AN INVESTIGATION ON DIRECT CURRENT EXTRACTION FROM MICROWAVE INDUCED PLASMA
It has been found that if two metal electrodes are immersed in a microwave induced 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.
Experimental investigations were made using mainly a commercial neon indicator lamp inserted into a waveguide and irradiated by a microwave signal of frequency 2.45 GHz. 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. Under optimal conditions, the lamp was ignited at about 0.5 W of incident microwave power, and 2 mA DC current into 500 Ohms DC load was obtained at 10 W of incident microwaves.
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.
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 used.
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.