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(ProQuest: ... denotes formulae omitted.)
The development of coaxial microwave components capable of handling high peak power without the presence of multipactor is an important part of component design for space and vacuum environments. Multipactor is a resonance type of discharge that can occur under vacuum conditions.1 The electric component of the electromagnetic field can have sufficient energy to cause the emission of electrons from the material surface. In a vacuum environment, the electron free path distance is greater than the electrode separation distance. This change in the distance relationship allows free electrons to impact the electrode surface. These collisions create a secondary electron emission from the electrode structure. The synchronization between the secondary electron emissions and the frequency of the electric field alternating polarity will accelerate the electrons back to the source electrode (see Figure 1). Under the appropriate conditions, the number of electrons will increase exponentially up to a saturation point, at which the electron density is large enough to block the electric field. This electron charge build up can cause a resonance type of breakdown in the form of a multipaction discharge.1 Multipaction discharge itself can also cause an additional saturation mechanism through its interaction with the electrodes.2
There are two main conditions that must be present for multipactor. First, the one way transit time between the two electrodes is an odd number of half cycles N, where N is an odd positive integer (N = 1, 3, 5 ...). Second, the secondary electron emission coefficient of the impact surface must exceed unity.
The multipaction discharge phenomenon is not always undesirable. In the 1930s, American Philo Farasworth designed an amplifier vacuum tube for television signal transmission, based on the multipactor. This tube was later superseded by Zworykin's Iconoscope.1 However, Farnsworth first derived the name multipactor from "AC Electron Multiplier,"2 originally describing hardware rather than the mode of the electron emission itself. Over time, in most space and vacuum applications, the term has come to define the harmonic electron breakdown.
Multipactor creates a sheet like cloud of electrons, which are oscillating between the two electrode surfaces.1 The discharge will heat the surfaces of the electrodes, increase signal noise, block the electric field and appear as a brief electron current between the two...