Observations of temperature rise during electron cyclotron heating application in Proto-MPEX

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) at ORNL utilizes a variety of power systems to generate and deliver a high heat flux plasma (~1 MW/m2 for these discharges) onto the surface of material targets. In the experiments described here, up to 120 kW of 13.56 MHz “helicon” waves are combined with ~20 kW of 28 GHz microwaves to produce Deuterium plasma discharges. The 28 GHz waves are launched in a region of the device where the magnetic field is axially varying near ~0.8 T, resulting in the presence of a 2nd harmonic electron cyclotron heating (ECH) resonance layer that transects the plasma column. The electron density and temperature profiles are measured using a Thomson scattering (TS) diagnostic, and indicate that the electron density is radially peaked. In the core of the plasma column the electron density is higher than the cut-off density (0.9x1019 m-3) for ECH waves to propagate and O-X-B mode conversion into electron Bernstien waves (EBW) is expected. TS measurements indicate electron temperature increases during 28 GHz wave application, rising (from ~5 eV to ~20 eV) as the neutral Deuterium pressure is reduced below 1 mTorr.