Measurements and Modification of Sheared Flows and Stability on the Compact Toroidal Hybrid Stellarator

Abstract

Sheared flows arising from spatially inhomogeneous, transverse electric fields are common phenomena found in space, laboratory, and fusion plasmas. These flows are a source of free energy that can drive or suppress instabilities. In space plasmas, numerous observations of electrostatic and electromagnetic instabilities at various scale lengths have been made. By contrast, in fusion plasmas, edge localized sheared flows provide a barrier against cross field particle transport and the presence of these flows are associated with enhanced confinement regimes (H-mode). Under- standing how these flows provide enhanced confinement is of critical importance to current and future fusion experiments. This work is an experimental investigation of sheared flow generation and the corresponding response of the plasma in a stellarator type fusion device. This work is performed in the Compact Toroidal Hybrid (CTH) stellarator device. The CTH stellarator is a five field period continuously wound stellarator run with 100ms long plasmas. Pri- mary plasma generation and heating is provided through Electron Cyclotron Resonance Heating (ECRH) with a secondary Ohmic heating system. Flow experiments are performed by modifying the radial electric field by inserting a biased electrode past the last closed flux surface. Plasma parameters are measured using a Triple Probe. Plasma flows are measured using a Gundestrup Probe. Flows in CTH are studied by examining the effects that three dimensional geometries have on fluid flows. The interpretation of probe measurements in highly shaped fields is achieved by transforming laboratory space positions to magnetic flux coordinate space positions. Biasing ex- periments will modify the edge electric fields, measure the induced flows, demonstrate the role electric fields play inducing flows and measure the enhancement or degradation of plasma stabil- ity in the presence of these flows. Instabilities that arise will be identified by examining various parameter scales to narrow down the vast spectrum of plasma instabilities.