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

2.1.4 Methods of suppression
Many of the factors mentioned in the previous section that affect the
multipactor threshold can also be utilised to suppress the discharge. The
without doubt easiest method of avoiding a breakdown is to pressurise
the component. The field strength required to achieve breakdown at
atmospheric pressure is in general much higher than at low pressures or
in vacuum. However, such a method is seldom feasible for components
that will be used e.g. in space, where the external environment is a high
vacuum. A small leakage can lead to slow venting of the component and
thus risking severe corona discharge when the pressure reaches the range
where the minimum breakdown field occurs.
Another way of suppressing multipactor is to amplitude modulate
the main carrier [21, 41]. If both signals are sinusoidal, the total field
can be written:
Etot = E1 sinω1t + E2 sin ω2t (2.23)
This means that the envelope of the signal will vary according to (see
also Fig. 2.8):
�
Eenv = E2 1 + E2 2 + 2E1E2 cos (ω1 − ω2)t (2.24)
When the total field strength is well above the multipactor threshold
(see Fig. 2.8), the secondary electron yield will increase quickly according
to Eq. (2.22). However, as soon as the voltage drops below the
threshold again, the electron loss will be large and according to Ref. [21]
all electrons will be lost in just a few RF cycles. However, whether or
not this is true also depends on the secondary yield properties of the
electrode material. For materials with a very high maximum secondary
yield, the number of electrons gained while above the threshold can be
greater than the losses incurred while below. In such a case, no suppression
is achieved and in some cases, the discharge may even become
more powerful than before the modulation carrier was added [42] (cf.
Fig. 2.9). Thus in order to successfully suppress a multipactor discharge
using amplitude modulation, it is vital that the material has a low maximum
secondary yield (preferably less than about 1.5). Due to the risk
of contamination, which can greatly increase the maximum secondary
yield, great care should be taken to assure a high level of cleanliness if
this method of suppression is to be used.
To AM-modulate the carrier is probably not feasible in most cases, as
it would require extra hardware to produce the AM-signal. However, the
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