Semester

Spring

Date of Graduation

2003

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mark E. Koepke.

Abstract

The objective of this work is the experimental study of the effect of parallel-ion-velocity shear on the destabilization and propagation of electrostatic ion waves. Shear in the magnetic-field-aligned (parallel) ion drift velocity of a single-drifting Maxwellian ion velocity distribution is created in the cylindrical barium plasma column produced in a Q machine. Standard, emissive, and single-sided Langmuir probes are used to measure plasma parameters and their fluctuations. Direct, noninvasive, measurements of the ion velocity distribution (yielding ion drift velocity and ion temperature) are performed using the laser-induced fluorescence technique. The use of laser-induced fluorescence to determine the radial profile of parallel ion drift velocity permits the measurement of the ion parallel-velocity shear without the weighting by the ion density contributions that are inherent in particle-flux-averaged measurements. Using an almost isotropic-temperature plasma, multi-harmonic ion-cyclotron waves are identified, characterized, and compared with theoretical predictions. Experimental evidence is presented that ion cyclotron damping can become inverted to result in net growth for sufficiently small values of the wavevector components ratio, k∥/k⊥, in the presence of shear in the parallel ion flow. Cases where, for larger values of k∥/k⊥ , the damping reduces without changing sign are presented. For other experimental conditions, low-frequency ion acoustic waves are identified, characterized and compared with theory. Their growth and propagation are measured in anisotropic plasma. Evidence is presented that shear in the parallel ion flow increases the wave frequency, that this increase is responsible for reducing ion Landau damping, and that increasing ion-temperature anisotropy both increases the growth rate and decreases the preferred propagation angle of these ion-acoustic waves. These experimental results serve to benchmark a theoretical model that predicts both shear-modified ion cyclotron and shear-modified ion acoustic waves. The results of this work provide experimental support for the inhomogeneous-parallel-ion-flow-model interpretation of multi-harmonic electrostatic ion-cyclotron waves and ion-acoustic waves observed in the auroral region. The results also broaden fundamental plasma physics concepts such as ion-cyclotron damping to include the possibility of inverse ion-cyclotron damping.

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