Bifurcation of the radial electric field in the presence of edge polarization in tokamaks

In guiding-center simulations of a tokamak plasma edge, bifurcations of bot h the radial electric field and the electrode current are observed for a th reshold value of the electrode bias voltage, in qualitative agreement with experiments. The simulations are neoclassical and Monte Carlo based, also c ontaining the perpendicular viscosity effect. Above the voltage threshold, an inward moving, solitary radial electric field structure is generated wit h a concomitant drop in the electrode current. The position, width, and hei ght of the soliton depend on the magnitude and radial variation of the cond uctivity (and, thus, on local plasma parameters), finite orbit effects, and also on the perpendicular viscosity and neutral density. The obtained E-r structures are narrower and higher than the ones measured in the edge polar ization experiments in the Julich tokamak TEXTOR-94 [S. Jachmich , Plasma P hys. Controlled Fusion 40, 1105 (1998)]. (C) 2001 American Institute of Phy sics.