Current Density and Time Resolved Emittance Measurement of AN Electron Beam Extracted from a Plasma Edge Cathode.

The Plasma Edge Cathode concept can produce high electron current densities while avoiding or delaying plasma closure of the extraction gap of an electron source. A plasma beam is generated by a surface flashover from a spark plug. An obstacle partially intercepts the emitted plasma jet at a distance of approximately 200 mm from the plasma source and causes a stationary transverse plasma boundary from which electrons are extracted. The plasma should experience negligible external electric forces as long as the extraction is space charge limited. Thus, it is theoretically possible to avoid the plasma closure of the extraction gap and to obtain an extracted electron current density over 100 A/cm^2. Experimental electron beam density measurements support this theoretical prediction. For small diameter electron beams with the anode area of approximately 5 cm^2, the extraction current is smooth and electron current densities up to 100 A/cm^2 were obtained repeatedly with a duration up to 2.5 mus. For a large-area anode of about 40 cm^2, an electron beam current density was measured from 2 to 7 A/cm^2. The pepper-pot method was used to measure the emittance of the extracted electron beam. An extraction gap of 15 mm together with an extraction voltage less than 20 kV yielded a relatively smooth pattern on the scintillator. The images were recorded with a framing camera. Then they were digitized with a scanner and the computer code determined the emittance. A two-frame intensifier camera with an exposure time down to 5 ns was built to record an electron beam pattern. A 50 ns exposure time was used to determine the effective emittance. Approximately half of the 100 shots taken had a suitable exposure and separated spots for evaluation. At higher extraction voltages the pattern was irregular and indicated some instability of the plasma surface. The measured emittance is on the order of 7 times10^{-6} m^2 rad^2. The brightness was found typically to be 1.5times10^5 A/m^{-2} rad ^{-2}. The trace space density measurements show nearly parallel electron beam. A major difficulty with the emittance measurements was the limited reproducibility from shot to shot.