Current density and time resolved emittance measurement of an electron beam extracted from a plasma edge cathode
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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 extemal electric forces as long as the extraction is space charge limited. Thus, it should be possible to avoid the plasma closure of the extraction gap and to obtain an extracted electron current density over 100 AJcnfi. Experimental electron beam density measurements support this theoretical prediction. For an electron beam with a diameter of about 2.5 cm the extraction current is smooth and electron current densities up to 100 A/cm were obtained repeatedly with a duration up to 2.5 jis for an extraction gap of 4 mm. For a 7.5 cm diameter beam an electron beam current density was measured from 2 to 7 A/cm^ at an increased extraction gap of 15 mm. 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 pattem on the scintillator. The images were recorded with a framing camera. Then they were digitized with a scanner and a computer code determined the emittance. A two-frame intensifier camera with an exposure time down to 5 ns was built to record the electron beam pattem. 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 pattem was irregular and indicated some instability of the plasma surface. The measured normalized emittance is on the order of EJ,^^ = 5 X 10"^m^ rad^. The normalized brightness was found typically to be B = 3 X10^ A m~^ rad"^. The trace space density measurements show a nearly parallel electron beam. A major difficulty with the emittance measurements was the limited reproducibility from shot to shot.