Experimental Tests of Quasisymmetry in HSX

Abstract

This paper discusses the results from experiments in HSX testing the properties of a quasisymmetric stellarator. HSX is a quasihelical stellarator with minimal toroidal curvature and a high effective transform. The high effective transform was verified from passing particle orbits as well as from the magnitude of the Pfirsch-Schlüter and bootstrap currents. The passing particle orbit shift, the helical structure of the Pfirsch-Schlüter current and the direction of the bootstrap current were all consistent with the lack of toroidal curvature. Good agreement was observed between data from plasma currents obtained by a set of magnetic pick-up coils and the results of the V3FIT code. Good confinement of trapped particles was observed with quasisymmetry. These particles may be responsible for a coherent global MHD mode that was detected during ECH at B = 0.5 T. It was found that the breaking of quasisymmetry increased the hollowness of the density profile and the damping of plasma flow while decreasing the core electron temperature, in good agreement with neoclassical models. At 0.5 T, anomalous transport appeared unaffected by the degree of quasisymmetry; more work is need to understand if this still holds at 1.0 T. The experimental energy confinement time and electron temperature profile could be reproduced reasonably well with a combination of neoclassical transport and a modified Weiland model for ITG/TEM turbulence.