Measurements and modeling of plasma flow damping in the Helically Symmetric eXperiment

Abstract

Fusion Technology 27, 273 ͑1995͔͒ using a biased electrode to impulsively spin the plasma and Mach probes to measure the rotation. There is a distinct asymmetry between the spin-up when the bias is initiated and relaxation when the electrode current is broken. In each case, two time-scales are observed in the evolution of the plasma flow. These observations motivate the development of new neoclassical modeling techniques, including a new model where the fast increment of the electric field initiates the spin-up process. The flow in the quasisymmetric configuration rises more slowly and to a higher value than in a configuration with the quasisymmetry broken, and the rise time-scale is in reasonable agreement with the neoclassical spin-up model. The flows decay more slowly in the quasisymmetry configuration than in the configuration with the quasisymmetry broken, although the decay rates are significantly faster than the neoclassical prediction.

Experimental Tests of Quasisymmetry in HSX

“…More details are given in the papers by Gerhardt [20] and Canik [21]. A common feature of the experiments is that a reduction in neoclassical transport could be observed by altering the magnetic field spectrum, in approximate agreement with theory, but in each case there remained a large level of anomalous transport that didn't appear to change between the configurations.…”

Flow Velocity Measurements Using ChERS in the HSX Stellarator

“…HSX has a helical direction of approximately constant magnetic field strength. This leads to reduced flow damping in the helical direction of symmetry [8]. In this paper ChERS measurements of large intrinsic flows in the direction of symmetry for HSX are shown for the first time.…”

Edge turbulence measurements in electron-heated Helically Symmetric Experiment plasmas

“…Ion temperatures measured by impurity Doppler spectroscopy are T i Ϸ 20 eV. 17 While an ion temperature profile measurement is not available, the balance of collisional electron-ion energy exchange with ion-neutral charge exchange is consistent with negligible ion transport, hence little expected ITG. The electron temperature and density place the electrons deep in the long mean free path regime across the entire minor radius.…”