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-III is 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 \left(T_\rme\right) temporal profile. Microwave absorption is dominant in the case of the X-mode, leading to a broader T_\rme 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 B_\rm0=0.075\,\rm\rm T). Traces of the electron-number density show a similar tendency, as revealed by T_\rme. These results suggest that the X-mode is the best candidate for efficient plasma pre-ionization at low filled gas pressures \left(\sim \rm10^-2\,\rmPa\right) in small tokamaks.