Experimental research of different plasma cathodes for generation of high-current electron beams

Abstract

The results of experimental studies of different types of cathodes—carbon-epoxy rods, carbon-epoxy capillary, edged graphite, and metal-dielectric—under the application of high-voltage pulses with an amplitude of several hundreds of kV and pulse duration of several nanoseconds are presented. The best diode performance was achieved with the edged graphite and carbon-epoxy-based cathodes characterized by uniform and fast (<1 ns) formation of explosive emission plasma spots and quasi-constant diode impedance. This result was achieved for both annular cathodes in a strong magnetic field and planar cathodes of a similar diameter (∼2 cm) with no external magnetic field. The cathodes based on carbon-epoxy rods and carbon-epoxy capillaries operating with an average current density up to 1 kA/cm2 showed insignificant erosion along 106 pulses of the generator and the generated electron beam current showed excellent reproducibility in terms of the amplitude and waveform.

High power microwave source for a plasma wakefield experiment

“…The BWO is driven by a high-current relativistic electron beam generated in a magnetically insulated foilless diode under the application of a negative polarity high-voltage (HV) pulse, produced by a semiconductor-opening-switches (SOSs)-based generator 24 with an internal impedance of 200 X. The parameters of the diode and the generated electron beam were described previously, 25 where a generator producing HV pulses at a matched load of $220 kV, $6 ns at FWHM, and 1.5 ns risetime was used. In order to increase the electron beam energy, the SOS-based generator was modified, resulting in the HV pulses of $350 kV, $6 ns at FWHM, and $0.5 ns rise-time obtained after the gas spark-gap switch at the matched (200 X) load.…”

“…An explosive emission plasma, formed at the sharp boundary of the edged graphite cathode (diameter of 28 mm) under the application of an HV pulse, served as a source of electrons that produced a %0.5 mm-wide annular electron beam. 25 This beam was transported through the SWS using a guiding magnetic field B ¼ 2:5 T (axial nonuniformity of $1% along $70 cm length), with a half period of $4.4 ms, produced by an external solenoid.…”

Wakefield excitation by a powerful sub-nanosecond 28.6 GHz microwave pulse propagating in a plasma filled waveguide

“…The device consists of an all solid-state high-voltage (HV) generator based on magnetic compression stages and semiconductor opening switches 22 (not shown in Fig. 1), a magnetically insulated foilless diode with an explosive electron emitting graphite cathode, 23 a solenoid producing a guiding magnetic field sufficient for strong magnetization of the generated cylindrical electron beam, a specially designed slow wave structure (SWS), and a horn antenna.…”

“…The HV generator (internal impedance of 200 X), described in Ref. 23, produces at its output a negative polarity of $320 kV and a rise-time pulse of $2.5 ns. To reduce the rise-time to $0.5 ns, a gas filled spark-gap switch is installed between the generator output and the cathode holder.…”

Experiments Designed to Study the Non-Linear Transition of High-Power Microwaves through Plasmas and Gases

“…A magnetically insulated foilless diode with a hollow graphite circular sharp-edged cathode is used to produce an annular electron beam [39]. The dimensions and exact location of the cathode are part of the design of each SR-BWO.…”