Comparative Investigation of End Plate and Limiter Biasing in the Hiei Tandem Mirror

It is demonstrated that the transition leading to the improved radial confinement is induced by the end plate biasing in the HIEI tandem mirror, in which the plasma is MHD stabilized, heated, and axially confined by the ICRF waves. On application of the bias voltage, the fluctuation level decreases, the density increases to over 10¹³ cm⁻³, and the plasma rotation changes significantly indicating a formation of the internal transport barrier. The observed strong nonlinear phenomena; the hysteresis change of the fluctuation level and the neutral line emission intensity, demonstrate bifurcation feature, which is similar to that in the previous limiter biasing experiment. This bifurcation feature may be explained by the model from the azimuthal momentum balance including the flow shear stabilization of the drift wave instabilities.