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

addition, it is difficult to make compact, light weight, and high gain
antennas for low frequency transmission. Many microwave components
are hollow metallic structures that guide the electromagnetic power. A
free electron inside the device will experience a force due to the electric
field and since there is no gas or other material stopping the electron,
it can accelerate to a very high velocity. Upon impact with one of
the device walls, the energetic electron can knock out other electrons
and under certain circumstances this procedure is repeated continuously
until the electron density is large enough to counter-act the effect of the
applied electric field and a steady state is achieved. A consequence of
this can be that the incident power is reflected instead of transmitted
to the intended load. Since many satellites lack sufficient protection
against reflected power in order to save weight, such reflected power can
cause severe damage to the high power stage of the system.
When a satellite is launched it carries fully charged batteries and
in the beginning, before the solar panels are deployed, they are the
only source of electric power. The capacity of these batteries is usually
low and may only last a couple of days, since a satellite in operation
will normally only lack access to power from the solar panels for a few
hours, at most, and consequently the batteries are made small in order to
save weight. If the solar panels are not deployed before the batteries are
exhausted, the satellite is permanently lost. Thus, a new satellite is often
taken into operation quickly after being put into orbit. A concern then
is that the satellite components may not be completely vented and there
is a risk for ordinary corona breakdown, which is more prone to occur
at intermediate pressures than at high and very low pressures. A corona
discharge is usually much more detrimental than multipactor and for a
certain range of pressures, the breakdown threshold for corona is lower
or much lower than for multipactor. Basic theory for ordinary corona
microwave discharge, when the mean free path of the electrons is smaller
than the characteristic length of the device, is well known [9]. However,
the intermediate range, between very low pressure and vacuum, has
received little attention and therefore one of the main topics of this thesis
is devoted to explaining what happens with the breakdown threshold at
these pressures.
Theoretical studies of the multipactor phenomenon have to a great
extent been performed using a one-dimensional model with a spatially
uniform approximation of the electromagnetic field. However, many
common RF devices involve structures where the field is inhomogeneous,
2