Observations of electron heating during 28 GHz microwave power application in proto-MPEX

The Prototype Material Plasma Exposure Experiment at the Oak Ridge National Laboratory utilizes a variety of power systems to generate and deliver a high heat flux plasma onto the surface of material targets. In the experiments described here, a deuterium plasma is produced via a ∼100 kW, 13.56 MHz RF helicon source, to which ∼20 kW of 28 GHz microwave power is applied. The electron density and temperature profiles are measured using a Thomson scattering (TS) diagnostic, and indicate that the electron density is centrally peaked. In the core of the plasma column, the electron density is higher than the cut-off density (∼0.9 × 10¹⁹ m^-3) for the launched mixture of X- and O-mode electron cyclotron heating waves to propagate. TS measurements indicate electron temperature increases from ∼5 eV to ∼20 eV during 28 GHz power application when the neutral deuterium pressure is reduced below 0.13 Pa (∼1 mTorr.).