AN INTERPRETATION OF LOW RADIATION COLLAPSES IN STELLARATOR PLASMAS

Radiative collapses in the Advanced Toroidal Facility (ATF) torsatron have been studied using the predictive transport code Proctr for reali stic shots with moderate radiation levels (P-rad/P-in approximate to 3 0%). Simulations to study the influence of density and of different im purity species on the occurrence of thermal collapses have been carrie d out and it has been found that the impurity cooling rate profiles pl ay a decisive role. A local breaking of the power balance is suggested as the possible origin of the thermal collapse and, once the collapse has started, the rate of increase of the local energy lost by radiati on seems to be the important parameter. The lack of efficiency in the radial energy transfer to sustain the local energy balance in the regi ons with enhanced losses leads to plasma collapse.