PROPERTIES OF A BIASED DIVERTOR SCRAPE-OFF LAYER PLASMA

The effect of biasing the divertor neutralization plates on the characteristic properties of the scrape-off layer (SOL) plasma is investigated using a 1-D fluid code along the magnetic field lines. The biasing effect is assumed to influence the e-folding density scale length lambda(e) which is then considered as a free parameter. The model includes radiative cooling in the divertor chamber, neutral reflux from the target plate and removal of neutral particles by ionization and pumping. Since pumping can only be done from the plenum region, we compute the escape probabilities to the plenum as a function of lambda(e) and then assume that a fixed fraction of the plenum neutrals is always pumped. We find that the average escape probability decreases with lambda(e). The plasma density and velocity distribution near the plate are considerably modified, however the influence on the temperature is modest. The flux multiplication factor can increase by as much as 50% for the range of lambda(e) considered. An increase in neutral pressure is possible for negative biasing.