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

Published online by Cambridge University Press:  01 February 2013

Jie Yang ,
Ting Shu  and
Yuwei Fan
Jie Yang*
Affiliation:
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, People's Republic of China
Ting Shu
Affiliation:
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, People's Republic of China
Yuwei Fan
Affiliation:
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, People's Republic of China
*
Address correspondence and reprint requests to: Jie Yang, College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, People's Republic of China. E-mail: [email protected]
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Abstract

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/cm2 were 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.

Keywords

Cold cathode plasmaExpansion velocityHigh-power microwave devicesOptical diagnostics
Type
Research Article
Information
Laser and Particle Beams , Volume 31 , Issue 1 , March 2013 , pp. 129 - 134
Copyright
Copyright © Cambridge University Press 2013

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