24P167-B
Material dependence of laser accelerated electrons temperatures
レーザー加速された電子の温度の材料依存性	 Yousuke Mishima1), Hideaki Habara1), Yasuhiko Sentoku2), Tomoyuki Iwawaki1), Kenshiro
Kikuyama1), Takuya Kouno1), Tomoya Morioka1), Kiyoshi Morita1), Mamiko Nishiuchi3), Akifumi
Yogo3), Alexander Pirozhkov3), Yuji Fukuda3), Koichi Ogura3), Tuyoshi Tanimoto3), Kiminori
Kondo3) and Kazuo Tanaka1)
三島陽介1), 羽原英明1)，千徳靖彦2)，岩脇智行1)，木久山健志郎1)，河野卓也1)，森岡朋也1)，森田清1)，西内満美子3)，余語覚文3)，Alexander Pirozhkov3)，福田佑仁3)，小倉浩一3)，谷本壮3)，近藤公伯3)，田中和夫1)1) Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
大阪大学大学院工学研究科電気電子情報工学専攻	  〒565-0871	  吹田市山田丘2-1
2) Physics Department, MS-220, University of Nevada, Reno, Nevada 89557, USA
ネバダ大学リノ校物理学部	  89557
3) Advanced Photon Research Center, Japan Atomic Energy Agency, Kidugawa 619-0215, Umemidai, Kyoto
8-1-7Japan
日本原子力研究開発機構関西光科学研究所	  〒619-0215	  木津川市梅美台8-1-7
We have investigated that the material dependence of electron acceleration by ultra-intense laser light.
Recent particle simulation shows that the average energy of fast electron decreases from the prediction ofponderomotive scaling if the intense laser light directly interacts with solid material when the laser has ahigh pulse contrast or long pulse duration enough to sweep out preplasma. Experiments have been
performed by using J-KAREN laser system (1020W/cm2, 50fs, 109 contrast). In the result, the electronenergy from Al target is about 40% larger than that from Au. This ratio well agrees with the ionization
degree from the calculation in the actual laser parameters, resulting that strong dependence of number offree electrons on the electron acceleration.

1. Introduction
In fast ignition of inertial fusion research, control
of input fast electron energy is important to heat theimplosion plasma core efficiently because low
energy electrons scatters in long scale corona
plasma and high energy ones pass the core plasma
without depositing their energy. The most efficientenergy range of electron slope temperature isthought about 1 MeV [1].
Recently Chrisman et al. provided the new model,
which hot electron temperature εh decreases by ns1/2
as increasing electron density ns of target material
as described below equation [2],! h = mec 2 (" os !1) " os nc / ns(1)

where me is electron mass, c is the light velocity, γos
is the electron’s relativistic factor, nc is the criticaldensity, and ns is solid electron density. Actually
their PIC simulation result is showing that ns1/2
dependence of electron slope temperature. If laser
system have high contrast ratio or pulse duration
enough to sweep out preplasma, the intense laser
can interact with solid electron density directly. Hotelectron temperature accelerated by J B model atcritical density plasma is generally characterized byPonderomotive-Scaling[3],
butelectrontemperature is said to get lower value than thescaling if the laser interacts with target directly. If
assuming equation (1) is valid in direct interaction
condition, lower atomic number material should gethigher electron temperature than that of higher one
when laser energy is transported to fast electrons
averagely. In this letter, we have performed theexperiment to see whether material dependence ofelectron temperature exit or not.
2. Experimental Setup
We have performed the experiment by using
J-KAREN laser system, Advanced Photon ResearchCenter. The experimental set up is shown in Fig. 1.
Laser intensity is ranging from 1019 to 1020 W/cm2
with 50fs pulse duration, 2~20J energy, 800nm

Fig. 1. Experimental Setup

Table 1. Ionization degree comparison between
Experiment and PIC simulation
AlAu
Cu
Experiment1318.5
10.4
Simulation
2112Fig. 2. Electron slope temperature against laser
focal intensity
wavelength and P polarization axis. Incident angle
was 45 ゜ and electron spectrometer (ESM) wasinstalled along the laser axis. Several targets wereused, Al, Cu and Au with thickness of 2, 10, 2.5µmrespectively.
3. Result
Laser intensity dependence of electron slope
temperatures for each material are shown in Fig. 2.
Au got slightly lower slope temperature than that of
Al. But the electron energy of Al target is about
only 40% larger than that from Au, though theatomic number of Al is one-sixth as large as that ofAu. So if substituting ns = Zni into equation (1),which Z is valence number and ni is ion density,ionization degree of each material can be estimatedby comparing two results. Ionization degree can be
described as below equation,
2Z n ! T $I Al! AlZ Auor Cu = Al i,Al ## Al &&ni, Auor Cu " TAuor Cu % I Auor Cu! Auor Cu(2)Assuming Al is fully ionized, the ionization	 

degree of Cu and Au are estimated as 10.4 and 18.5
respectively. Why the ionization degree of Cu got
lower than that of Al can be explained that the ion
density of Cu is 1.4 times higher than that of Al one.
Next, PICLS code [2][4][5], including field andcollisional ionization processes (Thomas-Fermimodel) and fully relativistic energy conserving
binary collision model, has been utilized to estimate
ionization degrees. In this simulation, laser intensity
is 5 1019W/cm2 with pulse duration is 50fs
(constant intensity after Gaussian rising) and P
polarization is used. The used thickness of Au andCu are 10µm and 40µm respectively. The result of
average ionization degree estimation is shown inFig. 3. Note that fully ionized preplasma is locatedin front of the target surface. The ionization degree
results of simulation and experiment are organizedin Table1 and it is showing little difference between
the two cases. It might be thought that ionization ofAu gets higher value if more laser energy isinjected.
4. Conclusion
If high intensity laser has high contrast ratio or
long pulse duration enough to sweep out preplasma,
the laser can interact with solid material directly
and the scaling of J B accelerated electron slope
temperature is modified as equation (1). In theexperiment conducted by J-KAREN laser, Au got
slightly lower temperature than that of Al. Theionization degree of Au and Al in experiment andPIC simulation shows almost same results. Due tothe results, the small ratio of slope temperature
between Al and Au can be considered valid andhigher ionization value of Au need much more laser
energy.
References[1] M. H. Key et al.: Phys. Plasmas 5 (1998) 1966
[2] B. Chrisman et al.: Phys. Plasmas 15 (2008) 056309[3] S. C. Wilks et al.: Phs. Rev. Lett. 69 (1992) 1383
[4] R. Mishra et al.: Phys. Plasmas 16 (2009) 112704[5] Y. Sentoku et al.: J. Comp. Phys. 227 (2008) 6846

Fig. 3. Average ionization degree Z on Au and Cu:
fully ionized preplasma exist in front of target