Study on Knock-on Tail Formation in Deuteron Velocity Distribution Function Due to ICRF-Heated Energetic Proton by Using Neutron Diagnostics in the Large Helical Device

Abstract

In fusion plasmas, energetic ions play a crucial role in plasma heating. Nuclear elastic scattering (NES) is a non-Coulombic scattering process that affects the energy transport between energetic and bulk ions. In the Large Helical Device (LHD), energetic protons produced by neutral beam injection (180 keV) formed a knock-on tail (KT) in deuterons via NES, and the DD neutron emission rate increased by one order of magnitude in relatively high-electron-temperature plasmas. Furthermore, the effect of NES among ion cyclotron range of frequency (ICRF) tail protons and bulk deuterons was investigated at high-electron-temperature plasmas in the LHD. It was found that DD neutron emission rate was increased by a factor of 2 to 4. Changes in ion temperature and plasma density cannot be the only reasons for the increase in DD neutron emission rate. The increment in DD neutron emission rate was reproduced by the Fokker-Planck simulation using the Boltzmann collision integral for NES by assuming the ICRF-tail protons having a high-temperature Maxwellian.