24D14
Neutron diagnostics in Fast Ignition Experiment
高速点火実験における中性子計測
Yasunobu Arikawa1, Hirokazu Hosoda1, Takahiro Nagai1, Mitsuo Nakai1, Takeshi Watari1,Hirotaka Nakamura1, Shinsuke Fujioka1, Hiroyuki Shiraga1, Hideo Nagatomo1, Tomoyuki Johzaki2,
Atsushi Sunahara2, Keisuke Shigemori1, Mayuko Koga1, Hirofumi Kikuchi1, Yoshinori Ishii1,Takehiro Sogo1, Shinji Ohira1, Zhe1 Zhang, Hiroaki Nishimura1, Yoichi Sakawa1, Tomoyuki
Iwawaki1, Kazuo Tanaka1, Kozue Watanabe1, Yuki Minami1, Kohei Yamanoi1, Toshihiko Shimizu1,Nobuhiko Sarukura1, Takahisa Jitsuno1, Yoshiki Nakata1, Junji Kawanaka1, Koji Tsubakimoto1,Noboru Morio1, Kiyonobu Sawai1, Tetsuji Kawasaki1, Shinji Urushihara1, Satoshi Matsuo1, Koichi
Tsuji1, Noriaki Miyanaga1, Yasushi Fujimoto1, Hirofumi Homma1, Takayoshi Norimatsu1, Osamu
Maegawa1, Tetsuo Ozaki3, Hitoshi Sakagami3, and Hiroshi Azechi1
有川安信1, 細田裕計1，長井隆浩1，中井光男1, 渡利威士1, 中村浩隆1, 藤岡慎介1, 白神宏之1, 長友英夫1, 城崎知至2, 砂原淳2, 重森啓介1, 古賀麻由子1, 菊池裕文1, 石井圭憲1, 十河雄大1, 大平真司1,Zhe Zhang1, 西村博明1, 坂和洋一1, 岩脇知行1, 田中和夫1, 渡辺梢1, 南佑輝1, 山ノ井航平1, 清水俊彦1, 猿倉信彦1, 實野孝久1,中田芳樹1, 河仲準二1, 椿本孝治1, 森尾登1, 澤井清信1, 川崎鉄次1, 漆原新治1, 松尾悟士1, 辻公一1,宮永憲明1, 藤本靖1, 本間啓史1, 乗松孝好1, 前川修1, 尾崎哲3, 坂上仁志3,
疇地宏1
1

Institute of Laser Engineering Osaka University, 2-6, Yamadaoka, Suitashi, Osaka 565-0871, Japan
2
Institute for Laser Technology, 2-6, Yamadaoka, Suitashi, Osaka 565-0871, Japan3
National Institute for Fusion Science, Japan, 322-6 Oroshi-cho, Toki-city, Gifu, Japan1
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 2-62
レーザーエネルギー技術総合研究所 〒565-0871 大阪府吹田市山田丘 2-63
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6

The fast ignition integrated experiment was conducted by using GEKKO XII and newly constructed LFEX.
A new neutron yield detector was developed for overcoming very strong x-ray pulse generated from fast
heating beam. Besides the x-ray signal, a large amount of neutrons generated by -n reaction from target
chamber or diagnostics instruments was confirmed as a more critical origin of the background signal. The
background signal was modeled by a MonteCalro simulation, and the detection error for fusion neutron
attributed by -n was evaluated. The maximum neutron yield observed was (3.5±1.3)×107, which is larger
than previous reported value.

1. Introduction
The fast ignition is one of the most ambitious
subjects in the inertial confinement fusion science.
The imploded fuel core was heated up by an ultra
intense short pulse laser. Kodama, et. al., havereported the 1000 times enhancement in neutron
yield by a heating pulse with around 300 J withthe pulse width of 0.7 ps [1]. Theobald. et. al.,have report a series of the experimental resultsfrom OMEGA and OMEGA EP facility with a 1
kJ , 10 ps heating pulse [2]. Fast heating electron
generates intense high energy x-ray pulse (we-ray), which makes neutron diagnosing
very difficult in fast ignition experiments.
These experimental results of neutron yield
include a large error bar originated from thebackground noises. The construction of the LFEX
with the world largest energy [3] was completed
in Institute of Laser Engineering OsakaUniversity (ILE), and fast ignition integrated
experiment was started in 2009. Many newly

