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

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.