Displacement of Pellet Deposited Material due to VB-induced drift

Up to now, the most suitable fuel supplying method of present tokamaks is the injection of cryogenic pellets. In fact the fuelling efficiency, defined as the fraction of the injected mass that remains in the confined plasma, is demonstrated to be much better for pellet injection than for gas puff, in particular due to the deep penetration associated with the pellet self-shielding capabilities. Nevertheless, the effectiveness of the system depends significantly on the poloidal location of the pellet launching point because the deposited material experiences a drift in the direction of the plasma major radius. The mechanism is the following. On each magnetic surface, the pellet deposits a spatially limited globule of cold and dense material where a charge separation takes place due to the electron and ion drifts in the toroidal magnetic field. Then, a vertical electric field builds up, which is cause of the drift of the high density blob towards the Low Field Side of the tokamak. This motion lasts as long as the pellet is in a situation of open-circuit. If this drift favours the matter penetration when the pellet is injected form the High Field Side of the torus, it expels it out of the discharge when the injection point is located on the Field Side.