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

tipactor events that are short-lived, like e.g. multicarrier multipactor,
where the discharges often are weak and of short duration. However,
the method is also sensitive to other sources of noise in the same way as
close-to-carrier noise and, thus, it is not recommended to use only third
harmonic and close-to-carrier noise detection in a test setup.
Reflected power
Mismatches in the transitions between microwave components imply
that some of the input power will be reflected. However, in a welldesigned
system, very little power is reflected. Components containing
high Q-value parts, like e.g. cavity resonators, are only well matched
at certain frequencies and minor changes in the component properties
can lead to detuning of the part. Multipactor is known to be able to
detune high Q-value components [70]. Thus the reflected power from
a component can be used as an indication of a multipactor event. To
study the absolute value of the reflected power is usually not a good way
of detection, since the input power can vary during a multipactor test
and consequently the reflected power will vary as well, as the reflected
power is a fixed fraction of the input power. This fraction is called the
return loss and is commonly measured in decibel. The return loss will be
a stable value, insensitive to power fluctuations, until the component is
detuned. Figure 6.3 shows an example where both close-to-carrier noise
and the return loss are monitored simultaneously during a multipactor
test. Both methods indicate a change around t = 50 s and thus it can
be determined with great confidence that a multipactor discharge was
initiated at that time.
The main advantage with this method is that it is quite reliable and
there is little risk that other phenomena will cause a mismatch that can
be confused with multipactor. However, for low Q-value components or
badly matched systems, the sensitivity of the method is low.
Electron monitoring
A new global method of detection was presented at the 4 th International
Workshop on Multipactor, Corona and Passive Intermodulation
in Space RF Hardware [71]. It is called the Electron Density Detection
Method, abbreviated EDDM, and uses a set of tri-axial cables as
a probe and electrons picked up by the probe are then monitored with
a high precision electron meter. The data is collected using a computer
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