Control of the cross field plasma flow in a simplified magnetic configuration (The Helimak)

Cross field plasma flow is measured in a simplified magnetic configuration [1] that re-creates essential aspects of the SOL of a tokamak. Flow velocities are measured via the Doppler shift of the spectrum of the main plasma ion. The magnetic field has a strong toroidal component and a weaker perpendicular component, and field line pitch is varied from 0.5$^{\circ}$ to 2.7$^{\circ}$ which corresponds to the pitch for q \textgreater\ 6 in tokamaks. The flow velocity is controlled by biasing conductors in contact with the plasma. The bias is varied from -50 V to 20 V in a plasma with unbiased plasma potential near 25 V. The perpendicular mass flow responds in the range -0.5 km/s to 1 km/s and saturates for bias less than -25 V. The flow is consistent in direction with the $E\times B$drift, and the inferred electric field is consistent in magnitude with that based on measured plasma potential. Flow control is exploited to explore turbulence supression by varying flow shear within a factor 2 of 1x10$^{4}$ s$^{-1}$. This flow shear is comparable with estimates for turbulence growth rate: 0.1 of the diamagnetic frequency and the autocorrelation time of the broadband turbulence. \\[4pt] [1] K.W. Gentle, Plasma Sci. Technol. 10, 284 (2008)