DIFFUSION COOLING OF ELECTRONS IN AN AC FIELD

Boundaries affect the measured values of transport coefficients 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 a rises essentially from a penetration of the 'boundary layer' (of thick ness of the order of the mean free path for energy exchange) throughou t 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 th eoretical point of view, and the Crompton et al. experiments motivated several theoretical treatments which successfully explained diffusion cooling, albeit for zero applied held and on the basis of the 'two-te rm' spherical harmonic representation of the velocity distribution fun ction. The present paper puts these theories in the context of the mod ern, generalised eigenvalue theory, which may be used as a basis for d escribing all swarm experiments. In addition, the earlier zero-field s tudies are generalised to the extent that an a.c. heating field is inc luded, 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.