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篠原 孝司; 石井 啓一*; 落合 謙太郎; 馬場 護*; 笹尾 眞實子*; 北島 純男*
no journal, ,
A neutron flux array is one of the important plasma diagnostics to obtain the information on fast ion population. The d-T neutrons produced by deuteron-triton reaction bring us the information on 1 MeV tritons in deuterium plasmas in present-day tokamaks. However, the number of d-T neutrons is small, so that higher detection efficiency is required for a neutron detector. Here, we propose a multi-stage neutron detection system, in which several sets of a scintillator and photo-multiplier tube (PMT) are placed in a line-of-sight. In order to evaluate the performance of the multi-stage detection system, we have carried out experiments on a two-stage detection system using a d-T neutron beam at FNS (Fusion Neutronics Source) of JAEA (Japan Atomic Energy Agency). The results have shown the concept of the multi-stage detection system works as expected. In the best setup, the test two-stage system reached about 1.8 times the efficiency of a single scintillator and photomultiplier tube system.
Bierwage, A.; 藤堂 泰*; 相羽 信行; 松永 剛; 篠原 孝司; 矢木 雅敏
no journal, ,
Energetic particle mode (EPM) dynamics are simulated using the hybrid code MEGA. The scenario considered is based on JT-60U shot E039672, just before the onset of a so-called Abrupt Large Event (ALE). The properties of modes with toroidal mode numbers n=1-4 are analyzed. The n=1 harmonic dominates when drive is relatively weak; e.g., after a relaxation event has taken place. The n=3 harmonic dominantes when drive is strong, but only for a relatively short time until the fast ions have been redistributed. In single-n simulations, the peak amplitude of the n=3 mode exhibits a sharp increase when the drive exceeds a certain threshold. This is shown to be due to a convective amplification process. The sharp transition is moderated when the n=1 mode is taken into account. It is concluded that both n=1 and n=3 modes play an important role during ALEs, but the trigger mechanism for the onset of an ALE is still obscure.