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

Using toroidally-spaced Langmuir probes, long-range fluctuation correlations have been measured in floating potential signals during biased discharges in the HSX stellarator. The increase in long-range correlations during biasing occurs in the floating potential signals, but not in the ion saturation current signals. This has been linked to zonal flow formation in the TJ-II stellarator, both in biased discharges and in naturally occurring improved-confinement discharges [1]. Measurements of the auto-bicoherence of the poloidal electric field signals show an increase in broadband 3-wave coupling during biasing, which is analyzed and compared to both biased and naturally occurring enhanced-confinement discharges in TJ-II [2]. Additional measurements of fluctuation moments in HSX are also presented.\\[4pt] [1] M.A. Pedrosa, et al, Phys. Rev. Lett. 100 (2008) 215003.\\[0pt] [2] B.Ph. van Milligen, et al, Nucl. Fusion 48 (2008) 115003