Electrode and Langmuir probe tools used for flow damping studies in the Helically Symmetric Experiment

Abstract

A system of electrodes and Langmuir probes has been developed for the measurement of plasma flow damping in the Helically Symmetric Experiment (HSX) stellarator. A biased electrode is used to apply a J Ã B torque to the plasma. The fast switching electrode power supply allows the electrode voltage to be applied in ϳ1 s, which is much faster than any of the plasma time scales; the electrode current is terminated on a similar time scale at the end of the electrode pulse. A pair of multitipped Mach probes have been designed to measure the plasma flows in a magnetic surface with good spatial and time resolution ͑⌬t Ͼ 20 s͒. The unmagnetized model by Hutchinson is used to analyze the Mach probe data, and radial force balance is found to be well satisfied during electrode bias. These probes allow for measurements of the transient response of the plasma flows and floating potential when the electrode is energized. An example of using the system for the estimation of viscous damping times in HSX is provided.

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

“…15 To measure the changes in the plasma flow and floating potential when the electrode is energized, a set of Mach probe diagnostics have been developed. 15 The probes, which are similar to the Gundestrup probes in the Tokamak de Varennes 16 and Texas Experimental Tokamak, 17 have six tips facing outward from the insulating body of the probe. The six tips are biased to ion saturation current ͑I sat ͒ at −180 V using battery packs.…”