Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Thomas K. Ishii
Joseph H. Battocletti
When a container filled with a gas is irradiated by a sufficiently strong microwave signal, an RF-type discharge occurs. It has been found that if two metal electrodes are immersed in such a produced plasma, under certain conditions, a DC potential difference develops across the electrodes, and a DC current flows through a resistor externally connected to the electrodes. This phenomenon suggests a method of microwave-to-DC energy conversion. The objective of this work was to experimentally determine factors which might be involved in the current extraction process, and to develop a theoretical model explaining the observed phenomenon.
Experimental investigations were made using mainly a commercial neon indicator lamp (type NE-2) inserted into a rectangular waveguide and irradiated by a microwave signal of 2.45 GHz. The output current was found to increase when the incident microwave power level was increased. By using an innovative lamp mount in which the electrodes are loaded by a variable reactance for microwave frequencies and a resistance for direct current, it was found that the magnitude of extracted current and its polarity could be changed by adjusting the impedance of the RF load of the lamp.
The incident microwave power required to break down the gas also varied with the RF load impedance. Under optimal conditions, the lamp was ignited at about 0.5 Watt of incident microwave power, and 2 mA DC current into a 500 Ohms DC load was obtained at 10 Watts of incident microwaves. A ratio of extracted power dissipated in the DC load to the microwave power absorbed by the discharge was found to increase with the incident microwave power level and reached about 2% at 25 Watts of incident power.
A theoretical model of the process consistent with the experimental data is proposed. It is based on the existence of a nonuniform microwave electric field distribution between extracting electrodes and the formation of unequal
plasma sheaths around electrodes
Numerical computation of the microwave electric field distribution for the particular structure of the NE-2 lamp is presented in the dissertation. The resultant field has been found to be highly nonuniform, and the position of its maximum has been determined to vary while adjusting the RF load reactance. The amplitude of the total electric field near the electrodes has been found to be several times greater than that of the incident wave. Therefore, the incident power for the gas breakdown is relatively low. The absolute value of the scattered electric field, as well as its gradient in the region between the electrodes, has been determined to be dependent on the RF impedance connected to the electrodes.
Such a nonuniform field produces a gaseous plasma characterized by an adequately nonuniform distribution of particle temperature and concentration. If the microwave electric fields at electrode surfaces are not equal, charged particles of the plasma diffuse toward the electrodes at different rates. This gives rise to different plasma sheaths around the electrodes, and when the external DC circuit is open, a difference in plasma sheath potentials appears across the electrodes. When the DC circuit is loaded, there is a flow of electrons through the external circuit from the high field electrode to the low field electrode. This process explains the origin of the extracted DC current. The value of the extracted current is determined by the microwave electric field distribution, geometry of the electrodes and discharge chamber, and properties of the gas.
The conclusion of this work is that microwave-to-DC conversion by a microwave induced plasma is possible. Although the power handling capacity and the energy conversion efficiency of the NE-2 lamp and other laboratory-built structures were not satisfactory, some design criteria have been developed to improve both of them. A high extracted current and improved efficiency can be obtained if the microwave electric field amplitude and its gradient are maximized, gas pressure is adjusted for minimum breakdown field, and extracting electrodes are large and form most of discharge chamber walls. A further increase of the extracted current can be obtained if the surface area of the high field electrode is much larger than that of the low field electrode. However, an excessive increase of the electrode surface areas ratio causes a drop in the conversion efficiency.
The findings of this dissertation regarding the microwave electric field distribution between electrodes of a NE-2 lamp can also be applied to improve the design of glow discharge microwave video detectors.