Passive and Active Plasma Emission Sources for High-current Electron Beam Generation

A combination of electrical and optical diagnostics has been used to investigate the time evolution of the two-dimensional expansion velocity distributions of the cathode plasma in pulsed high-power diodes. The perveance model based on the Child-Langmuir law was used to calculate the expansion velocity of the diode plasmas from voltage and current profiles. Additionally, a four-channel high speed framing camera was used to observe the formation and subsequent movement of the cathode plasma. More accurate and valuable information about the dynamics of the cathode plasma was also acquired by utilizing the digital image processing methods. Results from the experiments and theoretical analysis were compared. In this paper, the experiments have been performed using a high-voltage pulse generator with 200 kV output voltage and 110 ns pulse duration. Current densities up to 440 A/cm2were produced. The observation of the cathode plasma expansion in transverse direction indicated that the diode current was cathode-limited in the current rising stage (the first 60 ns of the current pulse). The perveance model should be modified taking in account the time dependent expanding plasma surface (i.e., not the whole cathode surface) for this period. The velocity in the direction parallel to the cathode surface (transverse velocity) was much larger than that in the direction perpendicular to the cathode surface (longitudinal velocity), and further, it dropped from 90 cm/μs to nearly 20 cm/μs rapidly. It was shown that, during the current flattop stage, the plasma filled out all the surface of cathode and the diode current was space-charge-limited. The values of the transverse velocity and longitudinal velocity were nearly the same and decreased relatively slowly. The satisfactory coincidence of experimental and calculated (both were in the range of 6–8 cm/μs) values of the cathode plasma expansion velocities was obtained.

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

Yang

Shu

Wang

2012

Physics of Plasmas

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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.

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Passive and Active Plasma Emission Sources for High-current Electron Beam Generation

Cited by 5 publications

References 11 publications

Plasma Cathode Research in Plasma Physics and Pulsed Power Laboratory

Plasma Cathode Research in Plasma Physics and Pulsed Power Laboratory

Time evolution of the two-dimensional expansion velocity distributions of the cathode plasma in pulsed high-power diodes

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

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