Date of Graduation

1994

Document Type

Dissertation/Thesis

Abstract

This laboratory investigation documents the influence of transverse, localized, dc electric fields (TLE) on the excitation of ion-cyclotron waves driven by magnetic-field-aligned current (FAC) in a Q-machine plasma device. A segmented disk electrode, located on axis at the end of the plasma column, is used to independently control TLE and FAC in the plasma (potassium plasma, {dollar}n\\approx 10\\sp9{dollar} cm{dollar}\\sp{lcub}-3{rcub}{dollar}, {dollar}\\rho\\sb{lcub}i{rcub}{dollar} {dollar}\\approx{dollar} 0.2 cm, {dollar}T\\sb{lcub}e{rcub} = T\\sb{lcub}i{rcub}{dollar} {dollar}\\approx{dollar} 0.2 eV). Ion-cyclotron waves have been characterized in both the weak-TLE and large-FAC regime and the strong-TLE and small-FAC regime. The existence of a new category of oscillation identified as the inhomogeneous energy-density driven (IEDD) instability is verified based on the properties of the waves in the latter regime. In the weak-TLE regime, current-driven electrostatic ion-cyclotron (CDEIC) waves with features in qualitative agreement with previous laboratory results have been observed at sufficiently large FAC. These waves have a frequency spectrum with a single narrow spectral feature located slightly above the ion-cyclotron frequency ({dollar}\\omega\\approx 1.2\\Omega\\sb{lcub}\\rm i{rcub}){dollar}. The waves are standing in the radial direction with peak oscillation amplitude located in the center of the FAC channel and are azimuthally symmetric (m = 0). Small magnitude TLE were found to have negligible effect on the characteristics of the waves. In the strong-TLE regime, a decrease in the threshold FAC level is observed. This transition in the instability threshold is accompanied by changes in the frequency spectra, propagation characteristics, and mode amplitude profiles. In the presence of strong-TLE, the ion-cyclotron waves propagate azimuthally in the {dollar}\\rm\\vec{lcub}E{rcub}\imes \\vec{lcub}B{rcub}{dollar} direction with {dollar}k\\sb\heta\\rho\\sb{lcub}i{rcub} = 0.4{dollar} and m = 1. The frequency spectrum becomes broadband and spiky, and shifts with the applied TLE strength. In contrast to CDEIC waves, the IEDD wave spectrum can extend to frequencies below {dollar}\\Omega\\sb{lcub}\\rm i{rcub}{dollar}. The waves have peak oscillation amplitude located off-axis in the region of maximum TLE.

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