Session RP1 - Poster Session VIII.
POSTER session, Thursday afternoon, October 30
Fran Hill Southeast Exhibit Hall, ACC
The quasi-helically symmetric (QHS) configuration of the HSX stellarator has strongly reduced parallel viscosity compared to conventional stellarators. We use a fast switching biased electrode system to induce plasma flows and a set of Mach probes to measure the flows. Results show that the flow evolution at bias turn on involves two time scales and two directions. The radial electric field is formed on the electrode voltage time scale (».001 msec.) and one component of the flow rises on or near that time scale. A second component of the flow takes longer (».5 msec.) to establish itself. The measured damping times of the bias induced flows are reduced in the QHS configuration by a factor of approximately two compared to configurations with the quasi-symmetry intentionally broken. We model the experiments using neoclassical theory involving parallel viscosity and ion-neutral friction, including a newly developed numerical calculation of the Hamada basis vectors for the 3D HSX geometry. The experimental results agree well with these neoclassical predictions.