Abstract

Scintillators, which are more tolerant of neutrons or γ-rays than semiconductors, are a promising candidate for soft X-ray (SX) diagnostics in high neutron flux environments such as JT-60SA or ITER. Although scintillators are tolerant of radiations, neutrons and γ-rays can cause scintillation light and become noise on SX signals. Therefore, a method to estimate the temporal effect by the radiations on SX signals and an appropriate design of the radiation shield based on the estimation are required. In previous studies, it has been proposed for estimating the effect by the radiations to calculate the absorption powers due to SXs, neutrons, and γ-rays in scintillators assuming that amplitudes of scintillation light are proportional to the absorption powers. In this study, an experimental examination of this proposal is conducted in the Experimental Advanced Superconducting Tokamak (EAST). It is shown that the proposal may be valid in the examination of EAST. In addition to results in EAST, initial results of a multi-channel scintillator-based SX diagnostic in the Large Helical Device (LHD) are introduced. Although a scintillator-based SX diagnostic in LHD observes oscillations of SXs by magnetohydrodynamic (MHD) phenomena successfully, the observed temporal effect on SX signals by neutrons or γ-rays is more significant than the expected effect, which is estimated by calculating the absorption powers. One of the possible reasons for the contradiction between the results in EAST and LHD is unexpected γ-rays around the scintillators in LHD. Although the temporal effect by the radiations is significant in the current system of LHD, the degradation of amplitudes of SX signals after the deuterium plasma experiments is not observed with the current level of the fluence. The scintillator-based SX diagnostic in LHD may work as a diagnostic to research MHD instabilities in deuterium plasma experiments without additional maintenance during an experimental campaign by making the pinhole larger or setting an additional radiation shield.

Highlights

  • Multi-channel soft x-ray (SX) diagnostic has been used as one of the main diagnostics in fusion plasma devices to research magnetohydrodynamic (MHD) equilibria and MHD activities1.For this purpose, semiconductors have been used in magnetic confinement devices

  • Though scintillator-based SX diagnostic in Large Helical Device (LHD) observes oscillations of SXs by magnetohydrodynamic (MHD) phenomena successfully, observed temporal effect on SX signals by neutrons or g-rays is more significant than the expected effect, which is estimated by calculating the absorption powers

  • The experimental examination of the proposal, which is suggested in previous studies9,10, to estimate temporal effect by neutrons and g-rays is conducted in Experimental Advanced Superconducting Tokamak (EAST)

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Summary

INTRODUCTION

Multi-channel soft x-ray (SX) diagnostic has been used as one of the main diagnostics in fusion plasma devices to research magnetohydrodynamic (MHD) equilibria and MHD activities. In addition to the experimental examination in EAST, initial results of multi-channel scintillator-based SX diagnostic in Large. The functionality of SX shield is examined in a discharge without NBIs as shown, which shows signals of (a) the plasma current and the line averaged electron density measured by the HCN interferometer, (b) the total neutron emission rate obtained with a fission chamber, (c) semiconductor-based SX diagnostic, and (d) scintillator-based SX diagnostic, respectively. The examination of the proposal is conducted in the shot shown, which shows signals of (a) the total neutron emission rate, (b) SXT1, and (c) SXT2, respectively. In this shot, NBs are injected repeatedly with intervals. Wn and Wγ are calculated with the total neutron emission rate ~ 3.3 × 1013 n s-1

EXPERIMENTAL EXAMINATION OF PROPOSAL
Observation of MHD phenomena
INITIAL RESULTS OF MULTI-CHANNEL
Effect on SX signals by neutrons and g-rays
SUMMARY
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