Rayleigh-Benard convection onset in a compressible fluid: He-3 near T-C

We present results on the onset of convective behavior of He-3 along the cr itical isochore where the fluid compressibility diverges as T --> T-c, the critical temperature. The aspect ratio of the Rayleigh-Benard cell was 57. The "adiabatic temperature gradient" that stabilizes the fluid layer was ob served and agreed with predictions. This is the first systematic study of t he convection onset in the compressibility-dominated stability regime. The initial slope of the convection current versus the reduced Rayleigh number was found to be independent of compressibility and to be the same as in exp eriments with a similar aspect ratio in liquid He-3, He-4, and water. [S003 1-9007(99)09283-2].