Simulation study of NBI heating in the time-evolving and multi-ion-species plasmas of LHD

Abstract

Numerical simulations of neutral beam injection (NBI) heating in the time-evolving and multi-ion-species plasma in the large helical device (LHD) are performed. Two typical impurity species in the high- experiment plasma, helium and carbon, are considered. The birth simulation using the HFREYA code has shown that the fast ion birth increases with the carbon impurity fraction in the tangential and perpendicular NBI heating cases. The GNET-TD code is used to simulate the slowing down of the fast ions, taking into account the plasma time evolution, impurity collisions and guiding-centre motion in the three-dimensional magnetic configuration. It is found that the fast ion confinement is deteriorated by the pitch-angle scattering with carbon ions, nevertheless the total heat deposition of the tangential NBI increases with the carbon fraction in the simulation of steady-state plasma. The time-dependent simulation with the carbon pellet injection has shown the deformation of the fast ion distribution function and an increase in the heat depositions after the carbon pellet injection. It has also been shown that the steady-state simulations underestimate the heat deposition in the core region right after the pellet injection.