Numerical Modelling of Limiter Biasing Experiments on TEXTOR-94

The 2D multifluid code TECXY has been used to model the experimental biasing of the toroidal belt limiter ALT-II on the tokamak TEXTOR-94 with respect to the first wall. It is well known that the edge flow pattern can be influenced by the poloidal electric drifts from imposing radial electric fields. The modelling with TECXY introduces imprinted bias currents in the scrape-off layer for the case of negative (limiter) biasing, and imprinted bias potentials for the case of positive biasing. This allows to simulate sufficiently well the experimental current-voltage characteristics for either biasing and also reproduces the essential features and trends of the observed plasma profiles in the SOL. A moderate negative biasing is found to be optimal for the exhaust efficiency of the pump limiter.