Confinement of high energy and high temperature ions in the MST reversed field pinch

Confinement of both high energy and high temperature ions has been investigated in the Madison Symmetric Torus (MST) Reversed Field Pinch (RFP), both in standard plasmas with magnetic stochasticity, and improved confinement plasmas in which stochasticity is reduced. We find that (1) energetic ions (produced by neutral beam injection) are very well-confined in the standard RFP plasma despite the presence of significant stochasticity. This is understood from theory and simulation, and has positive implications for feasibility of neutral beam injection and alpha-particle confinement; (2) when magnetic stochasticity is reduced the thermal ion confinement is substantially improved as well. This is evidenced from estimation of ion thermal diffusivity as well as from direct observations of long lasting periods of sustained high ion temperature. (author)