Modelling and measurement of the electrostatic potential perturbation in consideration of the non-adiabatic electron response to trace tungsten concentration in a tokamak

Abstract

Trace tungsten ions are almost unavoidable in tokamaks with the tungsten as the first wall material. In high performance plasma, tungsten ions are easily concentrated in the core with a finite gradient along the radius. Prevention of the concentration is an important task for long-pulse H-mode operations. In this paper, it is shown that the tungsten concentration generates electrostatic perturbations due to the bulk electron drift with dissipation. That leads to a phase difference between the electron and the tungsten ion density perturbations. The resultant electrostatic potential extends away from the resonant surface of an internal mode which is characterized by the mode frequency chirping down. This work indicates that the electrostatic potential is mainly determined by the electron kinetics as well as the tungsten-bulk ion collisions, and that the impurity ion dynamics is efficiently influenced by the electron perturbations. Modification of the electron kinetics is a candidate to control the tungsten transport in experiments.