Multipactor in Low Pressure Gas and in ... - of Richard Udiljak

Chapter 4
Multipactor in irises
A common microwave component is the waveguide iris, which is often
used as a shunt susceptance for the purpose of matching a load to the
waveguide. There are many different types of irises, but a typical configuration
consists of a step-like, short length, reduction of the waveguide
height. Similar structures also appear in other configurations, e.g. as
apertures in array antennas, as coupling slots in directional couplers,
and as irises in waveguide filters. As the field strength in the iris can
be very high and the gap height is small, there is a pronounced risk of
having a multipactor discharge.
So far in this thesis, all the models considered have been based on
the plane-parallel model with a spatially uniform harmonic electric field.
In general, most theoretical studies of the multipactor phenomenon have
been limited to this or similar approximations. However, many RF devices
involve more complicated electric field structures where predictions
based on the parallel-plate model are not applicable. This is true for
e.g. the waveguide iris, where the electric field will be a combination
of several different electromagnetic modes, most of which typically are
evanescent. However, due to the short length of the iris, these modes
will be of importance. Nevertheless, in this analysis, which is described
in more detail in paper D, it is only the importance of the random drift
due to initial velocity spread of the secondary electrons that has been
considered. Thus, as far as the field is concerned, a spatially uniform
harmonic field based on the parallel-plate model is used.
Experiments [60, 61] as well as numerical studies [61, 62] of multipactor
in an iris have shown that the discharge threshold increases with
decreasing length of the iris. It has been suggested that the reason for
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