Observation of a curvature-driven, trapped particle mode created by a potential barrier

Publisher Website
Google Scholar
Abstract
A mirror‐confined hot‐electron distribution is created using high‐power, short‐pulse electron‐cyclotron resonance heating (ECRH) in a single, good curvature (magnetically stable) cell of a plasma‐filled multiple mirror device. The hot electrons are observed to decouple good and bad curvature regions on the two sides of the hot‐electron cell. An unstable magnetohydrodynamic (MHD)‐like, rigid plasma motion to the walls occurs in the bad curvature region, with velocity comparable to that of a mode driven by the bad curvature alone. In some magnetic configurations the plasma restabilizes later in time. The hot‐electron distribution decays stably, independent of the other processes. The initiation of the instability is correlated with the appearance in the hot‐electron cell of a negative potential barrier. For the configurations in which plasma is restabilized, the restabilization is correlated with the decay of the potential barrier caused by ion trapping in the hot‐electron cell. An electron beam time‐of‐flight probe has been used to measure the potential barrier and its decay. The experimental results are compared to a curvature‐driven, trapped particle theory, including the effects of finite collisionality.