Effect of Plasma Biasing on Suppression of Electrostatic Fluctuation in the Edge Region of STP-3(M) Reversed Field Pinch

Abstract

A plasma biasing experiment using an insertable electrode has been carried out in the STP-3(M) reversed field pinch. The profiles of electron density n_e, electron temperature T_e and plasma potential V_s were measured with an electrostatic probe array in the edge region of the plasma. The effect of biasing is observed as an increase of edge electron density and suppression of fluctuation amplitudes in n_e, T_e and V_s. As a result, both the electrostatic particle and energy fluxes are reduced and confinement improvement is observed, which is caused by the formation of E×B velocity shear layer. The reduction in both fluctuation amplitudes and coherence between fluctuating quantities contributes to this improvement. The formation of velocity shear layer is dependent on the insertion depth of the electrode, which is possibly related to the location where additional power is deposited through biasing. The extension of plasma lifetime is also observed in the biased discharges.