Experimental investigation of electron beam formation in high-voltage low-pressure linear discharges

Energetic axial electron beams have been observed in a linear Z-pinch with a cathode recess operating in the pseudo-spark regime. The device is driven by a fast Marx generator. Target x-ray induced scintillation and Faraday cup measurements have been used for beam diagnostics. Experimental results in air at pressures between 5 to 100 mTorr and applied voltages 40-50 kV show two characteristic regimes of electron beam generation. At low p.d values a high energy electron beam is observed without electric breakdown of the gas. At higher values of the p.d product a second electron beam is seen to follow after a quiescent period, leading to gas breakdown and subsequent voltage collapse. The second beam is of lower characteristic energy but of much higher current than the first one. The time between the two events reduces until merging as pressure increases. At breakdown a multicomponent beam is observed with a high energy pulse just before the onset of the discharge current. This is followed by a low energy high current pulse which coincides with the voltage collapse. Experimental evidence indicates that the first beam is associated with the cathode recess and the second one is related to the gas breakdown mechanism. (author). 11 refs, 6 figs