There exists strong interaction between the plasma and channel wall in the Hall thruster,which greatly affects the discharge performance of the thruster.In this paper,a two-dimensional physical model is established based on the actual size of an Aton P70 Hall thruster discharge channel.The particle-in-cell simulation method is applied to study the influences of segmented low emissive graphite electrode biased with anode voltage on the discharge characteristics of the Hall thruster channel.The influences of segmented electrode placed at the ionization region on electric potential,ion number density,electron temperature,ionization rate,discharge current and specific impulse are discussed.The results show that,when segmented electrode is placed at the ionization region,the axial length of the acceleration region is shortened,the equipotential lines tend to be vertical with wall at the acceleration region,thus radial velocity of ions is reduced along with the wall corrosion.The axial position of the maximal electron temperature moves towards the exit with the expansion of ionization region.Furthermore,the electron-wall collision frequency and ionization rate also increase,the discharge current decreases and the specific impulse of the Hall thruster is slightly enhanced.
The distribution of magnetic field in Hall thruster channel has significant effect on its discharge process and wall plasma sheath characteristics. By creating physical models for the wall sheath region and adopting two-dimensional particle in cell simulation method,this work aims to investigate the effects of magnitude and direction of magnetic field and ion velocity on the plasma sheath characteristics. The simulation results show that magnetic field magnitudes have small impact on the sheath potential and the secondary electron emission coefficient,magnetic azimuth between the magnetic field direction and the channel radial direction is proportional to the absolute value of the sheath potential,but inversely proportional to the secondary electron emission coefficient. With the increase of the ion incident velocity,secondary electron emission coefficient is enhanced,however,electron density number,sheath potential and radial electric field are decreased. When the boundary condition is determined,with an increase of the simulation area radial scale,the sheath potential oscillation is aggravated,and the stability of the sheath is reduced.
In this work,physical models of neoclassical tearing modes(NTMs) including bootstrap current and multiple modulated electron cyclotron current drive model are applied.Based on the specific physical problems during the suppression of NTMs by driven current,this work compares the efficiency of continuous and modulated driven currents,and simulates the physical processes of multiple modulated driven currents on suppressing rotating magnetic island.It is found that when island rotates along the poloidal direction,the suppression ability of continuous driven current can be massively reduced due to current deposition outside the island separatrix and reverse deposition direction at the X point,which can be avoided by current drive modulation.Multiple current drive has a better suppressing effect than single current drive.This work gives realistic numerical simulations by optimizing the model and parameters based on the experiments,which could provide references for successful suppression of NTMs in future advanced tokamak such as international thermonuclear experimental reactor.
