24F01
Diagnostics of pedestal intensity of intense and short fast-heating laser pulse
with x-ray backlighting
X線バックライト法を用いた高速加熱用短パルス高強度レーザーの	 ペデスタル計測	 
Shinji Ohira1, Shinsuke Fujioka1, Atsushi Sunahara2, Tomoyuki Jozaki1, Hiroyuki Shiraga1,Hiroaki Nishimura1 and Hiroshi Azechi1
11
大平真司 , 藤岡慎介 ，砂原	  淳2，城崎知至1, 白神宏之1, 西村博明1, 疇地 宏1
1

Institute of Laser Engineering, Osaka University
2-6, Yamadaoka, Suita, Osaka 565-0871, Japan
大阪大学レーザーエネルギー学研究センター〒565-0871	  吹田市山田丘2-62Institute for Laser Technology
1-8-4, Utsubohonmachi, Nishi-ku, Osaka 550-0004,Japan
レーザー技術総合研究所〒550-0004	  大阪市西区靭本町1-8-4
Preformed plasma generated by a leakage pulse component from a high intensity laser pulse seriously
influences on heating performance of a compressed fuel in fast ignition scheme. We measured temporally
evolved plasma formation with x-ray radiography. It was found that plasma of 40-60µm in scalelength wasformed just before arrival of the main pulse. Intensity of the leakage component was estimated bycomparing the experimental result with hydrodynamic simulation.

1. Introduction
In fast ignition scheme [1], highly compressedfuel is heated by an extremely intense and shortlaser pulse (1019W/cm2/1ps) to cause ignition andburn. A gold cone attached to a fuel capsule is used
for guiding the heating-laser energy and thus
enables to transfer its energy efficiently to the fuel.
The heating laser also includes pre-pulse and/or
pedestal components, whose intensity ratio is
usually around 10-6~10-8 with respect to that of themain pulse. Here after we call these components
leakage pulses. These pulses will preform a plasma
inside the cone which is harmful for efficientheating of the fuel due to increase in density
scalelength and thermo-magnetic filamentations[2-4].
We observed the pre-plasma formation for thecase of LFEX (Laser for Fast ignition EXperiment)
laser irradiation in order to estimate quantity of theleakage component by comparing the experimentwith hydrodynamic simulation.

backlight technique combined with an x-ray streak
camera (XSC) and an imaging slit. The overall
spatial resolution was 48 µm. A gold foil of 10 µm
in thickness was irradiated by the LFEX laser. An
aluminum foil of 10 µm was irradiated with a laser
pulse from GEKKO-Ⅻ	  (300	  J	  /	  1	  ns /532	  nm) to generate an x-ray backlight source of1.59	  keV	  (Al+11 Heα line). Typical image fromthe x-ray steak camera is inset in Fig. 1.

2. Experiment
In the experiment LFEX laser was used. It
provided a laser pulse of 300 J in 1.5 ps at 1064 nm
in wavelength. Pulse contrast ratio of 10-8 wasexpected, but characterization of the leakage component has not yet been experimentally verified.
Figure 1 shows the experimental setup. Time-resolved 1-dimensional (1D) spatial distribution of
preformed plasma was measured with x-rayFig. 1. Experimental setup for preformed plasma
observation. The inset is a typical image from thex-ray streak camera

Abel inversion was used to drive the density
profile of the preformed plasma. Assuming radiallysymmetric distribution, absorption of x-rays along
the line of sight, in direction of l, is given by

T = exp #$! " µ! dl %& ,(1)where T is the transmission of x-ray, µ the mass
absorption coefficient in cm2/g, and ρ the density ing/cm3. Since the intensity of the leakage component
was predicted to be 1012~1013 W/cm2, temperatureof the preformed plasma is around several 10 eV.
Therefore mass absorption coefficient of gold incold state is still valid for 1.59 keV photons.
3. Analysis

Fig. 2. Density profile of the preformed plasma
estimated from the x-ray radiograph.

Figure 2 shows the observed 1D density distributionof the preformed plasma. The heating laser isincident from the right hand side and the origins of
the spatial and temporal axes are defined to be thesurface of the target and the onset of the main pulse,respectively. Preformed plasma became observable
at approximately 700 ps before the main pulse
arrival and the density scalelength at this time
becomes 40-60 µm. Upon this result, the time
duration of the leakage component is assumed to be
1 ns in the following simulations.
We estimated the intensity and spot size of theleakage pulse by comparing the experiment withsimulations made with a 2D radiationhydrodynamic code (STAR2D) [5]. First we
attempted to simulate the preplasma formation
assuming that the spot size of the leakage pulse on
the gold surface is the same as that of the LFEX
main pulse (i.e., 40 µm in diameter). Other
parameters of the leakage pulse were, respectively,
1 ns in duration and irradiance in the range of1011~1013 W/cm2. However, under these conditions
the simulated results could not replicate theexperimental results in terms of the density profile,
expansion velocity and the density scalelength. So,
assuming that the leakage component is attributed
to the amplified spontaneous emission, say, fromthe final amplifiers of LFEX system, thus having amuch larger spot size on the gold foil than that of

the main pulse, another simulation was made byusing 1D radiation hydrodynamic code (STAR1D)[6].
Figure 3 shows comparison between theexperiment and the 1D simulations. In this case aleakage pulse of 1 ns and 500 µm in spot diameterat 5×1012 W/cm2 were assumed. The simulation
reproduces the experiment fairly well.
In the presentation, these experimental results
and analysis with simulations will be discussed.

Fig. 3. Comparison of the density distribution
between the experiment and 1D simulations

Acknowledgments
The authors appreciate the dedicated technicalsupport of the staff at GEKKO-Ⅻ facility for thelaser operation, target fabrication and plasma
diagnostics.
This work was partly supported by the Japanese
Ministry of Education, Science, Sports, and Culture(MEXT), Grant-in-Aid for young Scientists (A) for“Non-LTE photo-ionized Plasma Generation with
Laser-Produced Blackbody Radiator (Grant No.
21684034)”.
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Rev. Lett. 104, 055002(2010)[3] N. Kumar, V. K. Tripathi and B K Sawhney, Phys.
Scr. 73, 659(2006)[4] H. Cai, K. Mima, A. Sunahara, et al., Phys. Plasmas
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