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Dissertation
Study of the plasma boundary using laser-induced fluorescence
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2020
DOI: 10.17077/etd.005310
Abstract
Sheaths play a crucial role in bounded plasmas by governing the loss of charged particles from the system. Understanding the physics of this region is critical to predicting steady-state plasma conditions and plasma-surface interactions, as well as to the interpretation of Langmuir probe and other probe-based measurements. Using Laser-Induced Fluorescence (LIF) diagnostics, I have made measurements of ion fluctuations and flow in the boundary region that are non-perturbative to the plasma. These techniques were used in conjunction with probe-based diagnostics to investigate two unresolved problems in low-temperature plasma physics: the affect of dielectric material surrounding a positively biased electrode surface, and the nature of ion-electron streaming instabilities in the presheath region near positively and negatively biased electrodes. The first measurement explored how the geometry of dielectric material surrounding a positively biased electrode effects the ion flow and sheath structure near that electrode. Results were compared with 2D particle-in-cell (PIC) simulations made by Brett Scheiner for this research. I found that dielectric material has a significant effect, producing a strong ion flow around 0.5 times the ion sound speed toward the positive electrode and forming a virtual cathode structure in the plasma potential. These measurements suggest that the ion presheath from the dielectric material surrounding the electrode encloses the electron sheath from the positive electrode, resulting in a virtual cathode that substantially influences the ion flow profile near the electrode. In the second measurement, standard linear theory predicts the presence of ion-acoustic instabilities when the ion and electron populations have a large enough flow-shift with respect to each other. This was tested using LIF measurements of ion fluctuation power spectra for both ion and electron sheaths, each of which have been predicted to result in unstable presheath flows. Narrow-bandwidth ion fluctuations were observed near 0.46fpi (460 kHz) for both sheath polarities, where fpi is the ion plasma frequency. The observed fluctuations were significantly stronger in the electron sheath case and were observed over a larger volume of plasma. Fluctuations were measured in the ion sheath case at locations far enough from the sheath that linear theory predicts stability, suggesting that ion-acoustic instabilities generated in the relatively small unstable region near the boundary reflect from the sheath and perturb a much larger volume of the plasma. This is expected to affect probes with both ion and electron rich sheaths, meaning that any dc biased probe may effectively act as an ion-acoustic wave antenna under certain conditions. These measurements are consistent with recent theory and particle-in-cell simulations.
A significant portion of this thesis involves the development and construction of the experimental device and diagnostics with which these measurements were made. This includes the vacuum system, LIF collection optics, plasma source, mechanical and electrical vacuum feedthroughs, as well as the diagnostic probes: Langmuir, emissive, and electric field.
Details
- Title: Subtitle
- Study of the plasma boundary using laser-induced fluorescence
- Creators
- Ryan Thomas Hood
- Contributors
- Frederick N Skiff (Advisor)Scott D Baalrud (Committee Member)Robert L Merlino (Committee Member)Steven R Spangler (Committee Member)Edward V Barnat (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Physics
- Date degree season
- Spring 2020
- Publisher
- University of Iowa
- DOI
- 10.17077/etd.005310
- Number of pages
- xvii, 130 pages
- Copyright
- Copyright 2020 Ryan Thomas Hood
- Comment
- This thesis has been optimized for improved web viewing. If you require the original version, contact the University Archives at the University of Iowa: https://www.lib.uiowa.edu/sc/contact/
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 126-130).
- Academic Unit
- Physics and Astronomy
- Record Identifier
- 9983949696002771
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