Plasma convection induced by toroidal asymmetries of the divertor plates and gas puffing

In the open field line region of the scrape-off layer (SOL), the plasma potential is to a considerable degree determined by the boundary conditions on the divertor plates. The idea is explored here that, by making the divertor plates slightly 'wavy' in the toroidal direction, toroidally varying potentials can be produced throughout the SOL and thereby convective plasma motion induced. This motion should lead to a broadening 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 strong convection only on the open field lines without causing enhanced transport inside the separatrix. The possibility is also considered of inducing plasma convection by varying the composition of the divertor plate in the toroidal and radial directions, so as to affect the plasma potential by virtue of the toroidal variation of the secondary emission coefficient. Mutual biasing of adjacent elements of the plates is the third option. Finally, the use of toroidally asymmetric gas puffing to induce toroidal asymmetry in the potential distribution is discussed.