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

Voltage [V]
10 2
µ=0
µ=0.75
10 0
Frequency − Gap product [GHz⋅mm]
µ=0.75
µ=0
Vacuum multipactor
Figure 3.11: Multipactor thresholds in low pressure argon for the two lowest
order modes (N = 1 and N = 3) with µ = 0.75 and µ = 0
respectively. Parameters used are: W1 = 23 eV, W0 = 3.68 eV,
σse,max(0) = 3, ɛ0 = 0, pd = 15 Pa·mm for N = 1, and pd =
7 Pa·mm for N = 3.
directly from the vacuum threshold without showing any maximum and
the same behaviour is seen in Figs. 3.9 and 3.10. Even with a low W1,
where the contribution of electrons from collisional ionisation is low, a
monotonically decreasing threshold is obtained. The cause of this distinct
lowered threshold is the partial thermalisation of the electrons due
to the collisions. The velocity spread results in a total impact velocity,
which is greater than the drift velocity alone and thus for the same
secondary electron emission, a lowered impact drift velocity is possible.
Even though the friction force requires a higher voltage to achieve the
same impact drift velocity, the thermalisation effect dominates, with a
lowered threshold as a result. This becomes clear in Fig. 3.11, where it is
apparent that it is not the electrons from ionisation that constitute the
main reason for a decreased threshold, rather it is a consequence of the
partial thermalisation. In the case with a high W1, the thermalisation
effect is also important, but without the contribution from collisional
ionisation, the behaviour would not be the same, which can be seen in
Fig. 3.12.
To summarise the key findings from the more advanced model, it
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