Observation of the Pitch Angle Distribution of High-Energy Ions and their Radial Transport in the Central Cell of GAMMA10 * )

Suppression of the radial transport of the particles caused by low-frequency density fluctuations is one of the important subjects in the GAMMA10 tandem mirror. In GAMMA10, plasmas are mainly produced and heated with Ion Cyclotron Range of Frequency (ICRF) waves. The plasmas heated with ICRF waves have strong temperature anisotropy, because the ions are accelerated perpendicularly to the magnetic field line. In the GAMMA10 experiments, two types of low-frequency density fluctuations are observed; one is a drift-type fluctuation and the other is a flute-type fluctuation. In this paper, we study the radial transport of ICRF-produced high-energy ions which is caused by the low-frequency density fluctuations

Effects of Toroidally Distributed Divertor Biasing on Scrape-Off-Layer Plasma in the QUEST Spherical Tokamak

A novel divertor biasing using four biasing plates that are arranged toroidally every 90° on the upper divertor plate is applied to low-density plasmas of the QUEST spherical tokamak. When some of these plates are biased in-phase by applying a sawtooth waveform voltage of 85-V amplitude and 50-Hz repetition, up to approximately 35% reduction of the particle flux to the divertor is observed during positive biasing. The input power for the flux reduction is approximately 0.2 kW for low-density tokamak plasmas produced by ∼130-kW electron cyclotron wave injection. Additionally, the signal of a plate probe placed in the low-field side of the mid-plane of the vacuum vessel indicates enhanced losses of fast electrons during positive biasing. The enhanced loss is attributed to small resonant magnetic perturbations produced by the bias-driven currents in the scrape-off layer. This novel divertor biasing is expected to provide a new experimental tool for studying divertor heat load control and fast electron confinement in a tokamak device