Effect of Impurity Ions on NBI Heating in LHD Plasmas

Abstract

The effect of impurity ions on NBI heating is numerically investigated in LHD plasmas. Helium and carbon are the two impurity species considered in the simulation. Fast-ion birth rates with various impurity fractions are calculated using the HFREYA code, taking into account ionization by the electron and ion impacts, and the charge exchanges with ions. The fast-ion birth rate is found to increase with increasing effective charge of plasma in the tangential injection NBI heating (beam energy: 180 keV) case, whereas the birth rate decreased in the perpendicular injection one (beam energy: 40 keV). The fast-ion slowing down is calculated using the GNET code, considering the Coulomb collisions with electrons, protons, and impurity ions. As the impurity fraction increases, the fast-ion confinement deteriorates because of the enhancement of the pitch-angle scatterings. Also, as the impurity fraction increases, the heat deposition per ion increases in the tangential injection NBI case and decreases in the perpendicular one.