Plasma and Fusion Research

Volume 14, 3402142 (2019)

Regular Articles

Reduction of the Neutron Induced Noise in the Compact Neutral Particle Analyzer for LHD Deuterium Plasma Experiments
Tetsuo OZAKI1), Shuji KAMIO1), Takeo NISHITANI1), Kenji SAITO1), Kunihiro OGAWA1,2), Mitsutaka ISOBE1,2), Masaki OSAKABE1,2), Makoto KOBAYASHI1) and the LHD Group1)
1)
National Institute for Fusion Science, 322-6 Oroshi, Toki 509-5292, Japan
2)
SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi, Toki 509-5292, Japan
(Received 10 January 2019 / Accepted 8 July 2019 / Published 14 August 2019)

Abstract

The compact neutral particle analyzer (CNPA) combined with the impurity pellet measurement is one of the few instruments that can directly measure the radial high energy particle distribution in the Large Helical Device (LHD). For this purpose, it is suitable to set CNPA near LHD. On the other hand, sufficient and heavy shielding against DD-neutrons, generated in deuterium experiment, is required, especially when the deuterium neutral particle beam injection heating is applied. The shield is insufficient due to the weight limitation on the stage. However, if the neutron energy is thermalized on the detector, the neutron noise can be estimated only from the total neutron yield, which is monitored by the 235U fission chamber, etc. In the experiments, the pure neutron noise on the CNPA has been measured by closing the gate-valve to avoid the charge exchange neutral particle signals. The neutron noise on the CNPA has been proportional to the total neutron yield over three-order magnitudes. This means that the neutron noise can be estimated from the total neutron yield. Therefore, the calibrated charge exchange neutral signal can be obtained simply by subtracting the estimated neutron noise from the measured signal in ordinary experiments.

Keywords

CNPA, neutral particle, LHD, NBI, neutron, neutron noise, activation

DOI: 10.1585/pfr.14.3402142

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