HIGH ENERGY LASER-PLSMA ACCELERATOR DEVELOPMENTS
AT JAERI-APR
AB

Hideyuki KOTAKI*, AMasaki KANDO, ATomonao HOSOKAI, AShuji KONDO,
A
Shinichi Masuda, AShuhei KANAZAWA, ATakashi YOKOYAMA,A
Toru MATOBA, ABCKazuhisa NAKAJIMAA
Japan Atomic Energy Research Institute, Kizu, Kyoto, 619-0215, JapanB
The Graduate University for Advanced Studies, Hayama, Kanagawa, 240-0193, JapanC
High Energy Accelerator Research Organization, Tsukuba,Ibaraki, 305-0801, JapanAbstract
Laser wakefield acceleration experiment using
100TW laser is planed at JAERI-Kansai. High quality
short pulse electron beams and the precise electron-laser
synchronization are necessary to accelerate the electron
beam by the laser. A microtron with a photocathode RF
gun was developed as a high quality electron injector. The
quantum efficiency(QE) of the photocathode wasmeasured to be 2×10-5. Emittance and a pulse width of theelectron beams were 6π mm-mrad and 10ps, respectively.
In order to produce a short pulse electron beam, and tosynchronize between the electron beam and the laser
pulse, the inverse free electron laser (IFEL) is planed.
One of problems of LWFA is a short acceleration length.
In order to overcome the problem, the Z-pinch plasma
waveguide has been developed for 1 GeV laser wakefieldacceleration.

2 HIGH QUALITY ELECTRON BEAMINJECTOR
It 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 µm in atransverse size. For the purpose, we have developed anelectron injection system consisting of a photocathode RFgun and a compact race-track microtron shown in Fig.1.

1 INTRODUCTION
The laser wakefield acceleration (LWFA)[1] 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 10 GeV/m wereobserved[2-5]. However, only a few of the injected
electrons were accelerated to high energy and themaximum energy gain has been limited at most to 100
MeV with energy spread of 100 % because of dephasing
and wavebreaking effects in highly dense plasmas wherethermal plasma electrons are accelerated. The first highenergy gain acceleration exceeding 200 MeV has been
observed with the injection of an electron beam at anenergy matched to the wakefield 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 oflaser 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.

Fig.1 Microtron with photocathode RF gun

2.1 Photocathode RF gun
The photocathode RF gun[6] is prepared as a highqulity electron source. The S-band RF gun has beendeveloped by the collaboration with BNL, KEK andSumitomo Heavy Industries, Ltd (SHI). This cavity wasbased on the gun developed by the BNL / SLAC / UCLA
collaboration[7] and was improved for high duty
operation at 60 Hz. A copper cathode is illuminated by an
UV light of 263 nm with an incident angle of 68˚,
delivered from a compact all solid-state Nd:YLF lasersystem (SHI PULRISE II). This laser can generate theoutput pulse energy of 200 µJ at 263 nm with fluctuationof 0.5 % and the pulse width of approximately 6 ps
FWHM. We have performed tests of the photocathode RFgun and the driving laser. The quantum efficiency of
2×10−5 and the energy of 3.5 MeV has been observed.

2.2 Microtron
The race-track microtron (RTM) manufactured by SHIis utilized as a booster accelerator. We installed thephotocathode RF gun as an electron source of the RTM
instead of the thermionic gun. The 3.5 MeV electron
beams from the photocathode RF gun enter the RTM. The
electron beam is accelerated 25 times circulation and theresonance condition is satisfied for K=1.5, provided byadjusting the gap. Assuming a femotosecond laser pulsewith λL = 400 nm, the second harmonics of a 20 fsTi:sapphire laser pulse splitted from a high peak powerpump pulse, the pulse energy required to produce anenergy modulation ∆E can be estimated as AL [µJ] ∆E2[MeV]. The energy modulation ∆E = 15 MeV can beproduced by a laser pulse with AL = 243 µJ. The setup ofthe bunch slice is shown in Fig. 3. The laser pulse isseparated by a beam splitter. One is for the IFEL and theother is for the LWFA. The electron beam interacts withthe laser pulse in the undulator, and the energy of electron
beam is modulated. The modulated electron beams areseparated in the chicane magnet. A high energy part of theelectron beam is guided to the laser acceleration chamber.
The expected pulse width of the modulated electron beam
will be 20 fs, that is the same as that of the laser pulse. Inaddition to this, the perfect synchronization between theelectron beam and the laser pulse.

Fig. 2 Emittance of the microtron.
energy of the electron beam becomes 150 MeV. Figure 2
shows an emittance of the accelerated electron beams.
The emittance is 6π mm-mrad at 50 pC/pulse. The
emittance is enough to focus as small as 50 µm of thespot size. The 10 ps pulse width of the electron beam ismeasured by a streak camera.

Fig.3 Setup of the bunch slice.

