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Cathode plasmas in pulsed high current vacuum diodes have been studied using optical interferometry and spectroscopy. Both aluminum and graphite cathodes were used and the diode current density was varied over a factor of ten. The cathode plasma inventory was seen to increase during the length of the pulse and the plasma density was seen to increase with increasing current density. Spectral line emission from H, CI, CII, and CIII was observed when either cathode was used. It is concluded that cathode plasma expansion is dominated by protons from cathode surface contaminants.