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

DEVELOPMENT OF PLASMA WAVEGUIDEUSING FAST Z-PINCH CAPILLARY DISCHARGE
AS. Masuda, AM. Kando, AsH. Kotaki, cT. Hosokai, AS. Kondo,
AS. Kanazawa, AT. Yokoyama, AT. Matoba, A80K.Nakajima
AJapanAtomic Energy Research Institute, Kizu, Kyoto, 619-0215, Japan
8 The Graduate University for Advanced Studies, Hayama, Kanagawa, 240-0193, JapancNERL, The University of Tokyo, Tokai, lbaraki, 319-1106, Japan
0 High Energy Accelerator Research Organization, Tsukuba, lbaraki, 305-0801, Japantransmitted laser beam profile at the exit of the capillary
Abstract
A Laser wake field acceleration (LWFA) experimentwith 100 TW laser is planed at JAERI-KANSAI.
Development of the plasma waveguide using a fast Z
pinch capillary discharge is one of the issues of this plan.
A 10 em long plasma wave guide is developed.
Preliminary results of PIC simulations show theacceleration gradient of the order of 1 GeV/cm isachieved.

was measured through a band pass filter (LlA = 1 nm) witha CCD camera.
lD-Model.llilllmllilllmllilllmllilllmiiiJIMililllmllilllmllilllmllilllmllilllm•

Plasma Shee!

1 INTRODUCTION
A short laser pulse propagating in a plasma excites theelectron plasma wave (laser wakefield) which canaccelerate an electron beam [1]. The laser wakefield
accelerator (LWFA) has high acceleration gradient, 100 to1000 times larger than that of the traditional rf
accelerators. Recent experiments [2-5] have demonstrated
the acceleration gradient of 10 GeV/m and the energy
gain of 300 MeV.
Acceleration length of LWFA is limited to thediffraction length of the laser. One of the issues for GeV
LWFA experiment is developing a plasma waveguide toextend acceleration length of LWFA. Several techniques
have been proposed to make the acceleration length
exceed the vacuum diffraction length [6-8]. A new plasma
waveguide using a fast Z-pinch discharge in a capillary isproposed [9]. We succeeded in guiding a 2 TW laser over2 em, 12.5 times larger than the diffraction length, in theZ-pinch plasmas. Development of the new plasma
waveguide with acceleration length up to 10 em has been
started at JAERI-KANSAI as shown in Section 2. Section
3 shows preliminary results of simulation study of theplasma waveguide. Section 4 summarizes this report.

Tim£Plasrna source: Gas,(a)NeRadius

2 DESIGN OF PLASMA WAVEGUIDEIn the previous experiment [9], we have used acapillary with an inner diameter of 1 mm and a length ofup to 2 em. The capillary was filled with helium. 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(FWHM). A DC discharge circuit was used to pre-ionize
helium gas. A Ti:Sapphire laser pulse A,L = 790 nm, 90 fs,
was focused to > 1xl0 17W/cm 2 on the front edge of thecapillary with a spot size of 40 f.lm in diameter. The(b)Figure 1: Temporal evolution of (a) fast Z-pinch capillarydischarge and (b) electron density profile.

-199-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

A high current fast Z-pinch discharge generates strong
azimuthal magnetic field, which contracts the plasmaradialy inward down to 100 f.!m 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 as
shown in Figure 1 (a) and (b). The concave electron
density profile is approximately parabolic to out of radiusof 50 f.!m, after which the density falls off (Fig.l(b )).
Pre-ionizatior

Wato

LASER

solenoid valve is used to supply gas into the capillary.
Pre-ionization by YAG laser is adopted to produce thestable plasma waveguide.

