Electron Emission Characteristics of High Temperature Discharge Cathodes in Magnetized Plasmas

The characteristics of electron emission from 0.03, 0.05 and 0.06 cm diameter tungsten filaments were studied to evaluate the performances as discharge cathodes for a magnetized plasma device. Thicker filaments required higher heating current but lower filament temperature to produce the same discharge current as a thinner filament. For 0.03 and 0.05 cm diameter filaments, the spiral shape filaments provided higher discharge current at the same temperature than the hairpin shape filament. When the filament diameter was 0.06 cm, there was no difference in the discharge current between the two filament shapes. Presence of an external magnetic field increased the discharge current for both types of filaments. The direction of the external magnetic field caused no effect on the discharge current emitted from the hairpin shape filament. For a spiral shape filament, the magnetic field produced inside of the spiral by filament heating current changed the electron emission characteristics due to a coupling to the external field. The field anti-parallel to the direction of the external magnetic field yielded higher discharge current than the parallel field.