Microwave-assisted pre-ionization experiments on GLAST-Ⅲ

GLAss Spherical Tokamak(GLAST-Ⅲ)is a spherical tokamak with an insulating vacuum vessel that has a unique single-passage capability for incident microwaves.In this work,electron cyclotron resonance heating(ECRH)-assisted plasma pre-ionization in GLAST-Ⅲis explored for three radio-frequency(RF)polarizations(the O-,X-,and M-modes)at different toroidal-field(TF)strengths and filled gas pressures.The optimum hydrogen pressure is identified for efficient plasma pre-ionization.A comparison of the plasma pre-ionizations initiated by the O-,X-,and M-modes shows prominent differences in the breakdown time,location,and wave absorption.In the case of O-mode polarization,microwave absorption occurs for a relatively shorter duration,resulting in a bell-shaped electron-temperature(Te)temporal profile.Microwave absorption is dominant in the case of the X-mode,leading to a broader Te temporal profile.The M-mode discharge contains features of both the X-and O-modes.Efficient plasma pre-ionization is achieved in the X-mode polarization for the intermediate TF strengths(with a central toroidal magnetic field B0=0.075 T).Traces of the electron-number density show a similar tendency,as revealed by Te.These results suggest that the X-mode is the best candidate for efficient plasma pre-ionization at low filled gas pressures(10-2 Pa)in small tokamaks.

Viscous effects on plasmoid formation from nonlinear resistive tearing growth in a Harris sheet

In this work,the evolution of a highly unstable m=1 resistive tearing mode,leading to plasmoid formation in a Harris sheet,is studied in the framework of full MHD model using the Non-Ideal Magnetohydrodynamics with Rotation,Open Discussion simulation.Following the initial nonlinear growth of the primary m=1 island,the X-point develops into a secondary elongated current sheet that eventually breaks into plasmoids.Two distinctive viscous regimes are found for the plasmoid formation and saturation.In the low viscosity regime(i.e.P_(r)■1),the plasmoid width increases sharply with viscosity,whereas in the viscosity dominant regime(i.e.P_(r)■1),the plasmoid size gradually decreases with viscosity.Such a finding quantifies the role of viscosity in modulating the plasmoid formation process through its effects on the plasma flow and the reconnection itself.

Effects of helium massive gas injection level on disruption mitigation on EAST

In this study,NIMROD simulations are performed to investigate the effects of massive helium gas injection level on the induced disruption on EAST tokamak.It is demonstrated in simulations that two different scenarios of plasma cooling(complete cooling and partial cooling)take place for different amounts of injected impurities.For the impurity injection above a critical level,a single MHD activity is able to induce a complete core temperature collapse.For impurity injection below the critical level,a series of multiple minor disruptions occur before the complete thermal quench.