Multipactor effect

The multipactor effect is a phenomenon in radio-frequency (RF) amplifier vacuum tubes and waveguides, where, under certain conditions, secondary electron emission in resonance with an alternating electromagnetic field leads to exponential electron multiplication, possibly damaging and even destroying the RF device.

A resonance between electron flight time and RF field cycle is a mechanism for multipactor development.

[2]: 91–105 [3] A single-surface multipactor event is also possible on a metallic surface in the presence of a crossed static magnetic field.

It will continue to accelerate and reach a maximum velocity half a cycle later, just as the voltage at electrode B begins to become negative.

The electric field is at zero and is beginning to point to the left so that the newly freed electrons are accelerated toward the right.

We know that resonance happens if the electrons arrive at the rightmost electrode after one half of the period of the RF field,

There are a number of geometry-based techniques to reduce or even eliminate the multipactor effect: One is through small-scale grooves which modify the direction of the electric fields exactly at the location of the electron emission, thereby kicking some of the electrons to directions that do not support the process.

Multipactor was identified and studied in 1934 by Philo 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.

The first application of computers to investigate multipacting is from the early 1970s[3] when it was discovered as a major SRF cavity performance limitation.