23P153-P
Hard X-Ray and Electromagnetic Pulse Harsh Environment in Fast IgnitionExperiments
高速点火実験における硬X線および電磁パルス過酷環境	 
Hiroyuki Shiraga, Mayuko Koga, Yoshinori Ishii, Takehiro Sogo, Osamu Maegawa, Yasunobu
Arikawa, Hirokazu Hosoda, Takahiro Nagai, Mitsuo Nakai, Shinsuke Fujioka, Hiroshi Azechi
白神宏之,	 古賀麻由子,	 石井圭憲,	 十河雄大,	 前川修,	 有川信安,	 細田裕	 計,	 長井隆浩,	 中井光男,	 藤岡慎介,	 疇地宏Institute of Laser Engineering, Osaka University
2-6 Yamada-Oka, 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	  大阪府吹田市山田丘2-6
In Fast Ignition experiments, targets are irradiated with ultra-intense short-pulse laser for heating of thecompressed fuel plasma. Intense hard x rays (γ rays) and electromagnetic pulse (EMP) are generated insuch experiments from hot electrons generated by the heating laser. Those bring serious backgroundproblems to x-ray imaging diagnostics and neutron diagnostics as well as malfunctioning of electronicdevices. Furthermore, intense γ rays causes some nuclear reactions producing neutrons which are obstacles
to the measurement of fusion neutrons. Plasma diagnostic instruments for Gekko-XII/LFEX laser facility
have been improved to be compatible with such hard x-ray and EMP harsh environment.

1. Introduction
FIREX project has been carried out for Fast
Ignition (FI) research at the Institute of Laser
Engineering, Osaka University [1]. In FI
experiments, targets are irradiated with ultra-intense
short-pulse laser for heating of the compressed fuelplasma. A new laser system, LFEX, has been
constructed and activated for this purpose as a fast
heating laser. Its final output will be 10 kJ in a 0.5 –20 ps pulse. In such experiments, intense hard xrays (γ rays) and electromagnetic pulses (EMP)
are generated from hot electrons created by theheating laser. Those bring serious backgroundproblems to x-ray imaging diagnostics and neutron
diagnostics as well as malfunctioning of electronic
devices. Furthermore, intense γ rays causes someother nuclear reactions producing neutrons which
are obstacles to the measurement of fusion neutrons.
Plasma diagnostic instruments for the experimentswith Gekko-XII/LFEX laser facility must becompatible with such hard x-ray and EMP harshenvironment.
The first campaign of the integrated experimentsof FI was performed in 2009, and the second one in2010. In 2009 experiment, we found that the plasma
diagnostics were in a very serious situation of γ-ray
background and EMP as descrived above, and it
was very difficult to find the real signals in the data
covered with intense backgrounds or noises. Then,
we have much improved the diagnostic instruments
including shielding and collimation as well as
redesigning of the instruments.

2. Fast Ignition Experiment on Gekko-XII andLFEX lasers
Implosion and heating experiments of FI targets
for FIREX-1 have been performed by operating
both Gekko-XII and LFEX lasers. Typical laser andtarget parameters were as follows. Gekko-XII laser
for implosion: 0.53-µm light with an energy of
1.5-4.5 kJ in total in a 1.5 ns nearly Gaussian pulse
in 2009, and a nearly flat-top pulse in 2010
experiment, nine beams among twelve. LFEX laser
for heating: 1.053-µm light with an energy up to1-2 kJ in 1-5 ps. The beam(s) were focused andinjected into a cone attached to a shell target. Shelltargets (CD: deuterated polystyrene): 500 µm in
diameter and 7 µm in thickness. A 10-20 µm
wall-thickness Au cone with an opening angle of 30
or 45 degrees. Outer surface of the Au cone wascoated with 10- µm-thick CH layer. Distance fromthe center of the shell to the cone tip was 50 µm.
Characteristics of the imploded and heated fuelplasmas were observed by using a variety of plasma
diagnostics. Dynamics of the imploded fuel plasma
was observed with ultrafast x-ray spectroscopic
imaging utilizing x-ray streak cameras [2-4] andx-ray framing cameras [5]. Fusion products wereobserved with detectors including a multi-channel
single-hit neutron spectrometer, ultrafast liquid
scintillator neutron detectors [6], filtered CR-39
detectors, and so forth.
3. X-ray Imaging Diagnostics

