Simulation of ECR startup and comparison to experimental observations in SUNIST

摘要

Electron cyclotron resonance heating is an effective method to startup the plasma current in spherical tokamaks (STs). It has been shown on many STs that the transient process at the beginning of ECR startup discharges can affect the startup results significantly, especially for short microwave pulses. This paper presents the experimental observations of two types of ECR startup on SUNIST: one with electron density spikes but very low plasma current, and the other one without electron density spikes but higher plasma current. The mechanism of the formation of spikes is studied by a series of parameter scan experiments, i.e., resonant layer scan and vertical field scan. It was found that the long wave length of microwave, the drift process of over dense electrons and the perpendicularly installed microwave antenna are all responsible for these interesting phenomena. The experimental configuration is modeled as an exciter-reflector system and is analyzed in one dimension. The continuous equations for each cell of the space between the inner and the outer wall are given. The reflection ratio of microwave is simply calculated with the assumption as optics. After careful tuning of the coefficients, the simulation results that are comparable to experimental observations are obtained thus the physical images conjectured from experiments are confirmed.