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

At altitudes where most satellites operate, the pressure is very low
and for most purposes it can be approximated as a perfect vacuum.
However, during the launch phase of a satellite and during its first days
of operation as well as at times when the satellite fires its attitude and
altitude control engines, the microwave parts may not be completely
vented and thus it is important to understand what happens with the
multipactor threshold with increasing pressure. This has been one of the
main topics of this thesis. It was found that for materials with a low first
cross-over energy, for the lowest order resonance, the threshold increases
with increasing pressure until reaching a maximum, after which it starts
to decline. The reason for the increase is the friction force experienced
by the electrons when colliding with the neutral gas particles. In all
other cases, for materials with a high first cross-over energy and in general
for the higher order modes, a monotonically decreasing threshold
is noted. This behaviour can be explained by the thermalisation of the
electrons, which leads to a higher total impact energy, as well as the
contribution of electrons from collisional ionisation, which reduces the
necessary secondary emission yield and consequently also the required
impact velocity. Improved quantitative results require a more detailed
investigation of the fraction of the electrons from impact ionisation that
actually contribute to the multipactor bunch, but that is left for a future
study. Furthermore, in order to make the model useful for all frequencygap
size products, an extension of the model to include also the hybrid
modes is necessary. Due to the complexity of such a study, it was not
included in this first analysis. However, this may be an interesting topic
for a future investigation.
In addition to designing with respect to the proper thresholds, most
hardware will require some type of testing to ensure compliance with
the standard. Such tests must be of good quality to avoid ambiguities,
which could disqualify a component that is multipactor free. In this
thesis a method that gives unambiguous and highly reliable test results
was presented. It is a method based on a weak amplitude modulation of
the input signal, which becomes very distinct in the multipactor signal,
since the generated noise power is a function of the difference between
input power and the multipactor threshold. By using this auxiliary
method of detection in connection with two other detection methods,
reliable test results can be obtained.
It is quite satisfying when looking back at the chapter on future work
in my licentiate thesis [75] and realizing that the two main topics men-
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