Abstract
Boundaries affect the measured values of transport coeffcients in all drift
tube experiments, to a greater or lesser extent, and nowhere is this more
apparent than in the experiment first devised by Cavalleri (1969) and
subsequently adapted by Crompton and coworkers in the 1970s. The phenomenon of
‘diffusion cooling’ is particularly striking and arises
essentially from a penetration of the ‘boundary layer’ (of
thickness of the order of the mean free path for energy exchange) throughout a
significant portion of the gas chamber. Although this is something of an
obstacle to extracting the classical diffusion coefficient from experimental
data, it is of great interest in its own right from a theoretical point of
view, and the Crompton et al. experiments motivated
several theoretical treatments which successfully explained diffusion cooling,
albeit for zero applied field and on the basis of the ‘two-term’
spherical harmonic representation of the velocity distribution function. The
present paper puts these theories in the context of the modern, generalised
eigenvalue theory, which may be used as a basis for describing all swarm
experiments. In addition, the earlier zero-field studies are generalised to
the extent that an a.c. heating field is included, as was the case for the
original Cavalleri experimental set-up. This field is found to enhance
diffusion cooling effects for a simple model cross section.
Subject
General Physics and Astronomy
Cited by
15 articles.
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