RF effects on current-driven plasma instabilities

Summary form only given. The Versatile Toroidal Facility (VTF) is a large laboratory plasma machine of 1 meter major radius used to out investigations of ionospheric plasma turbulence. LaB/sub 6/ hot-cathode electron emitters produce hydrogen plasmas of densities of 5.10/sup 17/ ions per cubic meter. Characteristics of the VTF plasma such as density gradients and field-aligned currents are in good agreement with those of the auroral and upper ionospheric regions. Spectral analysis has been performed on plasmas produced by the electron emitters. Interest has focused on the low frequencies below the lower hybrid resonance where ion acoustic and current-convective modes have been observed. Microwaves injected from a 3000 watt magnetron produce dramatic changes to the low frequency spectrum. First, the parametric decay instability intensifies the ion acoustic modes in the region of plasma heated by the microwaves. Second, the normally dominant current-convective modes are greatly suppressed in the heated region due to the oscillating electric field of the pump wave. When we probe beyond the heated region, these two pump wave effects are no longer observed, presumably because the microwaves are denied access to beyond the heated region due to the high plasma density. A theory which describes the suppression of current-convective modes in the presence of a pump wave will account for subtle differences in geometry between the ionosphere and the VTF machine. Results of recent experiments in the VTF machine and from Tromso, Norway are compared with the theory.