Compressional Cooling of He-3 to Millidegree Temperatures

A study of adiabatic compressional cooling of a two-phase mixture of solid and liquid He-3 has been made. Details of the experimental arrangement are given. Using a flexible elastic low temperature He-3 a cell that is compressed by liquid He-4 and precooled by a continuously operating dilution refrigerator, cooling to below 2 m degrees K was obtained from starting temperatures near 20 m degrees 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 He-3 a nuclear spin system. Application of the method to the production of bulk polarized He-3 nuclei is discussed.