Transient phenomena initiating phase transition in dilution refrigerator

Indigenous Development of a Millikelvin Refrigerator at VECC, Kolkata

“…Of the several unique features exhibited by helium, one of the most convenient for a low temperature physicist is that at sufficiently low temperatures (about 0.8 K), a mixture of 3 He and 4 He will spontaneously separate into two phases, with the lighter 3 He rich fraction floating on top of the heavier 4 He rich fraction. Since there is an interface between two phases, extra energy is required for particles ( 3 He) to go from lighter phase to heavier phase.…”

“…Below this temperature, the vapour is very small, and very little would evaporate. Because of the lower mass of the atom, 3 He has higher vapour pressure and therefore higher vaporization rate. Hence, we can reach further down to 0.3 K if we use 3 He instead of 4 He.…”

“…Because of the lower mass of the atom, 3 He has higher vapour pressure and therefore higher vaporization rate. Hence, we can reach further down to 0.3 K if we use 3 He instead of 4 He. Cooling below 0.3 K is not possible by conventional refrigeration technique; however, a dilution refrigerator can cool down to a few mK that can be maintained for several hours.…”

“…Since there is an interface between two phases, extra energy is required for particles ( 3 He) to go from lighter phase to heavier phase. As the still is pumped out, differences in vapour pressure between the two isotopes lead to 3 He being primarily removed from the dilute phase in the still. The 3 He that is pumped off at the still is pre-cooled through a series of heat exchangers in order to continue the cooling process.…”

“…Cooling below 0.3 K is not possible by conventional refrigeration technique; however, a dilution refrigerator can cool down to a few mK that can be maintained for several hours. It is based on the solution and separation of two isotopes of helium, 3 He and 4 He, and can produce a stable base temperature of few millikelvin to serve as a starting point for very low temperature experiments.…”