Role of poloidal-pressure-asymmetry-driven flows in L-H transition and impurity transport during MGI shutdowns

Abstract

Poloidal asymmetries in tokamaks are usually investigated in the context of various transport processes, usually invoking neoclassical physics. A simpler approach based on magnetohydrodynamics (MHD), focusing on the effects rather than the causes of asymmetries, yields useful insights into the generation of shear flows and radial electric field. The crucial point to recognize is that an MHD equilibrium in which the plasma pressure is not a flux function can be maintained only by contributions from mass flows. Coupling between the asymmetry-generated forces and toroidal geometry results in a strongly up-down asymmetric effect, where the flows exhibit a strong dependence on the location of the asymmetry with respect to the midplane. This location-dependence can be used as an effective control mechanism for the edge and thus the global confinement in tokamaks. It can also explain a number of poorly-understood observations. For instance, strong dependence of the low to high (L-H) confinement transition power threshold $P_{LH}$ on the magnetic topology can be qualitatively explained within this framework. Similarly, upper-lower midplane dependence of the poloidal flow direction after massive gas injections (MGI) naturally follows from this discussion. Similar arguments suggest that the ITER fueling ports above the midplane, to the extent they can generate a positive pressure asymmetry at the edge, are misplaced and may lead to higher input power requirements.