High-current carbon-epoxy capillary cathode

Abstract

The results of experiments on the reproducible generation of an electron beam having a high current density of up to 300 A/cm2 and a satisfactorily uniform cross-sectional distribution of current density in a ∼200 kV, ∼450 ns vacuum diode with a carbon-epoxy capillary cathode are presented. It was found that the source of the electrons is the plasma formed as a result of flashover inside the capillaries. It is shown that the plasma formation occurs at an electric field ≤15 kV/cm and that the cathode sustains thousands of pulses without degradation in its emission properties. Time- and space-resolved visible light observation and spectroscopy analyses were used to determine the cathode plasma’s density, temperature, and expansion velocity. It was found that the density of the cathode plasma decreases rapidly in relation to the distance from the cathode. In addition, it was found that the main reason for the short-circuiting of the accelerating gap is the formation and expansion of the anode plasma. Finally, it was shown that when an external guiding magnetic field is present, the injection of the electron beam into the drift space with a current amplitude exceeding its critical value changes the radial distribution of the current density of the electron beam because the inner electrons are reflected from the virtual cathode.

Temporal response of a surface flashover on a velvet cathode in a relativistic diode

“…1, 12 Many varieties of non-fabric cathodes have also been diagnosed optically, these include ferroelectric materials, [13][14][15] multicapillary dielectric cathodes, 16 carbon nanotube cathodes, 10 and carbon-epoxy cathodes. 17,18 The emission of carbon-epoxy cathodes has been compared in detail to velvet, carbon, and aluminum cathodes 17 indicating longer lifetimes of the carbon-epoxy cathodes at higher current densities. A limited investigation of the temporal response of the emission on the cathode face has been performed by a few authors, 1,3,5,11,15-17 but many of these studies include several variables in which the A-K gaps and electrode materials were varied.…”

Evaluating the Performance of a Carbon-Epoxy Capillary Cathode and Carbon Fiber Cathode in a Sealed-Tube Vircator Under UHV Conditions

“…The frame duration is 20 ns [2]. dominant mechanism [2]. There has been no evidence of gap closure from this type of cathode at current densities up to 5 kA/cm 2 , due to plasma formation occurring inside the carbon-epoxy capillaries.…”

“…The carbon-epoxy capillaries are adhered to the stainless steel substrate by Durabond 954 [2]. This epoxy is a metallic composite high expansion adhesive for high-temperature bonding.…”

“…In general, surface flashover depends on the rise time of the accelerating voltage and exhibits faster turn-ON over explosive emission, while explosive emission suffers from increased gas desorption from the cathode. In the case of surface flashover, plasma formation starts to occur with electric fields as low as 10 kV/cm [2].…”

“…Single capillary (left) and light emission from the CEM cathode (right). The frame duration is 20 ns [2]. dominant mechanism [2].…”