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

carriers, because the NLSQ method requires a good seed phase in order
to find a good local minimum. By combining these methods and using
the result of the best random phase approach as a seed phase for the
NLSQ optimisation, the best results are found. The problem with a
random phase approach is that for a large number of carriers, the number
of phase-sets needed to achieve a good result becomes discouragingly
high [52].
By using a genetic algorithm, a great improvement in finding the
worst case scenario in a short time has been achieved, especially for the
non-equally spaced carrier case. This type of optimisation scheme has
the advantage of being able to find not only a good local minimum as
in the NLSQ case, but the actual global minimum can be found. The
genetic algorithm was implemented by Merecki [40] and in addition to
improving the optimisation part of the software, he also implemented
the threshold of the hybrid modes (see Fig. 2.16) as well as many other
useful functions.
The main problem with multipactor in microwave systems is the
electric noise that is generated, which degrades the signal to noise ratio.
Thus the worst case may not always be the maximum power within the
T20 period. Depending on the material properties some other case may
produce a substantially larger amount of energetic electrons. Therefore,
WCAT also includes the possibility of finding the phase-set that will
produce the largest amount of electrons.
In addition to assessing the risk for a vacuum discharge, it can also
be of value to investigate if a multipactor free design may have a risk
of corona breakdown if the component is not thoroughly vented when
it is brought into operation. In WCAT this is analysed using the mean
carrier frequency and comparing the corona threshold with the in-phase
peak voltage. If the minimum of the Paschen curve is greater than this
voltage, then the corona margin is displayed in the output window of
WCAT. If the opposite is true, the pressure range within which there is
risk for corona discharge is presented (see Fig. 2.16).
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