This is a device for producing temperatures below 1 K. It was used for experiments on the properties of two-dimensional sheets of ions held just below the surface of superfluid liquid helium at temperatures down to 0.01 K.
It makes use of the properties of a mixture of the two isotopes of helium, the common He-4 and the very rare and expensive He-3. When the mixture is cooled below 4 K it liquefies and below 0.86 K it separates into two phases: a concentrated phase which is almost pure He-3 and a dilute phase which 94% He-4. The pure phase floats on the denser dilute phase. The He-3 atoms in the dilute phase are sufficiently far apart to act like a gas. To achieve cooling the He-3 is ‘evaporated’ across the boundary between the concentrated and dilute phases. There is a latent heat associated with this and thus the liquid cools. The He-3 is then evaporated as a true gas from the upper surface of the liquid, recompressed at room temperature and returned under pressure to the low temperature system so that the device operates continuously as a refrigerator for months on end. The whole system has many stages at different temperatures with heat exchangers between them. They are all suspended in a vacuum chamber (removed from the exhibit).

Notes:  Bibliography Barenghi CF, Mellor CJ, Muirhead CM and Vinen WF
Experiments on ions trapped below the surface of superfluid He-4
Journal of Physics C-Solid State Physics 19, 1135-1144 (1986).