3.1 X-ray framing cameras
Intense non-imaged hard x-ray signals wereobserved on gated-MCP based x-ray framing
cameras coupled with x-ray pinhole cameras.
Although this signal can be used as the heating time
indicator [5], it buries the thermal x-ray images of
the target at around the time of the heating beam
injection, the most important frames. We havedeveloped x-ray framing cameras coupled with totalreflection x-ray mirrors, which reflect only thermal
x-ray images below 5 keV. Those worked very wellin observation of the imploded core plasmas at thetime of the heating.

Fig.1. X-ray framing cameras coupled with totalreflection x-ray mirrors.

3.2 X-ray streak cameras
Time-resolved two-dimensional x-ray images of
the imploded core plasma was observed withMulti-Imaging X-ray Streak Camera [2]. The streak
tube has a cathode disk made of 1-mm-thickstainless steel with slits for choosing the image.
However, the disk material is almost transparent tohard x-rays of > 20 keV, and the whole surface
worked as cathode for such hard x-rays, resulting indischarges between the cathode disk and theacceleration mesh due to large amount of thephotoelectrons generated by intense hard x-rays. In
2010 experiment, the streak camera was equippedwith a hard-x-ray shielded photo cathode disk, andFig.2. X-ray streak camera data .

the discharge was successfully extinguished.
Furthermore, injection time of the heating beam
relative to the implosion was measured with this
x-ray streak camera with an accuracy better than+/-10 ps by using non-imaged hard x-ray signals.
4. Neutron Diagnostics
Neutron time-of flight detectors are the mostimportant diagnostic instruments for determining
the enhanced neutron yield and ion temperature in
FI experiments. Large hard x-ray signal and its
decay are obstacle to the fusion neutron diagnostics.
An ultrafast-decay liquid scintillator was developed
to solve the problem [6]. Also we found that notonly the DD fusion neutrons from the imploded andheated core but also neutrons due to (γ, n) reactions
and scattered γ rays were detected by time-of-flightneutron detectors. Signals of (γ,n) neutrons andscattered γ rays were analyzed by using
Monte-Carlo simulations. Such obstacle signals
were carefully eliminated and the net DD neutron
signals were extracted to determine the enhanced
DD neutron yield.
5. Electronic Devices and EMP
High-intensity laser-irradiated plasma emits
intense EMP in GHz-THz spectral regions. A highperformance electromagnetic shielding box wasintroduced to keep electronics and PC’s. Noise
amplitude was significantly reduced by a factor
more than 20, although further reduction is needed.
6. Conclusions
Various improvements in plasma diagnostics
worked well, and the plasma data weresuccessfully obtained in FI experiment with
Gekko-XII/LFEX.
Acknowledgments
The authors would like to thank Gekko-XII andLFEX laser operation crew, target fabrication group,
plasma diagnostics group, and computer operation
staffs for their great contributions to this work. Thiswork was supported by Interactive CoordinatedResearch Program with the National Institute for
Fusion Science, Japan.
References[1] H. Azechi, et al., Nuclear Fusion 49 (2009) 104024.
[2] J. Zhong, et al, Rev. Sci. Instrum. 79, 10E907 (2008)[3] H. Shiraga, et al, Rev. Sci. Instrum. 79, 10E920 (2008).
[4] M. Tanabe, et al., Rev. Sci. Instrum. 79, 10E908(2008).
[5] M. Koga, et al., Rev. Sci. Instrum. 79, 10E909 (2008).
[6] T. Nagai, et al., Jpn. J. Appl. Phys 50, 080208 (2011).