Speculations on multipaction.


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What does the damage?

Any charged particle accelerated by the RF field can generate secondaries when it impacts the cavity surface. Lightweight electrons damage the surface primarily by heating it. The surface temperature can rise well above the ambient for the cavity wall as a whole. Surface mobility of atoms at the surface is enhanced at temperatures well below the melting point for the bulk material. Mobile charged surface atoms can move around under the influence on the local electric and magnetic RF fields.

Ions on the other hand have significant mass compared to the surface atoms; they can directly transfer momentum to the mobile surface atoms and move them around. It is possible that ionic and electronic discharge damage can co-exist in the same component.

Of course, because the mass of ions is much larger than that of electrons, the time-of-flight across the gap will be much longer and that ionic multipaction may be expected to occur at lower frequencies, or higher field strengths, than electronic damage.

It is speculated that the visual appearance of cavities damaged by ions will be noticeably different from those damaged by electrons.

Pressurisation

One can work out the critical pressure at which multipaction may become a problem by finding the mean-free-path of the agent (electron, ion) in the gas filling the cavity. This mean free path needs to be much less than the distance across the gap for strong suppression of multipaction. There is a "critical pressure" at which the mean free path is of the same order as the minimum gap distance. Since the electric fields are accelerating the impaction agents, finding the mean free path for scattering is not a trivial problem. However, pressurisation of cavities is known to be highly effective at suppressing the worst effects, providing the energy of the accelerated agent, acquired in the accelerating field, is not sufficient to ionise the gas molecules on impact.

Titanium passivation.

Waveguide walls may be copper, silver, gold, or aluminium. Titanium is a refractory metal (it has a high melting point) and is also highly reactive, with a strong affinity for oxygen, forming TiO2 which in its crystalline form is called Rutile and has a conduction mechanism by hopping processes in the Ti 3d electron orbitals, particularly when non-stoichiometric. TiO2 has a very high melting point. It is likely to have a much lower secondary emission factor than bare metal surfaces.

The effect of coating a waveguide wall or cavity wall with Ti is therefore to put a very thin layer of TiO2 dielectric between the fields and the metal. This does not affect the electrical transmission-line properties, but greatly reduces multipaction.



Copyright D.Jefferies 1997, 1998.
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