Date of Award

Spring 2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Committee Director

Leposava Vuskovic

Committee Member

Svetczar Popovic

Committee Member

Alexander L. Godunov

Committee Member

James L. Cox, Jr.

Committee Member

John B. Cooper

Abstract

A partially ionized gas is referred to as either a plasma or a discharge depending on the degree of ionization. The term discharge is usually applied to a weakly ionized gas, i.e. mostly neutrals, where as a plasma usually has a larger degree of ionization. To characterize a discharge the plasma parameters, such as the rotational temperature, vibrational temperature, and electron density, must be determined. Detailed characterization of supersonic flowing discharges is important to many applications in aerospace and aerodynamics. One application is the use of plasma-assisted hydrogen combustion devices to aid in supersonic combustion. In conditions close to the real combustion environment, a cylindrical microwave cavity was used to study the effects of hydrogen and air admixtures to plasma parameters in an argon supersonic flowing discharge. Argon and hydrogen were chosen since their atomic and molecular structure are well documented in the literature. In addition, argon, as a noble gas, will help to decrease the penalty from ionization. However, the presence of hydrogen, nitrogen, and oxygen molecules leads to complex branching inter-radical chemistry, which may result in the decrease of the degree of ionization and the loss of combustion enhancing radicals. A qualitative description of the ionization loss was the main goal of this thesis. To complement the experiments a gas kinetic model was developed to explore the extent of ionization loss due to the addition of hydrogen and air.

The second goal of this thesis is to develop a supersonic flowing microwave discharge to validate Martian atmospheric entry models and explore the prospect of harvesting Martian entry plasma. The interactions between the Martian atmosphere and the Mars Landers have been a challenging issue from the very beginning of Mars exploration. During the entry phase, the friction between the atmosphere and probes cause thermal ionization and heating of the surrounding gas. An atmospheric and kinetic model was developed for Martian atmospheric entry plasma based on the existing Mars data. The entry plasma parameters vary considerably depending on the spacecraft's trajectory. In addition, we found that variations in the entry plasma composition were considerable and have to be included in various future harvesting schemes.

The experimental set-up included a de Laval nozzle in conjunction with a cylindrical microwave resonance cavity to create a Mach 2 supersonic flowing microwave discharge in the following gases: (1) Ar with up to 10% hydrogen and 45% air and (2) Martian simulated mixture composed of 95.7% CO2, 2.75% N2, and 1.55% Ar. Optical emission spectroscopy was employed to perform detailed measurements of the spectra of Ar, H, CO, and N2 . The gas temperature, vibrational temperature, and electron temperature along with the electron density were determined for both types of gas mixtures. We observed a decrease in the rotational and vibrational temperatures when hydrogen and air were added to an argon discharge. From analysis of the data for a pure air discharge, we determined that this decrease was due to the mixing of the different gas species. In addition, we found that the electron temperature did not change with the power density in the discharge, but it did decrease when hydrogen was added to a pure Ar discharge.

We developed a technique for finding the electron density by using the N2 second positive system. Direct indications of ionization loss were observed in the electron density measurements taken in the Ar/H2/Air discharges. In addition, in the Martian simulated mixture we found that the electron density measurements were consistent with those predicted by the atmospheric entry model. Both the experimental and model results for both types of gas mixtures indicate that the multispecies chemisty of the gases is very important to the characterization of a supersonic flowing discharge.

DOI

10.25777/ckj0-hb08

ISBN

9781109113129

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