Emittance Measurement Of An Electron Beam Extracted From A Plasma Edge Cathode

Abstract

current density of 100 A/cm2 [1,31. The Plasma Edge Cathode concept can possibly produce high electron current densities while avoiding or delaying plasma closure of the extraction gap of an electron source. An obstacle partially intercepts the plasma jet emitted from a spark plug 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. It should be possible to avoid the plasma closure of the extraction gap and obtain an extracted electron current density over 100 A/cm2 for practical plasmas. Experimental results partially support this theoretical prediction. Electron current densities up to 40 A/cm2 were obtained repeatedly with a duration up to 2.5 ps. The pepper-pot method was used to measure the effective emittance of the extracted electron beam. The images (50 ns exposure time) were digitized by a scanner and a computer code determined the effective emittance. An extraction gap of 15 mm together with an extraction voltage less than 20 kV yielded a relatively smooth pattern on the scintillator. About half of the 100 shots taken so far could be clearly scanned and evaluated. They have suitable exposure and separated spots. The effective emittance E' = 4x,, xim, and = 4y,, yims are of the order of 5 ~ 1 0 ~ m r a d = 1 . 6 ~ 1 0 ~ x m r a d with some shots down to 10-3mrad = 3 . 2 ~ 1 0 ~ x m r a d . At higher extraction voltages the pattern was irregular and indicated some instability of the plasma surface. The normalized brightness was found typically in the range from 2x 1 O6 to 2x lo7 A/m2rad2.