Influence of Gas Pressure and Accelerating Gap Length on the Emission Characteristics of Pulsed Forevacuum Plasma Electron Beam Sources

We have investigated the influence of acceleratinggap length (AGL) on the emission characteristics of a pulsed(500 mu s) forevacuum plasma-cathode electron sources based ona cathodic arc (CA) and on a constricted arc discharge (CAD) atdifferent pressure (4-30 Pa) and type of gas (Ar, N2, He). In thisresearch accelerating voltage is kept constant (8 kV). Highergas pressure and the use of Ar provide higher emission current(electron beam current) in the whole investigated range of theAGL (10-35 mm). For gas pressure below certain "boundary"pressure (25-27 Pa for He, 6 Pa for Ar and N2), an increasein the AGL leads to an increase in emission current and anincrease in average rate of rise (ARR) of emission current onthe pulse front for both discharge systems (at constant dischargecurrent). At pressure higher than the "boundary" pressure, theemission current and the ARR depend nonmonotonically onthe AGL, and at certain AGL their values reach maximum.However, in this case as the gas pressure and discharge currentincrease, the change in the emission current decreases. As aresult, the influence of the AGL on the emission current becomesinsignificant (within error bars) for Ar and N2at dischargecurrent of more than 20 A and/or gas pressure of more than 8 Pa.When using the discharge system based on the CAD, an increasein the AGL also provides a decrease in delay time for theappearance of emission current. The observed dependencies aredue to ionization processes in the accelerating gap and the beampropagation region.