PLASMA CONVECTION INDUCED BY TOROIDAL ASYMMETRIES OF THE DIVERTOR PLATES AND GAS PUFFING

In the open held line region of the scrape-off layer (SOL), the plasma potential is to a considerable degree determined by the boundary cond itions on the divertor plates. The idea is explored here that, by maki ng the divertor plates slightly 'wavy' in the toroidal direction, toro idally varying potentials can be produced throughout the SOL and there by convective plasma motion induced. This motion should lead to a broa dening of the SOL and to a reduction of the heat load on the divertor plates. An attractive feature of this technique is that it induces str ong convection only on the open held lines without causing enhanced tr ansport inside the separatrix. The possibility is also considered of i nducing plasma convection by varying the composition of the divertor p late in the toroidal and radial directions, so as to affect the plasma potential by virtue of the toroidal variation of the secondary emissi on coefficient. Mutual biasing of adjacent elements of the plates is t he third option. Finally, the use of toroidally asymmetric gas puffing to induce toroidal asymmetry in the potential distribution is discuss ed.