Overview of MST Research

MST progress in advancing the RFP for (1) fusion plasma confinement with minimal external magnetization, (2) toroidal confinement physics, and (3) basic plasma physics is summarized. Stellarator-like helical equilibria (QSH) appear spontaneously at high current. The Lundquist number appears a key parameter, unifying QSH onset versus current in both MST and RFX-Mod. Equilibrium modeling for the 3D state is corroborated by several diagnostics, including new 2-color SXR tomography. A 1 MW neutral beam injector stimulates beam-driven instabilities for the first time in the RFP. The observed modes are nonlinearly coupled, causing redistribution of the fast ions. Interestingly, beam-generated ions are accelerated to higher energy during sawtooth events, likely related to non-collisional ion heating. The tearing mode relaxation process is a rich story where single-fluid MHD appears to capture the sawtooth period dynamics, but extended MHD is needed to explain coupled momentum and dynamo processes. A kinetic momentum stress associated with magnetic turbulence has also been discovered. Theoretical analysis of micro-instability indicates micro-tearing will be dominant at high beta. MST diagnostics will assess such instabilities in high-performance plasmas with large Te gradients.