Numerical modelling of limiter biasing experiments on TEXTOR-94

The 2D multifluid code TECXY has been used to model the experimental biasin g 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 infl uenced 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 p otentials for the case of positive biasing. This allows to simulate suffici ently well the experimental current-voltage characteristics for either bias ing and also reproduces the essential features and trends of the observed p lasma profiles in the SOL. A moderate negative biasing is found to be optim al for the exhaust efficiency of the pump limiter.