THE PROBLEM OF PLASMA DENSITY INCREASING IN THE U-3M TORSATRON AFTER RF HEATING TERMINATION

INTRODUCTION One of most interesting effects that is observed during RF heating of a low density plasma in the U-3M torsatron is a chord-averaged plasma density increase after the heating termination [1]. There are three different versions explaining this effect. (1) The RF heating excites strong plasma instabilities causing a particle loss from the confinement volume. After heating termination the loss reduction results in the plasma density increase [2, 3]. (2) The RF heating gives rise to ionization of fuelling gas molecules outside the confinement volume resulting in a reduction of the flow of this gas inflowing into the confinement volume. After heating termination the RF screening vanishes and the fuelling gas is free to inflow from the region beyond the poles of the helical winding [4]. (3) A reduction of the toroidal plasma current with RF heating termination results in a toroidal electric field to occur. Under the action of this field, the inward drift of trapped particles and density rise come about [5].