Alfven wave studies on PRETEXT

Abstract

The properties of low frequency Alfven waves in hot, magnetically confined plasmas are quite unlike those in homogeneous media. Extensive theoretical studies have uncovered the Alfven continuum along with global Alfven eigenmodes (GAE). It has been suggested that such modes may be good candidates for rf heating below the ion cyclotron frequency. We present a unified investigation on PRETEXT of the structure of the global modes. These measurements are made using two phased toroidal antennas. The GAE are predicted to appear as resonances in the antenna loading resistance and have been observed earlier on TCA. We find that in addition to antenna resistance and inductance, signals from magnetic probes at the plasma surface also exhibit this resonant behavior. The plasma parameter dependence (i.e., the amplitude and location) of these resonancces is found in good agreement with the theory. Driven plasma density fluctuations with a rich spatial structure are predicted to develop at these resonances; we have observed this structure using a CO2 lser interferometer. Results from the laser and impedance measurements along with antenna phasing permit assignment of mode numbers. Since the two‐antenna configuration can simultaneously excite more than one mode in the plasma, the resulting interference effects demand careful interpretation of the spatial and temporal mode structures seen. These interference effects provide both a challenge and an opportunity for an optimum antenna design in an Alfven heating experiment.