Since its construction in the late 1970s1, this refrigerator has reached record low temperatures. This was achieved through the development at Lancaster of an immersed refrigerant technique for demagnetisation cooling using copper metal.2
Early experiments included the study of transport properties in 3He/4He solutions and also measurements of nuclear spin-lattice relaxation in copper down to 7 μK.
More recently, the experiments carried out on this refrigerator have centred on superfluidity in 3He at the extreme of experimental low temperatures. The transition temperature to superfluidity is just below 1 mK at zero pressure, and our measurements are made around 100 μK. Here, far below Tc, the normal fluid fraction is then so dilute that a condensate-dominated regime opens up where the quasiparticle excitations behave ballistically.
One of the unique features of these experiments is our use of shaped magnetic fields to control the transition between the A (high field) and B (low field) phases of the superfuid. We have looked at the properties of ballistic quasiparticles in both the A and B phase, and then used such quasiparticles to probe the interface between the two phases.
1. D. I. Bradley et al, Cryogenics 22 296 (1982)
2. D. I. Bradley et al, J. Low Temp. Phys. 57, 359, (1984)