Improved long-term electrical stability of pulsed high-power diodes using dense carbon fiber velvet cathodes

Abstract

The influence of fibrous velvet cathodes on the electrical stability of a planar high-power diode powered by a ∼230 kV, ∼110 ns pulse has been investigated. The current density was on the order of ∼123 A/cm2. A combination of time-resolved electrical and optical diagnostics has been employed to study the basic phenomenology of the temporal and spatial evolution of the diode plasmas. Additionally, an impedance model was used to extract information about this plasma from voltage and current profiles. The results from the two diagnostics were compared. By comparison with commercial polymer velvet cathode, the dense carbon fiber velvet cathode showed superior long-term electrical stability as judged by the change in cathode turn-on field, ignition delays, diode impedance, and surface plasma characteristics during the voltage flattop, a promising result for applications where reliable operation at high power is required. Finally, it was shown that the interaction of the electron beam with the stainless steel anode did not lead to the formation of anode plasma. These results may be of interest to the high power microwave systems with cold cathodes.

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

“…When compared with the velvet cathode, CF cathodes show almost constant diode impedance, whereas velvet cathodes do not [7], [8]. Optical measurements and scanning electron microscope (SEM) imaging strongly indicate that the dominant electron source mechanism of this cathode is explosive electron emission [3], [8]- [10].…”

Time-and-space resolved comparison of plasma expansion velocities in high-power diodes with velvet cathodes

“…By comparison with the commercial polymer velvet cathode, the carbon velvet cathode showed superior characteristics as judged by the low electric-field threshold ($10 4 V/cm), short time delay ($10 À8 s) in the onset of electron emission, and long-term electrical stability. 16,[19][20][21][22][23][24][25] One of the important dynamic issues of the cathode plasma is the expansion velocity which represents an interest not only for the study of cold cathode physics but also for the researchers of high current electron applications. Generally, the most common and simple method to investigate the temporal behavior of plasma in planar diodes is to compare the diode perveance P(t) with a one-dimensional model based on the classical Child-Langmuir law.…”

Performance of a SiC-nanowire-based explosive-emission pulsed plasma electron source

“…7) However, a velvet cathode has fatal limitations of short lifetime especially for repetitive operations. 8) This challenge stimulates great interest to employ emitters consisting of "velvetlike" slender structures with robust mechanical and thermal properties, including but not limited to the improved versions of velvets, 9,10) carbon fibers, 11) carbon nanotubes, 12) carbon-epoxy capillary cathodes, 13,14) and carbon-fiber-aluminum cathodes. 15) The use of an emitter as a pulsed plasma source has its own pros and cons.…”