23P177-B
Two-dimensional Particle-in-Cell Simulation on Forming Plasma Micro Opticsin Laser Wake Field Acceleration
レーザー航跡場加速におけるプラズママイクロオプティクス形成に関する2次元PICシミュレーション
Keigo Makito1, Alexei Zhidkov2,3, Shinichi Masuda2,3, Yoshio Mizuta1, Shohei Kajino1,Akinori Nishida1, Nobuhiko Nakanii2,3, Tomonao Hosokai2,3 and Ryosuke Kodama1,2,3
牧戸啓悟1，アレクセイジドコフ2,3，益田伸一2,3，水田好雄1，梶野祥平1，
西田明憲1，中新信彦2,3, 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, Japan2Photon Pioneers Center, Osaka University, 2-8 Yamadaoka, Suita-shi, Osaka 565-0871, Japan
3
Japan Science and Technology Agency (JST), CREST
1
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2-83
科学技術振興機構 (JST)，CREST

Generation of laser wake field using a high contrast laser pulse in argon gas is investigated by
two-dimensional particle-in-cell simulation including optical field ionization for control of electron beamcharacteristics. The quality of the wake field structure is strongly affected by ionization during the pulsepropagation. The structure disappears at higher gas density: the gas density threshold for the wake field
generation is found to be about 1×1018 cm-3.

1. Introduction
Sources of femtosecond mono-energetic
electron beams with low divergence and highcharge are required in the field of ultra-fasttransient imaging. Laser wake field acceleration(LWFA) may enable to develop such electron
beam sources. In order to produce electron beam
by LWFA, control of the wake field generationand electron injection is essential.
A plasma structure named “plasma microoptics” is to control the wake field. It has beenused in experiments with successful generation ofhigh quality electron bunches by focusing afemto-second intense laser to a helium gas target
with a controllable magnetic field [1, 2]. In termsof the gas species of the target, argon is reported
to produce that kind of structure at much higher
densities than helium targets [3]. Althoughexperiment on generating the plasma micro optics
using argon target has also performed, its
principle is not revealed yet because of its
complicated physical process. For clarifying thisproblem, as a basic research for forming theplasma micro optics, we firstly carry out
simulations on generation the wake field in argon
gas.
There are several physics involved in the wakefield generation: self-focusing, refraction of laser
light, and neutral gas ionization. Among all theseFig.1. Laser wake field generated in (a) fully ionizedplasma, (b) gas with ionization process.

factors, the ionization may play an important role
resulting in the difference of the wake fieldstructure as shown in Fig. 1. In this paper, wepresent results of two-dimensional particle-in-cell(PIC) simulations including optical field
ionization to investigate how this ionization
effect to the generation of the wake field during
the laser pulse propagation.
The optical field ionization rate α is given byfollowing:where ωA and EA are the atomic frequency andelectric field respectively, IH and Ii are theionization potentials of hydrogen and the atom
under consideration.

2. PIC Simulation.
Table I. Basic conditions of laser pulse
Wavelength
Intensity
Pulse duration
Spot size

800 nm
3×1019 Wcm-2
40 fs
16 µm in diameter

Wcm-2 and (b) its spot size of 32 µm in diameter.
The other parameters were fixed as shown in Table
I. As this figure shows, the wake field structure
disappeared in the higher intensity case. In this case,a steeper density gradient is made in the transverse
direction and stronger refraction results in the fasterwave-breaking. In contrast, with a wider spot size
the gradient becomes smaller with weakerrefraction and better structure of the wake.

We performed 2D PIC simulations of laser wakefield generation using high contrast laser pulse inargon gas. The code employed the Bunemanscheme for current weighting, variable particle
weight for calculating optical field ionization. The
code also employs moving window with theabsorbing boundary.
2.1 Conditions of simulation
The cell size of calculation mesh and time step
were set to 0.05 µm and 0.08 fs. And we used 4
particles per cell. Laser pulse whose conditions areshown in Table I propagated from right to left. The
pulse focused at 150 µm from its initial position
where argon gas density ramp starts increasing
linearly along the laser axis (X-axis); the densityramp length is 50 µm, then the plasma is uniform.
2.2 Gas density dependency
Spatial distribution of the laser wake field in thegas jets with different density, are shown in Fig. 2after the pulse propagated over 300 µm. All
coordinates are given in k = c/ω unit. The otherparameters were set as in Table I. In case of the low
gas density, the wake field has clear structure while
the high density case, its structure no longer exists.
The gas density threshold of the structure
breakdown is found to be about 1×1018 cm-3.
2.3 Dependencies on laser parameters
We also performed the calculations changing thelaser parameters. Fig. 3 shows the wake field
distribution at different (a) pulse intensity of 3×1020

Fig.2. Wake field generation in the gas with density(a) 0.7×1018 cm-3, (a) 2×1018 cm-3

Fig.3. Wake field generation using laser with (a) intensity
3×1020 Wcm-2, (b) spot size 32 µm in diameter3. Conclusion
In summary, we have performed 2D PIC
simulations on laser wake field generation after
high contrast laser pulses in argon gas toinvestigate the effects of ionization during thelaser pulse propagation, as a basic research forforming the plasma micro optics. The quality of
the wake field structure is strongly affected byionization during the pulse propagation. The
structure disappears in case of higher gas density,and higher laser intensity. The gas density thresholdfor the wake field generation is about 1×1018 cm-3.
Acknowledgments
This work was supported by CREST (CoreResearch for Evolutional Science and Technology)of the Japan Science and Technology Agency(JST).
References[1] T. Hosokai et al.: Phys. Rev. Lett. 97 (2006) 075004.
[2] T. Hosokai et al.: Appl. Phys. Lett. 96 (2010) 121501.
[3] M. Mori et al.: Phys. Rev. STAB. 12 (2009) 082801.