3 SIMULATION OF PLASMAWAVEGUIDE
We are developing particle-in-cell (PIC) [10] codes tosimulate the channel formation of the plasma waveguide,
the laser propagation, the wake field excitation and theelectron acceleration. Newton-Lorentz equations,m;={E+;xB)
drdt

bser

ur

(u = yv),(1)

describe the relativistic motion of the plasma particles andthe electron beam in the laser wake field. The laser andthe wakefield propagations are described by Maxwellequations

ldB
VxE+--=0edt
1 dE 4n.
VxB---=-Jc dtc(2)These differential-equations are translated to space- andtime-centered finite-difference equations. B and u areupdated by leap-frog method over E and r. Field
quantities, E and B, are staggered in space grid. Thisguarantees second order accuracy and space-timereversibility. The PIC algorithm consists of two phases: In
the particle push phase, the new particle position andvelocities are determined according to Eq. (1). In thefollowing field solve phase, the fields are updated,according to the particle motion. In order to push theparticles, the field quantities at the particle positions aredetermined by linear interpolation. Current density, j, oneach grid point is calculated from the particle position andvelocity with the volume weighting scheme.

Marx Gena:tot
Max 200k 1(a)Capillary

60

(b)

40

E'u

Figure 2: (a) Schematic drawing of the plasma waveguide.
(b) Equivalent circuit of the fast Z-pinch discharge.

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We have started to develop a 10 em long plasma waveguide. A fast rising high current generator is needed toproduce the fast Z-pinch discharge. Figure 2 shows adesign of the plasma waveguide using fast Z-pinchdischarge. A Marx generator stores the energy up to 68 J.
The maximum output is 200 kV. A laser triggered spark
gap switch (LTSG) is used to minimize jitter of the Marxgenerator. A 10 em long ceramic capillary is connected tothe center of a water filled disk capacitor (4 nF). FourLTSGs are symmetrically located on the disk capacitor togenerate axially symmetric current in the capillary. Ar20"'
3:j"''!.....

E~VIa:"'01020304050

Beam Line [J.IlTl]

Figure 3: Wakefield excited by the short laser pulse,
P=IOO TW, 20 fs. ne = 3xl0 18cm- 3•-200-

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

Preliminary results of 1D PIC simulations have beenobtained. Figure 3 shows a typical wakefield profile
excited by the short laser pulse. The laser power is 100
TW and pulse width is 20 fs. Spot size is 30 Jlm in theplama. The acceleration gradient reaches 1 GeV/cm at theplasma density ne = 3x10 18cm·3 • In the range of the plasma
density from 10 17 to 1019cm·3, the acceleration length from
1 to 10 em is required for the 1 GeV LWFA experiment(Fig.4).
Multi dimensional simulations are needed for more
realistic simulation such as the laser propagation in theplasma waveguide. Next issue is development of 3D PIC
code.

182486lOxlO-3

Electron density n, [em ]Figure 4: Wakefield strength as a function of the plasma
density.

discharge. We have succeeded in demonstrating
propagation of 2 TW, 90 fs laser pulses over 2 em in theZ-pinch plasma waveguide.
Development of the 10 em plasma waveguide at JAERIKANSAI is reported. 1D PIC simulations show theplasma parameter required for 1 GeV LWFA experiment.
The study of dynamics of the fast Z-pinch discharge and alaser guiding experiment are planed in this year. The
plasma waveguide will be installed into the beam line andLWFA experiment will be conducted next year.

5 REFERENCES[1] T. Tajima and J. M. Dawson, Phys. Rev. Lett. Vol. 43(1979) pp. 267-270[2] K. Nakajima, et. al., Phys. Rev. Lett. Vol. 74 )19950
pp. 4428-4431[3] A. Modena, et. al., IEEE Trans. Plasma Sci., Vol. 24(1996) pp. 289-295[4] H. Dewa, et. al., Nucl. Inst. And Meth. In Physics Res.
A410, 514 (1998)[5) M. Kando, et. al., Jpn. J. Appl. Phys. Vol. 38 (1999)
pp. L967-L969[6] K. Krushelnick, et. al., Phys. Rev. Lett., Vol. 78(1997) pp. 4047
[7] C. G. Durfee III and H. M. Milchberg, Phys. Rev.
Lett., Vol. 71 (1993) pp. 2409
[8] Y. Ehrich, et. AI., Phys. Rev. Lett., Vol. 77 (1996) pp.
4186[9] T. Hosokai, et. al., Optics Letters, Vol. 25 (2000) pp.
10[10] C. K. Birdsal and A. B. Langdon, Plasma Physics via
Computer Simulation, McGrow-Hill, 1985

4SUMMARY
The optical guiding development has resulted in a emscale plasma waveguide using the fast Z-pinch capillary_-201-