Overdense microwave plasma heating in the CNT stellarator

TL;DR

This study demonstrates successful overdense plasma heating in the CNT stellarator using 2.45 GHz microwaves, revealing mode conversion and tunneling effects that enable densities beyond the cutoff, with detailed analysis of plasma profiles and power dependence.

Contribution

It provides the first detailed experimental and numerical analysis of overdense microwave heating mechanisms in a low-field stellarator, highlighting mode conversion and tunneling processes.

Findings

Achieved plasma densities over four times the O-mode cutoff.

Confirmed mode conversion and tunneling via full-wave simulations.

Observed density increase with microwave power and magnetic field dependence.

Abstract

Overdense plasmas have been attained with 2.45 GHz microwave heating in the low-field, low-aspect-ratio CNT stellarator. Densities higher than four times the ordinary (O) mode cutoff density were measured with 8 kW of power injected in the O-mode and, alternatively, with 6.5 kW in the extraordinary (X) mode. The temperature profiles peak at the plasma edge. This was ascribed to collisional damping of the X-mode at the upper hybrid resonant layer. The X-mode reaches that location by tunneling, mode-conversions or after polarization-scrambling reflections off the wall and in-vessel coils, regardless of the initial launch being in O- or X-mode. This interpretation was confirmed by full-wave numerical simulations. Also, as the CNT plasma is not completely ionized at these low microwave power levels, electron density was shown to increase with power. A dependence on magnetic field strength was also observed (for O-mode launch) and discussed.