Numerical simulation of plasma profile changes in TEXTOR by externally driven radial polarization currents

An improved version of the 2D multifluid code TECXY is used to simulate polarization or biassing experiments on TEXTOR tokamak. In the numerical model the condition of globally ambipolar radial transport in the transition layer is replaced by externally driven radial polarization currents. They are produced by inserting and biassing an electrode with its tip a few centimeters inside the separatrix. The ensuing additional potential drop in the transition layer and the concomitant plasma profile changes are calculated for both signs of polarization. A comparison of modelling results with experimental measurements is presented. The fluid model reproduces some features of the L-H-like transition, but not the locally very high electric fields after bifurcation which are probably related to kinetic effects at very low collisionality.