Formation and Dynamics of Very Deep Negative Potential Well in the Small Tokamak Device CSTN-IV

We achieved a potential well more than fifty times deeper than the plasma electron temperature, down to -0.3 kV with the electrode biasing, by the cross-field radial arcing current of up to 150 A which flows between the small electron-emitting electrode (LaB₆) inserted to the plasma center and the stainless-steel wall of vacuum chamber. The empirical scaling of the radial resistance of plasma R _r on the radial arcing current I _b and the toroidal magnetic field B _T is found to be R _r I _b ^-1.2 B _T ^1.0. Structural dynamics of negative potential well formation is observed: the potential dip propagates from the plasma core to the periphery. The ion saturation current also increases, starting from the inner region. The fluctuation frequency in I _sat changes abruptly from 200 kHz to 30 kHz in a time of around 0.25 ms after negative biasing.