Spatially limited ion acoustic wave activity in low-pressure helicon discharges

Ion acoustic wave phenomena are studied and compared in two low-pressure argon discharges created by helicon sources. The wave amplitudes are spatially localized near the edge of a plasma column as the amplitudes of the "mirror waves" that are separated from the helicon source frequency by the ion wave frequency. Dependencies of the ion wave on radial position, pressure, input power, and magnetic field are investigated. Measurements of the wavelength show that the wave is traveling azimuthally at approximately the ion sound speed in the direction of electron gyration. Although the wave spectra are indicative of a parametric decay phenomenon, it seems more likely that the radial plasma pressure gradient drives an ion acoustic instability which then modulates the helicon source.