The Knee of the Transition from TL to FSCL Operation Space Charge Effects on Thermionic Emission

M. C. Lin, Yung-Chiang Lan, J. Y. Hsu, H. M. Chung, D. S. Chuu

Research output: Contribution to journalConference articlepeer-review

Abstract

Thermionic emission cathodes are widely used in microwave tubes. In the temperature limited (TL) region, the Richardson-Dushman equation, corrected for the Schottky effect applies. In the fully space charge limited (FSCL) region, the Child-Langmuir law applies. Longo found that the following simple relation described the emission from an impregnated cathode over a wide range of voltages and temperatures: 1/JOP = 1/JFSCL + 1/J TL (1) However, the knee of the transition curve from TL to FSCL operation described by Longo's equation is much broader than that found for most cathodes. Vaughan proposed the relation 1/JOPn = 1/JFSCLn + 1/JTLn (2) where n can be adjusted to provide the best fit to data. A value of 6 to 10 was given as being appropriate for good electron guns. Fortgang employed a simple model based on the cathode having a work function distribution function to fit the experimental data measured from TL to FSCL regime. He found that a single work function distribution could fit the I-V curve over a range of cathode temperatures very well. In this work, we study the space charge effects on thermionic emission via a self-consistent approach. The thermionic emission process is described by the Richardson-Dushman equation, corrected for the Schottky effect. Space charge effects are described by Poisson's equation including relativistic effects. The approach enables us to make detailed calculations over a wide range of voltages and temperatures. The knee of the transition curve from TL to FSCL operation is obtained self-consistently.

Original languageEnglish
Number of pages1
JournalIEEE International Conference on Plasma Science
Publication statusPublished - 2003 Oct 17
Event2003 IEEE International Conference on Plasma Science - Jeju, Korea, Republic of
Duration: 2003 Jun 22003 Jun 5

All Science Journal Classification (ASJC) codes

  • Atomic and Molecular Physics, and Optics
  • Condensed Matter Physics
  • Electrical and Electronic Engineering

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