Multipactor effect
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The Multipactor effect is a phenomenon in radio frequency (RF) amplifier vacuum tubes and waveguides, where, under certain resonant conditions, secondary electron emission may multiply and destroy the RF device.
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[edit] Description
Multipactor is an effect that occurs when electrons accelerated by radio-frequency (RF) fields are self-sustained in a vacuum (or near vacuum) via an electron avalanche caused by secondary electron emission. The impact of an electron to a surface can, depending on its energy and angle, release one or more secondary electrons into the vacuum. These electrons can then be accelerated by the RF fields and impact with the same or another surface. Should the impact energies, number of electrons released and timing of the impacts be such that a sustained multiplication of the number of electrons occurs, the phenomenon can grow exponentially and may lead to operational problems of the RF system such as damage of RF components or loss/distortion of the RF signal.
[edit] Mechanism
The mechanism of multipactor depends on the orientation of an RF electric field with respect to the surface.
There are two types of multipactor: two-surface multipactor on metals and single-surface multipactor on dielectrics.
[edit] Two-surface multipactor on metals
This is a multipactor that occurs in the gap between metallic electrodes. Often, an RF electric field is normal to the surface. A resonance between electron flight time and rf field cycle is a mechanism for multipactor development.
The existence of multipactor is dependent on the following three conditions being met: The average number of electrons released is greater than or equal to one (this is dependent on the secondary electron yield of the surface) and the time taken by the electron to travel from the surface from which it was released to the surface it impacts with is an integer multiple of one half of the RF period and the average secondary electron yield is greater than or equal to one.
[edit] Single-surface multipactor on dielectrics
This is a multipactor that occurs on a dielectric surface. Often, an RF electric field is parallel to the surface. The positive charge accumulated on the dielectric surface returns electrons back to the surface.
[edit] History
Multipactor was discovered in the 1920s by Philo T. Farnsworth, the inventor of electronic television, who attempted to take advantage of it as an amplifier. More commonly nowadays, it has become an obstacle to be avoided for normal operation of particle accelerators, vacuum electronics, radars, satellite communication devices, and so forth. A novel form of multipactor has been proposed (Kishek, 1998), and subsequently experimentally observed, in which charging of a dielectric surface considerably changes the dynamics of the multipactor discharge.
[edit] References
More information on multipactor may be found in:
- J. Rodney M. Vaughan, Multipactor, IEEE Trans. Electron Devices, vol. 35, No 7, July 1988.
- Rami A. Kishek, Y.Y. Lau, L.K. Ang, A. Valfells, and R.M. Gilgenbach, Multipactor Discharge on Metals and Dielectrics: Historical Review and Recent Theories, Physics of Plasmas 5 (5), 2120 (1998).
Online
- Study of the Multipactor effect in multi-carrier operation inside space microwave components Ph. Mader, J. Puech, H. Dillenbourg, Ph. Lepeltier, L. Lapierre, J. Sombrin. PDF Accessed Decembaer 2006
- Breakdown In Waveguides Due To The Multipactor Effect. H.M. Wachowski, Aerospace Corp El Segundo California, May 1964. Accessed December 2006
