Energy Distribution and Transfer in Flowing Hydrogen Microwave Plasmas.

This thesis is an experimental investigation of the physical and chemical properties of a hydrogen discharge in a flowing microwave plasma system. The plasma system is the mechanism utilized in an electrothermal propulsion concept to convert electromagnetic energy into the kinetic energy of flowing hydrogen gas. The plasmas are generated inside a 20 cm ID resonant cavity at a driving frequency of 2.45 GHz. The flowing gas is contained in a coaxially positioned 22 mm ID quartz discharge tube. The physical and chemical properties are examined for absorbed powers of 20-100 W, pressures of 0.5-10 torr, and flow rates of 0-10,000 mu -moles/sec. A calorimetry system enclosing the plasma system to accurately measure the energy inputs and outputs has been developed. The rate of energy that is transferred to the hydrogen gas as it flows through the plasma system is determined as a function of absorbed power, pressure, and flow rate to +/-1.8 W from an energy balance around the system. The percentage of power that is transferred to the gas is found to increase with increasing flow rate, decrease with increasing pressure, and to be independent of absorbed power. An energy transfer efficiency of 50% has been reached for the described apparatus at a flow rate of 8,900 mu-moles/sec and a pressure of 7.4 torr, and for a absorbed power range of 20-80 W. An optical spectroscopy system to measure the line intensities and widths of the plasma emission has been developed. The electron density, plasma frequency, percent ionization, atomic electronic temperature, and molecular rotational and vibrational temperatures of the plasma are determined as functions of absorbed power, pressure, and flow rate. The electron densities are found to range from 10^{12}-5 times 10^{13} cc^{-1}. The percent ionization is found to range from 0.001-0.1%. The atomic electronic temperature is found to range from 3500-6500 ^circK. The molecular rotational temperature is found to range from 225-850^ circK. The molecular vibrational temperature is found to range from 4000-17,000^circ K. A simplistic heat transfer model that characterizes the energy transfer of the plasma-wall interactions utilizing the calorimetry and spectroscopy results has been developed to lend some insight to the dominant processes involved.