Anomalous transport in a toroidal plasma

Abstract

Experimentally obtained data from the toroidal Blaamann device have been analysed. Blaamann is a simple magnetised torus with no rotational transform, and plasma is generated by a discharge from a hot filament emitting electrons. The data were obtained using a three pin Langmuir probe in combination with a stationary reference probe. Sampled floating potential and electron saturation currents from 19 different positions along the horizontal central line in a poloidal cross section, enabled us to study the turbulent transport within the plasma column. Several statistical properties of the data were investigated. The average transport of flux is outwards, with large fluctuations in signals near the filament, but decreasing with distance from the source. A clear and systematic relation between skewness and kurtosis was also identified. Through use of cross correlation functions and the conditional sampling technique we were able to investigate the nature of the anomalous transport, as well as identify coherent structures. These structures appear to be burst-like in nature, with a lifetime of ∼100 μs. There is some indication that these structures are vortex-like. The coherent structures propagate around the poloidal cross section along with the rest of the plasma column, rotating with the E×B/B^2-velocity, and at the same time transporting plasma radially out of the column, giving rise to spiral trajectories.