23P168-B
Formation of a Transient Plasma Micro Optics
for Laser Wake Field Acceleration
レーザー航跡場加速のためのプラズママイクロオプティクス形成
Yoshio Mizuta1, Akinori Nishida1, Shinichi Masuda2,3 ,Keigo Makito1, Shohei Kajino1,
Nobuhiko Nakanii2,3,Masaki Kando4, Michiaki Mori4, Hideyuki Kotaki4, Yukio Hayashi4,
Alexei Zhidkov5, Sergei Bulanov4, Tomonao Hosokai2,3, and Ryosuke Kodama1,2,3
水田好雄 1、西田明憲 1、益田伸一 2,3、牧戸啓悟 1、梶野祥平 1、中新信彦 2,3、神門正城 4、森 道昭 4、小瀧秀行 4、林由紀雄 4、アレクセイジドコフ 5、セルゲイブラノフ 4、細貝知直 2,3、兒玉了祐 1,2,3
1

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita-shi, Osaka 565-0871, Japan2
Photon Pioneers Center, Osaka University, 2-8 Yamadaoka, Suita-shi, Osaka 565-0871, Japan
3
Japan Science and Technology Agency (JST), CREST,
4
Kansai Photon Science Institute, Japan Atomic Energy Agency, Umemidai 8-1, Kizu, Kyoto 619-0215, Japan1
大阪大学大学院 工学研究科 〒565-0871 吹田市山田丘 2-1
2
大阪大学 光科学センター 〒565-0871 吹田市山田丘2-8
3
科学技術振興機構 (JST), CREST
4
日本原子力研究開発機構 〒619-0215 京都府木津川市梅美台 8-1

The transient plasma micro optics (TPMO) is important part of the two-staged laser wake- field acceleration
scheme, because the electron self-injection and acceleration can be controlled independently. We have observed
TPMO structures created by laser pulses with the pulse duration of nanoseconds and picoseconds using
interferometry. A density profile of plasma cavities created by the nanosecond pulse pedestal was measured. Inaddition, temporal evolution of a plasma channel created by a picosecond pulse was also observed. The channel
length exceeded 600m, which was three times the Rayleigh length.

1. Introduction
We study laser wake-field acceleration (LWFA)
for aiming at a repeatable and high quality electron
source. By controlling the waveform of intenselaser pulse with nanoseconds and picoseconds
pre-pulses, we have demonstrated generation ofquasi-monoenergetic, hundreds MeV electron
beams with excellent emittance by laser wake fieldacceleration[1,2].
Nanosecond pre-pulse may
produce plasma cavity in a gas target. The plasma
cavity acts like optical element of focusing or
defocusing laser light. Picosecond pre-pulse
propagates in the cavity and it creates long plasmachannel. We called this process as a transient
plasma micro optics (TPMO). Femetosecond pulse
is guided in the long plasma channel. As a result,high quality electron beam is generated. TPMO
plays important role in LWFA. Until now, it has notbeen studied much. In this paper, we investigatedthe cavity and channel creation in details. We
measured the density profiles of plasma created bylaser pulses with the pulse duration of nanoseconds
and picoseconds in a high-density gas jet target
using picoseconds time-resolved interferometer.

2. Experimental Setup
The experiment was performed at JLITE-X
800nm Ti:Sappire CPA laser system at theJAEA-KPSI (Kansai Photon Science Institute,Japan Atomic Energy Agency) [3,4]. An outline of
the experimental setup is given in Fig. 1.A laser
pulse with a diameter 30 mm is focused to theposition of the ~100m front edge of the slit gas
jet height of 1.5 mm form nozzle exit with f/5.9
off-axis parabolic mirror. In order to study theeffects of the TPMO formation by the laser pulse,we controlled the nanosecond order laser pre-pulse
by tuning the Pockels cell behind the regenerative
amplifier of the laser system [4]. We also controlled
pulse duration 300 fs~6 ps by turning the distanceof the grating pair in the pulse compressor, toobserve the effect of pulse duration. The focal spot
size is 16m in full width at 1/e2 of maximum with
186m Rayleigh length. The maximum laser
intensity on the target is estimated to be 3.5×1015
W/cm2 .We used an Ar gas jet target, which wasprovided through a supersonic nozzle by a pulsevalve (Smartshell Co.Ltd). The stagnation pressureof the pulse valve was 2.00 MPa. The neutral gas
density is estimated to be 1.5×1019 cm-3 .

Fig.1 Schematic diagram of the experimental setup

To observe the formation process of a TPMO, weused time-resolved interferometer. The length of theprobe laser is 400 nm. The interferograms weretaken by a CCD camera.
3. Experimental Results
Fig.2 shows interferogram taken during thepropagation of a CPA pulse through the gas jet.
Laser pulse comes from the left side.
The corresponding electron density distribution isalso shown Fig.3. We are calculating about theabsolute number of the electron density now. The
electron density distribution can be calculated fromthe phase shift. IDEA code was used to obtain aphase shift map. A fast Fourier transform technique
was used to retrieve the plasma-imposed phase shift.
The phase-shift map is processed then with an Abelinversion algorism [5]. The maximum channel
length was 600m. It was three times the Raleighlength.

Fig.2 Interferogram

Fig.3 Electron density distribution

4. Conclusion
In conclusion, we have studied the formation
process of a TPMO useful for laser wake field
acceleration. We have investigated the TPMO
creation in detail upon using the picoseconds
time-resolvedinterferometry.
Thedensity
distribution of a TPMO was measured. The channel
length was 600m, which was three times theRayleigh length. Our measurements prove theplasma behavior as TPMO.
Acknowledgments
This work supported by the Japan Society for thePromotion of Science Global Centers of Excellence
program(GCOE). This work was also supported byCREST (Core Research for Evolutional Scienceand Technology) of the Japan Science andTechnology Agency(JST). We also are grateful toJAEA for providing the laser system.
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