Compressional cooling of He3 to millidegree temperatures

A study of adiabatic compressional cooling of a two-phase mixture of solid and liquid He³ has been made. Details of the experimental arrangement are given. Using a flexible elastic low temperature He³ cell that is compressed by liquid He⁴ and precooled by a continuously operating dilution refrigerator, cooling to below 2 m°K was obtained from starting temperatures near 20 m°K. Frictional heating is remarkably low. Calculations are presented which investigate the fraction of solid formed and which suggest rather long thermal equilibrium times in the solid He³ nuclear spin system. Application of the method to the production of bulk polarized He³ nuclei is discussed.This research was supported by the U.S. Atomic Energy Commission under contract AT(11-1)-34, P.A. 143.This work is based on the thesis of R. T. Johnson, submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Illinois.