Dynamic Behavior of Potential in the Plasma Core of the CHS Heliotron/Torsatron

Abstract

During combined electron cyclotron and neutral beam heating, an abrupt drop and rise in potential by about one half of the central electron temperature ( $\ensuremath{\sim}400$ V) was observed in the core of the compact helical system heliotron/torsatron plasma. Drastic changes of radial electric field between positive and negative states occur over periods as short as 60 $\ensuremath{\mu}$s, which is much shorter than the energy confinement time scale of about a few milliseconds. A nonlinear relation between the radial electric field and radial current is obtained. This is the first experimental observation of a microsecond time scale spontaneous transition based on an electric field bifurcation in toroidal helical plasmas.