High-energy neutral particle measurements simulating α particle diagnostics in Large Helical Device

Abstract

It is very important to investigate the confinement of α particles, which is produced by the nuclear reaction in ITER or fusion reactor. In Large Helical Device (LHD), it is possible to perform the simulation experiment of the α particle heating by using the ion cyclotron resonance heating (ICH) because high-energy particles generated by ICH are well confined in the plasma. High-energy particles including α particle are emitted not only by the charge exchange but also by the MHD instability as the Alfven eigen mode. Their particles give damage to the plasma wall in addition to create a poor plasma confinement. Therefore, the suitable method for measuring α particle distribution should be established immediately. For this purpose, the silicon surface barrier detector-type neutral particle analyzer (SD-NPA) and the position sensitive detector are installed in LHD. The neutral particles (mainly hydrogen), which are generated by the charge exchange between the high-energy ion and the background neutrals, can be observed by using them. However, a few neutral helium particles can be observed because α particle (or fully ionized helium) can emit only by double charge exchange process. Therefore, we also introduce the pellet charge exchange system (PCX). The diagnostic pellet is injected to the plasma in order to obtain the charge exchange neutral particle, which is produced by the charge exchange reaction between the ablated pellet cloud and α particle or high-energetic particle. Much hydrogen flux can be obtained around the ion cyclotron resonance region by the PCX. Similar results can be also obtained in SD-NPA. The helium distribution measurement in helium plasma is demonstrated. Here the reliability of PCX as the particle diagnostic and the possibility of α particle (helium) diagnostic are described.