Transition from electrostatic drift waves to drift-Alfven waves in a helicon plasma

Abstract

Summary form only given, as follows. The transition from drift wave to drift-Alfven wave dynamics is studied in a laboratory plasma experiment. Measurements were conducted in the new, linearly magnetized helicon plasma device VINETA. The device has a total length of 4.5 m and a diameter of 0.4 m, immersed in a set of 32 magnetic field coils, which produce a maximum magnetic field of 0.1 T on the device axis. The Argon plasma is produced by helicon wave heating. This highly efficient plasma production mechanism provides high ionization degrees (10-50%) at relatively low rf heating powers (1-2.5 kW cw). The radial plasma Gaussian-shaped density profiles in the helicon discharge are only weakly dependent on the heating power. At relatively low heating power, the plasma-B is small and the drift waves destabilized in the strong plasma density gradient region are of electrostatic nature. Increasing the rf heating power results in a density increase and thereby in increased plasma-β, which eventually exceeds the critical value of the electron-to-ion mass ratio where the drift waves are predicted to become electromagnetic due to the coupling to self-consistently excited Alfven waves. The present paper discusses recent measurements performed in both β-regimes. Equilibrium plasma parameters are recorded by electrostatic probes, which are compensated to account for the rf plasma potential variations. Density fluctuations are detected by negatively biased Langmuir probes and magnetic fluctuations are measured by Hall sensors as well as by standard Bdot probes. On the basis of single, saturated drift modes the transition from the electrostatic to the electromagnetic regime is documented by simultaneous measurements and cross spectral analysis of density and magnetic field fluctuations. In this way, the coupling between drift wave dynamics and Alfven waves is explicitly demonstrated. Attention is also paid to the feed-back effects of the magnetic fluctuations on the stability of drift waves.