2.3 Bunch slicing and synchronization
We apply the energy modulation technique to
production of an ultrashort slice of a few 10 femtoseconds
duration from a electron bunch of a few picoseconds
duration delivered by the RTM. The mechanism of energy
modulation is based on the inverse free electron laser(IFEL)[8,9], which generates the efficient energy
exchange between electrons and laser fields in an
undulator when the laser wavelength λL satisfies theresonance condition of free electron lasers, given by λL= λu (1+K2/2)/(2γ2), where λu is the undulator period, γ isthe Lorentz factor, and K = eB0λu/(2π mec) =0.934λu[cm]B0[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[8]2min(Mu / M L ,1)
21+ K /2
where AL is the laser pulse energy, α 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 forthe microtron beam at E=150 MeV with an expectedenergy spread of 0.1 % using the undulator with λu = 3.3cm, Mu =61.5 and the maximum K value of 1.8. The(∆E )2 = 4παAL E L

K /2

3. OPTICAL GUIDING IN PLASMA
The length of the laser acceleration is limited to thediffraction length of the laser. In order to increase thelength, we have developed a stable cm-scale plasmachannel produced by an imploding phase of fast Z-pinch
discharge in a gas-filled capillary without wall ablation. A
high current fast Z-pinch discharge generates strong
azimuthal magnetic field, which contracts the plasma
radially inward down to 100 µm in diameter. The
imploding current sheet drives the converging shock wave
ahead of it, producing a concave electron density profilein the radial direction just before the stagnation phase.
The concave profile is approximately parabolic to out aradius of 50 µm, after which the density falls off. We
succeeded in guiding a 2TW laser over 2cm in the Zpinch plasmas[10]. We have used a capillary with an
inner diameter of 1 mm and a length of up to 2 cm. With
this configuration, we obtained a discharge current of 4.8
kA with a rise time of 15 ns and a duration of 70 ns(FWMH). The capillary was filled with helium. A DC
discharge circuit was used to preionized helium gas. ATi:Sapphire laser pulse λL = 790 nm, 90 fs, was focusedto >1×1017W/cm2 on the front edge of the capillary with aspot size of 40 µm in diameter. The transmitted laser
beam profile at the exit of the capillary was measured
through a band pass filter (∆λ = 1 nm) with a CCD

camera.

4 DESIGN OF GEV LWFA
The electron beam line for the LWFA experiment isshown in Fig. 4. The electron beam from the microtron isdelivered through the IFEL, the chicane magnet and theplasma waveguide. The electron beam interacts with thelaser wakefield in the plasma waveguide. When aGaussian driving laser pulse with the peak power P [TW]and wavelength λ0 [µm] is focused on the spot size r0[µm] the maximum axial wakefield yields
P[TW ]λ 2 [µm](eE ) = 8.6 × 10 r [µm]τ [ fs]γ40z max

20L0

where τ [fs] is a pulse duration of the laseer pulse, γ0 =(1+a02/2)1/2 takes account of nonlinear relativistic effects,
and a0 = 6.8×λ0 P1/2/r0 is the laser strength parameter forthe linear polarization[11]. Several effects limit theenergy gain in a single-stage of laser-plasma accelerators;
laser diffraction, electron dephasing, pump depletion andlaser-plasma instabilities. The maximum energy gains
limited by these effects is given by∆Wdif [GeV] = 0.85 P [TW] λ0 [µm] / (γ0 τL [fs]),∆Wd [GeV] = 0.01 P [TW] τL2 [fs] / r02 [µm],∆Wpd [GeV] = 0.91×10-3 τL2 [fs] γ02 / λ02 [µm],where ∆Wdiff is the limited energy gain by the diffraction

length, ∆Wd is the limited energy gain by the dephasing
length and ∆Wpd is the limited energy gain by the pumpdepletion length. The energy gain of LWFA by the 40 TW,
50 fs laser is shown in Fig. 5. The energy gain is 1 GeV
with the 3 cm plasma waveguide at the laser spot size is30 µm.

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.
Recently commissioning of the injector system has beensuccessfully conducted to produce a 150 MeV. The
emittance and the pulse duration of the electron baems
were measured to be 6π mm-mrad and 10 ps, respectively.
We will construct the femtosecond bunch slicing stage asa part of the laser acceleration test facility to generate a
femtosecond electron pulse and to trap the electron baemin the wakefield accelerating phase. The optical guiding
development has resulted in a cm-scale plasma
waveguide using the fast Z-pinch capillary discharge. We
have succeeded in demonstrating propagation of 2 TW, 90
fs laser pulses over 2 cm in the Z-pinch plasma
waveguide. The laser wakefield acceleration experiments
will be demonstrated to achieve the high energy gains
more than 1 GeV as well as high quality beamacceleration.

Photocathode RF-Gun

REFERENCES

Microtron

Microtron

IFEL Energy Modulator
Laser Pulse

Laser Pulse Compressor

Plasma Waveguide
PlasmaWaveGuideElectron SpectrometerFig.4 Electron beam line for LWFA

Fig.5 Energy gain of LWFA by 40TW, 50fs laser.

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