nozzle of thegas jet and a fringe pattern were imaged onto a chargedcoupled-device (CCD) camera with an image intensifier
whose gate time is 5!ls. A phase shift of the interferogramisAI/)=J(N2 -N1 )w dL,cwhere A<l> is the phase shift in rad, N 1 is the refractive
index in vacuum, N 2 is the refractive index in gas, ro is thefrequency of the light and L is the interaction length. If
the refractive index in gas is ng at 1 atm and 273 K, thephase shift isA</J(2n = ng -1)

273 L ·10-3.

P.r. 1t .w-9 'where p is the pressure in atm, T is the temperature in K,
L is the interaction length in mm and A is the wavelength
in nm. The gas density is calculated from the Abelinversion of the fringe shift assuming the axisymmetry.
The result at the backing pressure of 25 atm is shown inFig. 5. It is observed that an uniform density regime isgenerated at 1.5 mm from the nozzle.

5 CONCLUSION
We have developed the compact microtron with thephotocathode RF gun as a high quality electron beam
injector for the laser wakefield acceleration experiments.
The emittance and the pulse duration of the 150 MeV
electron baems were measured to be 61t mm-mrad and 10ps, respectively. We will construct the femtosecond bunchslicing stage as a part of the laser acceleration test facility
to generate a femtosecond electron pulse and to trap the0.6 -'r-----;-----,r-----;----;-------r----1-0.6-0.4-0.2

0.00.20.4

0.6x [mm]Fig.5 Contour plot of the gas density.
The nozzle is located at x=O, y=O.
electron baem in the wakefield accelerating phase. Thegas jet is set up in a vacuum chamber to test it. We havesucceeded to measure the density of the gas jet. The laser
wakefield acceleration experiments will be demonstrated
to achieve the high energy gains more than 1 GeV as wellas high quality beam acceleration.

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