Semester

Summer

Date of Graduation

2009

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mark E. Koepke.

Abstract

It was previously found that sheared ion-flow alters the dispersion of waves in plasma [Koepke et al., 1994, 1998a]. Many aspects of sheared ion-flow effects were demonstrated for ion-acoustic waves [Teodorescu et al., 2002a], ion-cyclotron waves [ Teodorescu et al., 2002b; Koepke et al., 1999; Amatucci et al., 1998] and drift waves [Kaneko et al., 2003]. This dissertation further investigates ion-cyclotron waves and ion-acoustic waves in the WVU Q machine plasma in the presence of sheared parallel (magnetic-field aligned) ion-flow and further investigates ion-cyclotron waves in the WVU Q machine plasma in the presence of sheared perpendicular ion-flow.;Experiments were performed to measure the time evolution of the density profile, plasma-potential profile and ion-velocity distribution function is measured, upon onset of the wave. The wave is observed to heat the ions perpendicular to the magnetic field. Evidence is presented that wave-particle resonance is responsible for this heating. Because it increases the perpendicular ion-temperature, this wave-particle interaction is observed to decrease the shear in the perpendicular ion-flow and decrease density gradients that coexist with the gradient in plasma potential. Comparison to the predictions of a theory for the inhomogeneous energy-driven instability [Ganguli et al., 1985; Gavrishchaka et al., 1996; Koepke et al., 1994; Amatucci et al., 1996] indicates that the final equilibrium state is considered stable if ion-temperature anisotropy is not included but unstable when it is included. This interpretation is supported by the observation that the damping rate, measured immediately after reducing suddenly the parallel electron drift velocity below threshold, decreases with increasing ion-temperature anisotropy. This work illustrates the importance of including ion-temperature anisotropy in the interpretation of plasma equilibrium, for example, in the case where ion-temperature anisotropy is observed to exist in the ionosphere and magnetosphere.;Previously ion-cyclotron waves were observed to spontaneously grow also in the presence of sheared parallel ion-flow and small values (nu de << (me/mi) ½nute) of parallel electron-drift-velocity [Teodorescu et al., 2002]. For large values of shear dViz/dx ≈ Oci, waves were shown to appear at multiple harmonics of the ion gyro-frequency. Here, spectral analysis of many time series measurements indicates that the relative phase between the individual harmonics is random, with random scattering much larger than the individual error bars, thus suggesting that each harmonic is independently excited. For a case of a lower value of parallel-velocity shear, two distinct spectral modes, with characteristics consistent with predictions from theory [Gavrishchaka et al., 2000] made subsequent to the observation, are simultaneously excited. One mode is observed above the local ion-cyclotron frequency while the other is observed below the local ion-cyclotron frequency. These two modes are shown to have equal but opposite values of azimuthal wavenumber, consistent with m = +/-1 azimuthally propagating modes. Comparison to theoretical predictions suggests that the frequency and azimuthal wavenumber of each mode is such that ion-cyclotron damping was reduced below its zero-shear value.;Previously electrostatic ion-acoustic waves were observed to arise spontaneously in the presence of small values (nude ≤ nute) of parallel electron-drift-velocity, and shear in the parallel ion-drift-velocity [Agrimson et al., 2001; Teodorescu et al., 2002a]. Here, the growth rate and propagation characteristics of the ion-acoustic waves are measured in the presence of a controllable density gradient and in the presence of controllable ion-temperature anisotropy and compared to predictions from theory [Gavrishchaka et al., 1998; Spangler, 2002]. The presence of a density gradient is shown experimentally to have a destabilizing influence on the equilibrium and to cause the waves to propagate at a larger angle relative to the magnetic-field direction. The presence of ion-temperature anisotropy is shown to have the opposite effect and to decrease the growth rate of the ion-acoustic wave.;The application of these results to observations in the ionosphere and magnetosphere, where sheared flow, density gradients, ion-temperature anisotropy and electrostatic waves are often simultaneously observed, is discussed with emphasis on mode propagation characteristics needed for interpretation of space observations.

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