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

Chapter 5
Multipactor in coaxial lines
The coaxial line is a very common and important component in microwave
systems. It is a transmission line that consists of an inner
cylindrical conductor and a coaxial outer conductor. A constant crosssection
is maintained by means of a dielectric medium, which is contained
between the conductors. In some space applications, as well as in
other systems, the dielectric medium has been partly omitted in order to
save weight or to reduce the dielectric losses. In a vacuum environment,
the line may become evacuated, which makes it exposed to the risk of a
multipactor discharge.
The electric field in a coaxial line is nonuniform, which makes analytical
analysis difficult, since the the equation of motion for an electron becomes
a non-linear differential equation. However, multipactor in a coaxial
line has been studied experimentally [63] and numerically [64–67]. In
these studies it was found that two different types of resonant multipactor
can occur, namely a two-sided discharge between the outer and
the inner conductors and the one-sided analogue on the outer conductor.
In an attempt to understand the effect of varying the relative inner
radius, i.e. varying the characteristic impedance of the coaxial line,
different scaling laws were suggested in these studies. Another study focused
on the current due to the multipacting electrons and treated this
as a radially oriented Hertzian dipole in order to determine the electric
field generated by the multipactor discharge [68].
In paper E resonant multipactor in a coaxial line is analysed by
means of an approximate analytical solution of the non-linear differential
equation of motion, which in a large range of microwave frequencies
and amplitudes agrees very well with the numerical solution. As
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