Electron and ion heating physics for the Material Plasma Exposure eXperiment plasma source concept

The Material Plasma Exposure eXperiment (MPEX) is a proposed high-intensity linear divertor simulator based on radio frequency (RF) physics and technology to address the fusion plasma-material interaction difficulty. RF heating has never been demonstrated at these unexplored high density, low temperature, and high collisionality regime in linear plasma machines, so new RF heating schemes, challenges, results, and understanding have emerged. We report recent electron and ion heating physics highlights on the Proto-MPEX experiment as a demonstration of various heating concepts for MPEX:

• ion temperatures of 10-15 eV with 15-25 kW of RF power
• core electron temperatures of 10-25 eV with 10-35 kW of microwave power
• improved electron and ion heating physics understanding of the effects of cyclotron, Landau and collisional damping

We have observed and modeled these results for a variety of different heating schemes: magnetic beach ion cyclotron heating, electron Bernstein waves (EBW), upper hybrid (UH), and electron cyclotron heating (ECH). For observed core ion cyclotron heating, finite electron temperature needs to be included in the model to match experimental observations of core ion temperature. The model also illustrates possible electron heating mechanisms at the Alfven resonance.

EBW modeling and experiments show that beam focusing and reduction in magnetic mirror trapping are necessary for efficient core heating. UH collisional damping can be controlled both experimentally and numerically by magnetic field or helicon power to obtain core or edge power deposition and may reduce effects of magnetic mirror trapping. ECH may have low single pass absorption, and both UH and ECH require higher frequencies to access higher density plasmas. Neutral pressure control is important for all these electron heating schemes.