Multipaction

Updated January 19, 2009

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New for December 2008! Multipaction is the phenomenon that can cause breakdown due to high electric fields in a vacuum or near vacuum. The word "multpaction" is an example of portmanteau in engineering (the combination of the words "multiple" and "impaction".) Note that in Europe, the word "multipactor" commonly replaces "multipaction" as in the "multipactor effect".

Here's a link to the European Space Agency's Multipactor calculator page.

http://multipactor.esa.int/

Thanks to Simon for suggesting the link, and most of the content on this page! Regarding the ESA multipactor calculator, he wants you know the following...

Working as a microwave engineer at the European Space Agency ESA, I've been producing free software for microwave engineers working in the European space community for many years. I'd thought it to be a good idea to let you know about the Multipactor Calculator program available on the webpage, which includes many RF tools; mismatch calculator, skin depth calculator, waveguide matching transformer (single step), waveguide insertion loss calculator (rectangular and circular), venting and outgassing calculator, shielding effectiveness calculator (venting holes & slots), EMC radiated emission & susceptibility analysis, units convertor and a pressure units convertor. The explanation of the Multipactor or Multipaction phenomena as a particular RF breakdown problem in vacuum ( space, vacuum tubes, accelerators, etc.) is also given on this webpage.

What is multipaction?

The following explanation was supplied by Simon:

Multipaction is an electron resonance effect that occurs when RF fields accelerate electrons in a vacuum and cause them to impact with a surface, which depending on its energy, release one or more electrons into the vacuum. These electrons can then be accelerated by the RF fields and impact with the same or another surface. When 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 will grow exponentially and may lead to operational problems. In RF space systems, multipaction will cause loss/distortion of the RF signal (increase of noise figure or bit-error-rate) and can damage RF components or subsystems due to excess RF power being reflected back or dissipated by them.

In accelerators, multipaction will produce electron clouds preventing the undisturbed buildup of  accelerator energy levels. Multipaction will also cause an increase of local pressure which could lead to a really destructive corona breakdown.

The existence of multipaction is dependent on the following four conditions being met:

1. The mean free path of the electrons should be (much) greater then the spacing between the opposing surfaces, which is normally only the case in good vacuum and without any further obstruction in the way (no other di-electricum).
2. The average number of electrons released is greater than one which is dependent on the secondary electron yield of the surface, which in turn is dependent on the field strength (RF power) between the surfaces.
3. The time taken by the electron to travel from the surface from which it was released to the surface it impacts with, is to be an integer multiple of one half of the RF period (resonance).
4. The availability of free electrons to start of the release of secondary electrons.
   (In space, free electrons are released from the surfaces by high energy particles, while during on-ground testing they are provided by a radioactive source (strontium 90) or an electron gun)