Plasma expansion and fast gap closure in a high power electron beam diode

Abstract

High power electron beam generation studies were carried out in a planar diode configuration to investigate the effect of the accelerating gap, diode voltage, and anode-cathode materials on the electrode plasma expansion. The diode voltage has been varied from 145–428 kV, whereas the current density has been varied from 208–2215 A/cm2 with 100 ns pulse duration. It was found that the diode voltage and current follow the bipolar space-charge limited flow model. The anode and cathode plasma expansion velocities were calculated using the perveance data. The plasma expands at 11 cm/μs for 34 mm anode-cathode gap and the plasma velocity decreases for smaller gaps. It was found that the plasma expansion velocity increases significantly due to the cathode edge contribution and the edge contribution is particularly important during the beginning and the end of the accelerating pulse when the diode voltage and the corresponding electric field are comparatively low. It was also observed that the diode current increased by a factor of 3 when anode material was released into the accelerating gap due to the electron beam bombardment.

Electron beam current in high power cylindrical diode

“…One can see from Table I that the plasma expansion velocity increases with the AK gap as observed in the case of a planar diode. 14 Figure 13 displays experimental diode current for three different AK gaps as a function of diode voltage. It is interesting to note that the initial emission length decreases with the radial AK gap.…”

Application of high speed frame camera on the intense electron beam accelerator: An overview

“…In the field of IEBA, the vacuum diode is used to generate electron beams, and the phenomenon of gap closure of the vacuum diode should be avoided, which adversely affects the diode performance, limits the duration of the electron beam pulse, and results in poor efficiency of coupling between the electron beam diode and the IEBA (Roy et al, 2009). So the gap closure of diode is widely researched (Li et al, 2008;Robert et al, 2001;Sampayan et al, 1990).…”

Explosive emission and gap closure from a relativistic electron beam diode

“…DARHT Axis-1 is unique for these studies because it has a relatively large A-K gap and does not have a solid or perforated anode as refs [2,9,10,12,13], reducing the contribution of the anode plasma to the gap closure velocity. Initial estimates indicate the closure velocity for this relativistic diode is ~ 10 cm/µs.…”

“…[2,9] to mitigate gap closure, in relatively small diodes (~8 mm) with voltages >100 kV and currents >2 kA. Gap closure measurements have been performed on the KALI 5000, 3 cm diode with voltages up to 430 kV and currents of 17 kA [10]. Numerous velvet and graphite cathodes were studied with a solid SS anode; initial gap closure velocities as high as 40 cm/µs were measured then slowing to 10 cm/ µs.…”