2-DIMENSIONAL TRANSPORT AND WALL EFFECTS IN THE THERMAL-DIFFUSION CLOUD CHAMBER .2. STABILITY OF OPERATION

In this paper, the second of a series of two presenting a detailed des cription of thermal diffusion cloud chamber operation, we address the operational stability of the vapor-gas mixture in a diffusion cloud ch amber with respect to density profile extrema and the accompanying pos sibility of buoyancy-driven convective flow disturbances. We examine c onditions for stable operation (no convective flow disturbances) in th e central portion of the cloud chamber, as well as conditions necessar y for stable operation in the vicinity of the cloud chamber wall. We f ind that the total density profile in the central portion of the cloud chamber can pass through a density minimum ever though the density at the upper plate surface is less than the density at the lower plate s urface. This local density profile inversion can result in unstable (c onvective) behavior that propagates through the cloud chamber. Further more, we find that local extrema in the total density profile near the chamber wall can lead to subtle, convective flows that are difficult to detect yet can exert a profound influence on nucleation in the cent ral portion of the cloud chamber. We have developed a simple method to estimate the limiting total pressure in a cloud chamber that will sup port stable operation. From results of our investigations based upon t his method, it appears that the thermal diffusion cloud chamber is bes t suited for experiments at higher temperatures where the accessible t otal pressure range is largest. Finally, we find that results of our i nvestigation into the effects of total pressure and kind of background gas on nucleation in diffusion cloud chambers involving the low molec ular weight alcohols and hydrogen and helium background gases cannot b e explained on the basis of these kind of density disturbances occurri ng within the diffusion cloud chamber. Also, for (relatively) low vapo r pressure materials such as 1-pentanol or other high molecular weight alcohols and alkanes stability limitations may preclude nucleation me asurements at low temperatures using a diffusion cloud chamber altoget her. (C) 1997 American Institute of Physics.