Isotope effects on instabilities driven by ion temperature gradient and tungsten ions in tokamak plasmas

摘要

Isotope effects on instabilities driven by ion temperature gradient (ITG) and impurities in tokamak plasmas in the presence of tungsten ions are numerically studied. It is revealed that the tungsten ions significantly modify the isotope scaling of the maximum growth rates of the instabilities with respect to the main or effective ion mass number M_i or M_(eff) [=(1 - f_z)M_i + f_zM_z] with f_z (=Zn_z-_e) being impurity charge concentration. The most reasonable scaling is deduced as γ_(max) = M_(eff)~(-1.5±β) with β ~ 0.8, for ITG driven modes, whilst γ_(max) = M_i~(-0.4±β_1)Z_(eff)~(1.2±β_2) holds with β_1 ~ 0.2, β_2 ~ 0.4 for tungsten impurity modes, in significant contrast with the case of light or intermediate impurities where the scaling is γ_(max) = M_(eff)~(-0.5) and γ_(max) = M_i~(0.5)Z_(eff)~(1.5) for ITG and impurity modes, respectively. These results suggest that existence of tungsten impurity would enhance (weaken) the isotope effect of instability driven by ITG (tungsten impurity ions), which is beneficial (harmful) for improvement of confinement when hydrogen isotopes are used in plasmas. The results might also provide hints on studies of particle and energy transports and discharge performance, particularly, in ITER-like wall machines.