High Rayleigh number convection with gaseous helium at low temperature

This article presents an experimental set-up to study the turbulent regime of free thermal convection in a Rayleigh-Benard cell. Helium gas around 4 K is confirmed in a cell of aspect ratio 0.5. With a thermocouple technique we can measure temperature differences across the cell as low as 0.2 mK, which allows to test the adiabatic gradient effect. Covering a large range of Rayleigh numbers (10(3) to 5 x 10(12)), Nusselt numbers from 1 to 10(3) are obtained. The results are compared with previous works. They show a departure from the 2/7 power law above Ra = 3 x 10(10).