Analytical and Experimental Studies of Thermionically Emitting Electrodes in Contact with Dense, Seeded Plasmas

Citation

Koester, John Kent (1970) Analytical and Experimental Studies of Thermionically Emitting Electrodes in Contact with Dense, Seeded Plasmas. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/VK0B-6955. https://resolver.caltech.edu/CaltechTHESIS:04202011-112053777

Abstract

The electrode current-voltage characteristics and the limiting cathode current density for thermionically emitting electrodes in contact with high-pressure, seeded, non-equilibrium MHD plasmas were determined analytically and experimentally. The theoretical model was based on the coupling of the adsorption phenomena of alkali metal seed particles onto electrode surfaces with that behavior due to the plasma in the electric boundary layer adjacent to the electrode surface. The desorption rates of electrons and seed atoms and ions (which were given by quoted functions of the surface temperature, the surface degree of coverage, and the electric field at the surface) were related to the appropriate boundary conditions for the governing continuum-type plasma equations in the electric boundary layer. An algorithm was given for the simultaneous solutions of both the surface state and the electric boundary layer. Machine computed results were presented for a potassium-seeded argon plasma (at 2000°K and one atmosphere pressure) in axially-symmetric stagnation flow over a tungsten electrode and displayed the effects of varying the parameters: surface temperature and seed fraction. These results indicated an order of magnitude increase in the thermionic limiting current density due to coupling effects. Experimentally, the electrode phenomena were studied in a non-equilibrium discharge produced in the same plasma-electrode system as used for the machine computations. The electrode behavior was determined from voltage probes, photomultiplier measurements, and photographs. As the thermionic limit was approached, a transition to an arc mode of cathode operation occurred at some breakdown voltage drop. The experimental current-voltage characteristics for the cathode agreed with the machine results, and the predicted enhancement of the thermionic limiting current density was observed experimentally.

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