Convection and Critical Dynamics in Helium Three Superfluid Helium Four Mixtures

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The proposed measurements, to be carried out using superfluid helium mixtures, will address a number of questions of considerable current interest. Using a pioneering flow visualization technique exploiting the interaction of thermal neutrons and helium three, they will obtain the first images of flows near the onset of convection in liquid helium. Additional measurements will use the very high temperature resolution obtainable at low temperatures and a variable geometry technique developed in this laboratory to further characterize these flows. Superfluid mixtures have a unique and very interesting parameter range in which the hydrodynamic instabilities of steady convection will change character. Using appropriate geometries, they will explore in detail these instabilities, the way they lead to chaos, and the way they are affected by the steady convection patterns. In a second set of measurements using the neutron imaging technique, they will establish the helium three profiles which are caused by the flow of heat in a superfluid mixture. By these measurements, they hope to understand recent data which show a striking violation of current theories describing the heat flow in these mixtures. The importance of the theories involved--Two-fluid Hydrodynamics and Dynamic Renormalization Group Theory--requires a careful examination of the heat flow problem in mixtures using a direct, but non- perturbative technique such as the neutron probe.