Fast ion confinement and stability in a neutral beam injected reversed field pinch

Abstract

The behavior of energetic ions is fundamentally important in the study of fusion plasmas. While well-studied in tokamak, spherical torus, and stellarator plasmas, relatively little is known in reversed field pinch plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in the Madison Symmetric Torus with an intense 25 keV, 1 MW hydrogen neutral beam injector (NBI). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3–5% D2) both demonstrate that at low concentration the fast ion population is consistent with classical slowing of the fast ions, negligible cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops; simulations predict a super-Alfvénic 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. The sharp reduction in amplitude, however, has little effect on the underlying magnetic island structure. Simultaneously, beam driven instabilities are observed as repetitive ∼50 μs bursts which coincide with fast particle redistribution; data indicate a saturated core fast ion density well below purely classical predictions.