FORMATION AND EVOLUTION OF A WALL PROTECTIVE PLASMA SHIELD IN A HARD DISRUPTION

The behaviour of the material ablated from the wall in a hard disruption is analysed, in order mainly to ascertain its capability of protecting the wall from more severe damage. The investigation is limited to the fast first phase of the disruption, during which most of the bulk plasma thermal energy is given off. Within the ablated material an interaction region and a transmission region are defined. The first region (the 'virtual limiter') is where the bulk plasma thermal energy is dumped and the associated kinetic energy flux changed to radiation flux. The second region (the 'plasma shield') is where the radiative energy flux is attenuated through interaction with ablation matter, thus preventing more serious injuries to the wall. The whole problem is investigated in terms of a simplified mathematical model, which is shown to have a similarity solution. By making further approximations, an explicit solution is derived in full analytical terms. Applications are made to ITER type and IGNITOR type tokamaks.