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 accelerating gap length (AGL) on the emission characteristics of a pulsed ( $500~\mu $ s) forevacuum plasma-cathode electron sources based on a cathodic arc (CA) and on a constricted arc discharge (CAD) at different pressure (4–30 Pa) and type of gas (Ar, N2, He). In this research accelerating voltage is kept constant (8 kV). Higher gas pressure and the use of Ar provide higher emission current (electron beam current) in the whole investigated range of the AGL (10–35 mm). For gas pressure below certain “boundary” pressure (25–27 Pa for He, 6 Pa for Ar and N2), an increase in the AGL leads to an increase in emission current and an increase in average rate of rise (ARR) of emission current on the pulse front for both discharge systems (at constant discharge current). At pressure higher than the “boundary” pressure, the emission current and the ARR depend nonmonotonically on the AGL, and at certain AGL their values reach maximum. However, in this case as the gas pressure and discharge current increase, the change in the emission current decreases. As a result, the influence of the AGL on the emission current becomes insignificant (within error bars) for Ar and N2 at discharge current of more than 20 A and/or gas pressure of more than 8 Pa. When using the discharge system based on the CAD, an increase in the AGL also provides a decrease in delay time for the appearance of emission current. The observed dependencies are due to ionization processes in the accelerating gap and the beam propagation region.