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

3.1.3 Main results
The main result found in paper B is that a higher microwave power is
required to initiate breakdown in a low pressure gas, since the collisions
tend to slow down the electrons. By combining the low pressure multipactor
graph with the corona threshold curve, it was concluded that
with increasing pressure, the required threshold will first increase and,
after reaching a plateau, it will make a smooth transition to the low
pressure branch of the Paschen curve. This behaviour is confirmed by
the investigations made by Gilardini [53] for materials with a low first
cross-over point, close to the ionisation energy of the gas, and for N = 1,
i.e. for the first order of multipactor.
For materials with a higher first cross-over energy and for higher
order multipactor, Gilardini found no initial increase in the multipactor
threshold, instead a monotonically decreasing breakdown voltage was
seen. A possible explanation for the differences between the result obtained
by the simple analytical model and the result of Gilardini is also
presented in paper B and it is suggested that the reason is that for materials
with a higher W1 and for N > 1 the contribution of electrons from
impact ionisation decreased the required W1 (see Fig. 3.4). However,
as will be seen in the next section, electron contribution from impact
ionisation is not the only reason for this behaviour. The collisions will
also cause an electron velocity spread, which will result in a larger total
impact velocity and thus a lower voltage is needed to achieve the
necessary first cross-over energy.
A comparison was made with experiments by Höhn et al. [18] in low
pressure argon as well as with PIC-simulations by Gilardini [54] in the
same gas (see Fig. 7 in paper B). However, the fd-product chosen by
these authors was located in the middle of the right boundary of the
first multipactor zone, an area dominated by the hybrid modes [39].
The simple model used in the presented analytical approach is not applicable
to these modes and consequently the behaviour found in the
experiments and simulations could not be confirmed. Furthermore, the
impact energy of the electrons at this fd-product is several times higher
than the ionisation energy of argon and thus a significant contribution
of electrons from collisional ionisation would be expected. In addition,
the required W1 would be reduced due to the electron velocity spread
and thus a behaviour similar to curves (b) or (c) in Fig. 3.4 should be
expected and it is also what is found.
To further analyse multipactor in a low pressure gas, a better ana-
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