Letters: Dynamics of H-mode-like edge transitions brought about by external polarization

By setting up radial electric fields in the edge of a tokamak plasma by means of electrodes it is possible to trigger edge rearrangements which are similar to L-H transitions. The radial currents due to the finite orthogonal conductivity provide a poloidal torque which spins up the plasma against the viscous force opposing the poloidal rotation. It is found, in agreement with theoretical predictions, that this force first increases with the rotation speed and then, above a critical rotation speed, decreases. Under conditions in which the applied torque exceeds the maximum opposing force, a bifurcation from low field to high field is inevitable and results in many of the typical L-H transition signatures. The measured radial conductivity is compared with neoclassical estimates.