developed neutron detector were installed toovercome the harsh -ray back grounds problem
in 2010. In this paper the neutron yield
diagnostics and the analysis of the background infast ignition experiment in 2011 will bepresented.
2. Background from -n reaction
The great amount of neutrons which larger thanfusion neutron yield was observed from many kindsof detectors. These neutrons were confirmed-n (photo disintegration) reaction invarious materials in such as the iron in the targetchamber or experimental instruments, deuterium infusion fuel. The scattered -ray from the target bay
concrete wall ('-ray) was also found to be criticalbackground. The background signals from-ray, ′-n neutrons were modeled using
Monte Carlo simulation code MCNP-5. In thesimulation model, a point -ray source was set at thecenter of the target chamber and assumed to have anexponential decay-spectrum with the temperature of 5
MeV based on the typical electron spectrum measured
by an electron spectrometer. The anglar distribution ofthe -ray was measured by using Ag-activated glassdose monitors. The simulated -n profiles agreed very
well with the experimental results. The fluctuation of
the detected neutrons originated from -n neutron wasestimated and added as a -n error. The details about
the analysis are discussed in Ref. [4].

3. The neutron yield in the fast heating
experiment
The DD-neutron signal could be distinguished fromthe background in only 5 shots in over 30 shots byusing the newly developed liquid scintillation detector[4, 5]. These shots were conducted over a period of
two consecutive weeks after completion of thedetector, and the energy of the LFEX was less than
500 J. Figure 1 shows the observed signal of themaximum neutron yield. The detector was constructedwith a fast decay liquid scintillator developed in ILE[5] and a gated photomultiplier tube. The gate open
time is after 70 ns of -ray arrival, and fusion neutron
is seen at 150 ns. The pulse height of the signal seen at
150 ns was determined as the DD fusion neutron withthe yield of (3.5±1.2)×107. The blue dashed lineshows -n neutrons, and the fluctuation of that within
10 ns around DD fusion arrival time was estimated.
Then 6×106 -n error was added, the final conclusionof the neutron yield was (3.5±1.3)×107.

where 0 ps is corresponding to the timing of
maximum compression. The neutron enhancementfrom heating laser was observed only in the casethat core was heated within 50 ps at maximumcompression. This fact confirms the neutron
enhancement was attributed from fast heating. This
neutron yield was larger than previously reported
value [1,2].
However the ion temperature or the ion densityof the fuel have not be measured in the experiment2010. We are still lack of the information to fully
understand fast heating mechanism. The next
integrated experiment is being planned to be
conducted at the summer in 2012 with the heatingenergy over 2.5 kJ with the pulse width of 1 ps. The
neutron collimator for suppress -n neutron
backgrounds will be developed, and the time
resolution of the detector will be improved tomeasure the ion temperature by neutron spectrum
Doppler broadening. The ignition equivalent ion
temperature by fast heating will be observed.

Figure. 2. Relationship between neutron yield andLFEX injection time. Green dashed line is the neutronyield obtained in the experiment in 2002 [2].

Acknowledgement
Authors gratefully acknowledge the support of theGEKKO XII operation group, the LFEX development
and operation group, the target fabrication group, andthe plasma diagnostics operation group of the Instituteof Laser Engineering, Osaka University.
Figure. 1. The signal of the liquid scintillator(black) and simulation (red) observed in fast
ignition experiment in 2010.
The injection time of the fast heating laser wasmonitored by using multi-image x-ray streak
camera with a shield slit made by Tungsten. The
implosion shell trajectory and the x-ray signal fromheating laser were measured. The relative time
between the time intensity of the x-ray emission
from the core becomes maximum (here we refermaximum compression time) and heating time wasmeasured with the accuracy of 8 ps. The neutron
yield against heating time was plotted in Fig.2,References[1] R. Kodama, et. al., Nature, 418, 933-934 (2002)[2] W. Theobald, Phys. Plasmas, 18, 056305, (2011)[3] H. Shiraga, et. al., to be published in PlasmaPhysics and Cont. Fusion,[4] Y. Arikawa, to be published in, Phys. Plasmas,[5] T. Nagai, et. al. Jap.J.App.Phys.50,080208 (2011).