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

σse,max the zones become wider and zones previously suppressed by the
losses can appear (cf. Fig. 5.11). This behavior is very similar to that
noted for the parallel plate geometry [46]. The lower (left) envelope is
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0.2 0.4 0.6 0.8 1
V /V
ω max
1.2 1.4 1.6 1.8
Figure 5.10: Number of electrons after 200 rf-cycles. The vertical straight
lines indicate the lower and upper theoretical envelopes according
to Eq. (5.12). Parameters used: Ri/Ro = 0.7, f = 1.5 GHz,
σse,max = 1.3, vT/vmax = 0.01, and W1 = 50 eV.
obeyed, but the upper can be exceeded since a non-zero initial phase will
yield a lower impact velocity and a concomitant higher upper threshold.
In Figs. 5.10 and 5.11 the velocity spread is quite low, vT/vmax =
0.01. When increasing this ratio, a greater portion of the electrons
can have a large negative initial phase, which leads to overlapping of
the multipactor regions when σse,max is large (see Fig. 5.12). On the
other hand, the increased velocity spread also increases the losses, which
especially affects the higher order resonances, since the phase-focusing
effect gets weaker with increasing mode order. If σse,max is low, the
losses are not sufficiently compensated for and this leads to suppression
of the higher order modes (cf. Fig. 5.13). This is a result similar to
that found in the parallel plate case [46]. The corresponding behaviour
is seen also for single-sided multipactor (see paper F).
For single-sided multipactor it was noted that for the first order
mode, the envelope of the breakdown zones Vω,o = v1/2 was not obeyed.
This is confirmed by the PIC-simulations, where the first order mode
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