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Measurements and Modeling of Plasma Flow Damping in the HSX Stellarator
| Author: | Gerhardt S.P. |
| Coauthor: | D.T. Anderson, J.N. Talmadge, J.M. Canik, and the HSX Team |
| Institution : | HSX Plasma Lab, U. of Wisconsin-Madison |
| Abstract text: | The quasi-helically symmetric (QHS) configuration of the HSX stellarator is predicted to have strongly reduced parallel viscosity compared to conventional stellarators. We use a fast switching biased electrode system to induce plasma flows and a set of Mach probes to measure the flows. Toroidal and poloidal arrays of absolutely calibrated Ha detectors allow a determination of the neutral density and the ion-neutral friction component of the flow damping. Results show that the flow evolution at bias turn-on or turn-off involves two time-scales and two directions. We model these experiments using neoclassical theory involving parallel viscosity and ion-neutral friction, including a newly developed numerical calculation of the Hamada basis vectors for the 3D HSX geometry and an inclusion of the effect of toroidally asymmetric neutrals. The relaxation phase at the end of the pulse is modeled assuming that the electrode current is terminated based on the modeling of Coronado and Talmadge; this calculation predicts two time scales for the flow to decay which can be considered the \"normal modes\" of the system. The plasma spin-up and electric field formation phase is modeled assuming that the electric field formation is the initiating event; this predicts a third time scale, intermediate to the decay time scales. The spin-up time is longer in the quasisymmetric configuration than a configuration with the quasisymmetry broken, and the time scale is consistent with the spin-up model. The flow decay rate is reduced in the QHS configuration compared to the symmetry-broken case, but is faster than the neoclassical prediction in both cases. These results illustrate the reduction of flow damping in a stellarator with quasisymmetry. |
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