Coaxial cables and connectors, inherently, have a limited power handling relative to microwave tubes. With regard to coaxial cables and connectors, Vaughan’s statement that multipactor as “essentially a medium power phenomenon” is not true in the majority of the cases. This type of impact does not meet the second required condition for multipactor, since the secondary electron emission coefficient is less than one. The impact velocity of the emitted electron is of such a magnitude that it penetrates too deep inside the second electrode to cause secondary electron emission from its surface. 1 Vaughan noted that multipactor is essentially a medium power phenomenon.1 The reason for this statement is that under very high energy, multipactor is not possible. The focus of Vaughan’s work with multipactor was in the field of high and very high power microwave tubes. For further reading on the history of multipaction discharge analysis, current theory and practical information, see the works of Vaughan, 1 Kishek 2 and Ming Yu. This single surface discharge phenomenon on the dielectric surface is not applicable for coaxial lines. Single surface multipaction discharge can also occur on dielectric component surfaces, if there is a bias DC field, electric or magnetic, and a high frequency electric field is present parallel to the dielectric surface. Additionally, in some high power tubes, multipactor can emit visible light and X-rays. In the case of a high mode (N mode) multipactor, several electron cloud sheets (exactly 2N-1) will oscillate between the surfaces under steady state conditions. 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 electrodes. Multipactor creates a sheet like cloud of electrons, which are oscillating between the two electrode surfaces. Over time, in most space and vacuum applications, the term has come to define the harmonic electron breakdown. 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. This tube was later superseded by Zworykin’s Iconoscope. In the 1930s, American Philo Farnsworth designed an amplifier vacuum tube for television signal transmission, based on the multipactor. The multipaction discharge phenomenon is not always undesirable. Second, the secondary electron emission coefficient of the impact surface must exceed unity. 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 …). There are two main conditions that must be present for multipactor. 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. 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. 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). These collisions create a secondary electron emission from the electrode structure. This change in the distance relationship allows free electrons to impact the electrode surface. In a vacuum environment, the electron free path distance is greater than the electrode separation distance. 1 The electric component of the electromagnetic field can have sufficient energy to cause the emission of electrons from the material surface. Multipactor is a resonance type of discharge that can occur under vacuum conditions. 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.
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