Gyrokinetic particle-in-cell simulation of Alfvénic ion-temperature-gradient modes in tokamak plasma

Abstract

A linearized gyrokinetic particle-in-cell code is developed to study the excitations of electromagnetic instabilities due to finite ion temperature gradients in tokamak plasmas. The code employs the δf scheme to solve the gyrokinetic equations for ion dynamics; while electrons are modeled as massless fluid. The instability threshold condition of the Alfvénic ion-temperature-gradient (ITG) mode as well as the finite β stablization of electrostatic ITG mode are studied. Here β=8πp/B2 is the ratio between the plasma and magnetic pressure. In the β→0 limit, our code recovers the electrostatic results. As β increases, the destablization of kinetic ballooning mode due to finite ion temperature gradient is observed. The simulation results also reveal two possible critical values of ηi, where ηi=d ln Ti/d ln n is the ratio between the density scale length to the temperature scale length; one of the critical values is for the predominantly electrostatic ITG mode and one for the Alfvénic ITG mode, where ηi,cAITG<ηi,cESITG. The transition domain between the Alfvénic ITG mode and the ITG mode, meanwhile, is found to depend on β and ηi.