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

software and the time evolution of the electron density can be studied.
The probe does not have to be located inside or close to the area where
the discharge will take place. Even though it is advantageous for higher
sensitivity to have the probe pointing at the critical gap it is also reliable
when located outside the device under test and thus it can be used
for waveguides and coaxial transmission lines. It is not completely clear
from the paper, however, how the electrons escape from a completely
confined device like a typical waveguide or coaxial cable and one will
have to assume that there must be some small opening somewhere in
the system, where electrons can leak out.
Furthermore, the method can be used to quantitatively measure the
amount of generated electrons. however, this requires some kind of calibration
of each test setup and that may prove to be problematic. When
used in this way, the method can no longer be viewed as a global method,
which can detect a multipactor event anywhere in the system, instead it
has become a local method. Among the main advantages of the method
is the low cost involved, since no expensive microwave instruments are
needed.
Residual mass
A very slow global method of detection is to detect the gas molecules,
which are outgassed from the device walls due to the electron bombardment
during a multipactor event. The gas molecules consist of residuals
of water, air and contaminants, and using a mass spectrometer,
the different molecules can be identified. It has been noted [31] that a
detectable increase in the water spectrum can be seen during a multipactor
discharge. The major drawback of this method of detection is
its inability to detect fast multipactor transients (not enough molecules
are released from the walls) and thus it is not a suitable method for
multicarrier multipactor studies. Another disadvantage is that there is
a certain delay between onset of the discharge and indication in the instrumentation.
However, it can be useful as a diagnostic tool together
with one or two of the other described methods.
6.1.2 Local methods
In cases where it is not sufficient to only confirm the existence of a
discharge in the system, but also to determine the exact position, local
methods of detection will have to be used. The two most common local
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