Oscillation phenomena of a plasma produced by an AC driven hot cathode

Amplitude of oscillating discharge current has been measured for Bernas-type ion source equipped with an alternating current heated hot tungsten filament cathode. Measurements have been made by changing the filament diameter from 0.2 mm to 0.7 mm, and by changing the DC discharge current from 0.1 A to 1.5 A. The fluctuation amplitude of the discharge current was found decreased as the diameter of the filament was enlarged. The fluctuation amplitude took the minimum value at a frequency from several hundred Hz to 1 kHz depending upon the filament diameter and DC discharge current. For example, the relative oscillation amplitude was the smallest at 1 kHz for 0.7 mm diameter filament. Increase in discharge current had reduced the minimum oscillation amplitude, and increased the frequency at which the oscillating amplitude took the minimum. Fundamental mechanisms relevant to these characteristics of oscillation amplitude of discharge current from an AC driven hot cathode filament are discussed.