Coherent Density Fluctuations in the HSX Stellarator

A multi-channel interferometer system is used to measure equilibrium density profile and its fluctuations in the HSX stellarator. Low-frequency, coherent density fluctuations are observed in certain quasi-helically symmetric (QHS) plasma conditions and has characteristic frequency of 15kHz. The mode is observed for small displacement of the 1$^{\mathrm{st}}$ harmonic O-mode ECRH location inward from the magnetic axis. This mode is also observed on magnetic fluctuation signal, using external coils, which shows n$=$1. When HSX is operated without quasi-helical symmetry (mirror configuration), a coherent electrostatic mode at 28 kHz is observed. While the coherent mode in QHS plasmas shows ballooning effect, the coherent mode in Mirror plasma exhibits an anti-ballooning characteristic. Mode radial structure can be obtained from inversion of interferometer measurement when the m number is known. Under certain Mirror conditions, the coherent modes display strong bi-coherence on Langmuir probe signals. Detailed characterization of the observed coherent modes will be reported and their identification will be explored. \textit{*Supported by USDOE grants DE-FG03-01ER54615 and DE-FG02-93ER54222.}