Measurements of bicoherence and long-range correlations during biasing in the HSX stellarator

The effect of biasing on edge fluctuations has been studied in the HSX stellarator using Langmuir probes. Biasing leads to an increase in the bicoherence of the poloidal electric field, as well as a simultaneous increase in long-range correlations of the floating potential, measured using toroidally separated Langmuir probes. These observations are consistent with zonal flow formation. A comparison has been made between experiments in an optimized quasi-symmetric magnetic configuration (quasi-helically symmetric - QHS) and a configuration with the symmetry intentionally degraded (mirror). The observed differences are insignificant and most likely unrelated to the change in configuration.