Explanation of the L-H mode transition induced by applied voltage

The response of the edge plasma to an applied voltage is analysed. When the anomalous transport is ambipolar, the radial current is determined by neoclassical fluxes. An equation is given for the neoclassical radial current which is valid across the transition between the plateau regime and the collisional viscous regime. The current first increases with the applied electric field E 1 and then decreases owing to either the resonant velocity band being pushed to the tail of the ion velocity distribution or a decrease in viscous damping. Thus the same current can be driven by two different electric fields, E 1 and E 2 . This allows an equilibrium field profile to be obtained, where E changes abruptly from E 1 to E 2 at some intermediate radius, while maintaining current continuity. It is shown unambiguously that a continuous electric field profile becomes unstable when the applied voltage exceeds a critical value, the instability being driven by negative incremental resistivity. The profile then bifurcates and forms a branch with a discontinuous electric field. Qualitatively, the predicted behaviour agrees well with that observed in tokamaks where an H mode has been induced using a biased electrode. Quantitative comparisons with TEXTOR results are made and the predictions are found to be in qualitative agreement with measurement. The implications for spontaneous L-H transitions are discussed