Speaker: Jay Anderson, University of Wisconsin-Madison
Abstract: The envisioned burning plasma experiment, regardless of magnetic concept, relies on sufficient confinement of the charged fusion products for plasma self heating. As such, the confinement of fast ions and their impact on the bulk plasma are crucial issues. While well-studied in tokamak, ST and stellarator plasmas, relatively little is known in RFP plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in MST with an intense 25 keV, 1 MW hydrogen neutral beam injector (300 MW/m^2 at injection port). Fast particles are confined much better than thermal particles in the stochastic RFP magnetic field, and a significant population of fast ions develops during NB injection. TRANSP simulations predict a super-Alfvenic ion density of up to 25% of the electron density with both a significant velocity space gradient and a sharp radial density gradient. There are several effects on the background plasma including enhanced toroidal rotation, electron heating and an altered current density profile. The abundant fast particles affect the plasma stability. Fast ions at the island of the core-most resonant tearing mode have a stabilizing effect, and up to 60% reduction in the magnetic fluctuation amplitude is observed during NBI. Simultaneously, beam driven instabilities are observed for the first time in the RFP. Repetitive 50 us bursts of m=1 modes have scaling signatures of both Alfvenic and continuum energetic particle modes. The dominant modes are n=4 (EP-like) and n=5 (AE-like), which nonlinearly couple to an n=1 mode. The feedback of the altered plasma stability on the fast ion confinement is investigated.