22P135-P

Development of Associated Particle Coincident Counting – Neutron Spectrometer
toward Deuterium Plasma Diagnostics in LHD
LHD 重水素プラズマ診断のための共役粒子同時計数型中性子スペクトロメータの開発
Yuta Nakamuraa, Yosuke Yamamotoa, Hideki Tomitaa, Mitsutaka Isobeb, Kunihiro Ogawaa,
Sadayoshi Murakamic, Jun Kawarabayashia and Tetsuo Iguchia
中村悠太a, 山本洋輔a，富田英生a，磯部光孝b，小川国大a，
村上定義c，河原林順a, 井口哲夫aa

Graduate School of Engineering Nagoya University
Furo-tyo, Chikusa-ku, Nagoya 464-0073, Japan
名古屋大学大学院工学研究科 〒464-8603 名古屋市千種区不老町b
National Institute for Fusion Science, Oroshi-cho, Toki, 509-5292, Japan
核融合科学研究所 〒509-5292 土岐市下石町
cGraduate School of Engineering, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan
京都大学大学院工学研究科 〒606-8501 京都市左京区吉田本町
Toward deuterium plasma diagnostics in LHD, we have developed a neutron spectrometer based on coincident
detection of the scattered neutron and recoiled proton. To evaluate the applicability of the spectrometer, neutronenergy spectrum was estimated using Monte Carlo simulation model based on FIT code. The predicted neutron yield inLHD indicated that a counting efficiency of the spectrometer should be above 10 -6 counts/neutron to obtain thespectrum with statistical precision of less than 10% in the integration time of 1 s.

1. Introduction
In deuterium plasmas, neutrons around energy of 2.5
MeV (DD neutron) are emitted as accompanying
products of DD fusion reaction (D+D➝3He+n). In thecase that target deuterium plasma is heated by neutralbeam injection (NBI) heating, the neutrons are mainly
produced by beam-plasma interactions because of
their high DD reaction rate. Thus, the neutron energy
spectrum emitted from the deuterium plasma
especially reflects the velocity distribution of fast ion
in high-temperature plasma. For deuterium plasma
experiments that are now being planned in the world
largest heliotron device, Large Helical Device (LHD),
at National Institute for Fusion Science (NIFS), Japan,
we have proposed a neutron spectrometer based on
coincident counting of associated particles [1]. Weshow the detection principle of the spectrometer andits basic specifications. And we discuss the expected
neutron spectra from deuterium plasma heated by NBI
in LHD by using Monte Carlo simulation model basedon FIT code [2].

radiator, the neutron energy is derived from sum of thedeposit energies in the radiator and the recoiled proton
detector and the scattered neutron energy measured bya time-of-flight technique.
We made a prototype spectrometer and measured thedetector response to monoenergetic DD neutron. Theenergy resolution of 6.3% and the detection efficiencyof 3.3×10-7 counts/neutron were experimentallydemonstrated for DD neutron, respectively.

2. Detection Principle and Results from theProto-type Spectrometer
Figure 1 shows schematic diagram of the proposed
neutron spectrometer named "associated particle
coincident counting – neutron spectrometer". Thespectrometer is based on coincident detection of thescattered neutron and recoiled proton from a plastic
scintillator as the incident neutron target, or a radiator.
In the case of elastic scattering of neutron in the

3. Model Calculation of Neutron Spectrum inLHD
We developed a detailed Monte Carlosimulation model for the neutron energy spectrum
based on FIT code [2]. The FIT code analyzes anenergy deposition and a slowing down process of ahigh energy ion based on its birth profile and plasma
temperature and density profiles. Here, the slowing
down process is treated by the steady state solution ofFig. 1. Schematic diagram of the associated particle
coincident counting – neutron spectrometer.

1mn u n22m3 He1Q  K  mnV 2 
2m3 He  mn

En 

 cos  V(1)2mn m3 HeQ  K mn  m3He 

where mi is the mass of ion i, V is the relative velocityof ion and K is the energy of ion before reaction.
Figure 2 shows arrangement of neutral beam
injectors in LHD and a position of the spectrometerconsidered in this calculation. Figure 3 shows thetypical calculated spectrum on the tangentially
viewing spectrometer at 6-T port. In this calculation,
deuterium plasma (the ion density of 1.7×1019 m-3) was
heated by one neutral beam injection (NB#2) with anacceleration voltage of 150 keV and a port-throughinjection power of 2.75 MW. The maximum neutron
yield was roughly estimated to be about 108 neutrons/s
at the position of the spectrometer. Thus, a counting
efficiency of the spectrometer should be above 10-6
counts/neutron to obtain the spectrum with statisticalprecision of less than 10% in the integration time of 1 s,i.e. counting rate of more than 102 counts/s.
NB#5

NB#1
NB#2

NB#3

Spectrometerat 6-T port

NB#4

Fig. 2. Arrangement of neutral beam injectors in LHD
and a position of the spectrometer in this calculation.

Intensity (X 104 neutrons/s)

Fokker-Plank equation. On each position in theplasma i.e. the normalized minor radius ρ, themaximum velocity of the fast ion, the pitch angle ofthe fast ion averaged over magnetic surface(reprehensive pitch angle), and the fast ion density
were obtained by the FIT-DD code. The energy ofneutron emitted from the plasma and reached to thespectrometer was calculated by8Total :1.2 X 10 7 neutrons/s

64
20
1.8

2
2.2
2.4
Neutron Energy (MeV)

2.6

Fig. 3. Typical calculated spectrum on the tangentially
viewing spectrometer at 6-T port with the neutral beam
injection heating of NB#2.

4. Conclusions
We have developed the associated particle coincident
counting – neutron spectrometer as a useful diagnostic
tool for dynamics of fast ions in LHD deuterium
plasma. The Monte Carlo simulation model for theneutron energy spectrum based on FIT code predicted
the neutron yield as about 108 neutrons/s at 6-T port in
LHD. Thus, a counting efficiency of the spectrometershould be above 10-6 counts/neutron to obtain thespectrum with statistical precision of less than 10% in
the integration time of 1 s. To achieve the detection
efficiency of 10-6 counts/neutron, we will consider theinstall of additional set of three detectors to a whole
spectrometer system. In addition, we will estimate theapplicability of the proposed spectrometer based on themodel calculation.

Acknowledgments
This work is performed with the support and under
the auspices of the NIFS Collaboration Researchprogram (NIFS08KOBH001).

References[1] H. Tomita, H. Iwai, T. Iguchi, M. Isobe, J.
Kawarabayashi, and C. Konno, “Development ofneutron spectrometer toward deuterium plasma
diagnostics in LHD” Rev. Sci. Instrum. 81, 10D309(2010).
[2] S. Murakami, N. Nakajima, M. Okamoto, "Finite
beta Effects on the ICRF and NBI Heating in theLarge Helical Device", Fusion Technology, 27 Suppl.
S, 256-259, (1995).