Evidence of electron heating at different radial locations on Proto-MPEX

Abstract

This paper reports on the observation of controlled underdense electron heating for high heat flux applications in the Prototype Material Plasma Exposure eXperiment. The goal of this experiment is to demonstrate the source and heating concepts to create a high intensity plasma at material targets to study plasma-material interactions in similar plasma conditions to magnetic fusion divertors. A 80 kW helicon source at 13.56 MHz is used for the plasma density production and a 15 kW microwave source at 28 GHz is used for electron heating. For electron densities ∼5 × 1018 m−3, the electron temperature at the target location increases from 2 eV to 5 eV and the core heat flux increases by a factor of 5. It is shown that the radial profile of the heat flux delivered at the target can be controlled by either the background plasma density or the magnetic field. Analysis suggests that the electron heating is due to heating near the upper-hybrid resonance layer and that this heating scenario may scale up to divertor-relevant electron densities ∼5 × 1019 m−3 by increasing the microwave driving frequency to 105 GHz.