Recent advances in quasi-poloidal stellarator physics issues

Abstract

The quasi-poloidal stellarator (QPS) hybrid has been developed using a stellarator optimization approach that has proven to be compatible with both low aspect ratio and significantly reduced neoclassical transport relative to anomalous levels. A unique characteristic of this type of quasi-symmetry is a reduced viscous damping level for poloidal plasma flows. Since the plasma-generated E × B and diamagnetic flows are nearly poloidal, minimal parallel flows (and viscous stress) are required to achieve parallel pressure balance in comparison with configurations such as the tokamak, in which the plasma induced flows are nearly perpendicular to the direction of minimum viscosity and relatively larger parallel flows are required. In addition to this impact on neoclassical flows it is also anticipated that quasi-poloidal symmetry will minimize resistance to self-organized plasma-turbulence-driven shear flows and ease access to enhanced confinement states. In order to test these and other transport issues, the QPS device has been designed with a high degree of flexibility by allowing variable current capability not only in its vertical and toroidal coilsets but also in each separate modular coil group. Numerical optimizations have demonstrated that this flexibility can be used not only to modify transport properties, such as the poloidal viscosity, but also to directly suppress magnetic islands.