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4.9: Breakdown

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    46124
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    Breakdown occurs when the electric field is strong enough that a stray electron strips electrons from atoms or molecules resulting a cascading effect producing a conducting plasma of electrons. There are two types of effects that cause breakdown. One of these is the multipactor effect which occurs at the interface of a metal or dielectric with a vacuum or low pressure gas where electrons are stripped from the metal or dielectric. The other is the corona effect which occurs in a gas, even at fairly low pressures, in which electrons are stripped from atoms or molecules in the gas. The plasma of electrons shorts conducting surfaces at different potential, and in the worst case causes destruction of components. But even before a sufficiently dense plasma is established, effects include noise generation, increase in ohmic losses, and nonlinear signal distortion since breakdown is strongly nonlinear effect the number of free electrons created is a strong nonlinear function of electric field and thus voltage.

    4.8.1 Multipactor Effect

    The multipactor effect is due to the secondary emission of electrons when a fast moving electron in a vacuum or low pressure gas impacts a metal or a dielectric. If the energy of the free electron is above a first critical field \(E_{c1}\), an electron in the material will be excited and sometimes will be emitted from the material. The initial electron will penetrate the material by several lattice constants but follow a zig-zag path. If the energy of this electron is below a second critical value \(E_{c2}\), then it will only penetrate one or a few lattice constants and will itself be emitted back into the vacuum (or low pressure gas). If the electron has an initial energy above \(E_{c2}\) then it will penetrate the material a considerable distance and will become trapped. The actual material and the surface finish have a significant effect on electron emission.

    The number of electrons that leave the surface for each impacting electron is called the secondary electron emission yield (SEY) and this includes the initial electron which may or may not be emitted. If SEY is greater than one then the number of free electron will avalanche and could eventually lead to device destruction. Before that occurs, there is a balancing effect which limits growth in free electron numbers. Principle among these is the field that is produced by the free electrons themselves which counters the field created by an externally applied voltage potential. The reference geometry used in measuring the multipactor effect is a parallel plate structure with a voltage \(V\) between the plates that are separated by a distance \(d\). This geometry generally has the lowest thresholds for the multipactor effect. For other geometries, i.e. non-parallel plate geometries, the high field region where the secondary electrons are created may not be where the plasma, i.e. the region where the free electrons accumulate, is located. Thus the growth in the number of free electrons is curtailed. A further limiting effect is the frequency of the signal applied to the plates where a high frequency signal could reverse limiting the build-up of free electrons. Thus the applied voltage, \(V_{\text{multipactor}}\), required to initiate the multipactor effect is a function of \(d\), material, material finish, and frequency \(f\). If there is a free electron between the plates then the electron will accelerate and acquire velocity and thus energy. The voltage required to initiate impact ionization. It is found experimentally that there is a minimum voltage \(V_{\text{min}}\) required for the effect to be initiated. Then there is a linear relationship between \(V_{\text{multipactor}}\) and the product \(f\cdot d\). Materials and material finish also have an impact on ionization For example cleaned mirror-finish silver has a first critical energy, \(E_{c1} = 130\text{ eV}\) whereas silver exposed to the air has \(E_{c1} = 20\text{ eV}\). Guidelines for the thresholds for multipactor ionization for various materials are given in [50].

    4.8.2 Corona Effect

    Multipaction describes ionization of materials in a vacuum or in when the environment has a low pressure gas. As the pressure of the gas increases the molecules in the gas will breakdown and this is called the corona effect. If \(\mathsf{e}\) is an electron and \(\mathsf{M}\) is a molecule then the corona ionization process is described by

    \[\label{eq:1}\mathsf{e}+\mathsf{M}\to\mathsf{e}+\mathsf{e}+\mathsf{M}^{+} \]

    and the molecule takes on a positive charge after an electron has been stripped off. The competing process which limits the number of free electrons available is when a free electron combines with a molecule and in air these interactions principally involve nitrogen \(\mathsf{N}\) and oxygen \(\mathsf{O}\):

    \[\label{eq:2}\mathsf{e}+\mathsf{N}_{2}^{+}\to\mathsf{N}+\mathsf{N}\quad\text{and}\quad\mathsf{e}+\mathsf{O}_{2}\to\mathsf{O}^{-}+\mathsf{O} \]

    The minimum energy required for the corona effect is heavily dependent on gas pressure. At very low pressures ionization is solely due to multipaction. Above a critical but low pressure multipaction is replaced by the corona effect as as the density of molecules increases it becomes easier to ionize more molecules. The minimum energy required for the corona effect occurs when the pressure in multiples of standard atmospheric pressure is approximately equal to the frequency in gigahertz. Further increase in the pressure reduces the free main path of an electron and fewer free electrons are able to acquire the requisition energy [50].

    4.8.3 Summary

    Breakdown can lead to device destruction but even at very low levels it can cause intermodulation products, i.e. passive intermodulation distortion, that obscure small received signals. Breakdown is a particular problem when there are high fields, especially in filters with high energy densities because of resonance. Breakdown can be a problem in basestations, radars, and satellites as these often operate with EM powers of kilowatts to megawatts, and even higher powers when operated with low duty cycles.


    4.9: Breakdown is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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