Dilution Refrigerator

Dilution Refrigerator

Dilution refrigerators provide continuous cooling to temperatures as low as 2 mK without moving parts in the low-temperature region. Details can be found e.g. in the books by Lounasmaa and Pobell, and the references therein.

A "'dilution refrigerator"' is a cryogenic device first proposed by Heinz London. The first dilution refrigerator was built in 1964 in the Kamerlingh Onnes Laboratorium in Leiden and further developed by Hall, Wheatley, Niinikoski, and Frossati. The refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4. When cooled below approximately 870 millikelvin, the mixture undergoes spontaneous phase separation to form a 3He-rich phase (the concentrated phase) and a 3He-poor phase (the dilute phase). At very low temperatures the concentrated phase is practically pure 3He and the dilute phase contains about 6.6% 3He and 93.4% 4He.

Fig.1 is a schematic diagram of a classical dilution refrigerator.

The working fluid is 3He which is circulated by pumps at room temperature.

Fig.2 zooms in on the low-temperature part.

The 3He enters the cryostat at a pressure of a few hundred millibar. In this example it is precooled by liquid nitrogen at 77 K and a 4He bath at 4.2 K. Next, the 3He enters a vacuum chamber where it is further cooled to a temperature of 1.2-1.5 K by the so-called 1K bath, which is a pumped 4He bath. At the 1K bath the 3He gas is liquefied. The heat of condensation is removed by the 1 K bath. Next the 3He enters the main impedance. This is a capillary with a large flow resistance. Next the 3He is cooled by the so-called still (to be explained later) which is at a temperature of around 500 to 700 mK. Subsequently the 3He flows through a secondary impedance and one side of a set of counterflow heat exchangers where it is cooled by a cold flow of 3He on the other side. Finally the pure 3He enters the mixing chamber which is the coldest spot of the machine where the cooling power is produced.

In the mixing chamber two phases of 3He-4He mixtures are in equilibrium: the so-called concentrated phase (practically 100% 3He) and the dilute phase (about 6.6% 3He and 93.4% 4He). The boundary between the two phases, is called the phase boundary. In the mixing chamber the 3He flows from the concentrated phase through the phase boundary into the dilute phase (is diluted). The heat, needed for the dilution, is the useful cooling power of the refrigerator. The 3He leaves the mixing chamber in the dilute phase. On its way up the cold 3He cools the downward flowing 3He via the heat exchangers until it enters the still. At the dilute side the 3He flows through superfluid 4He which is at rest. The pressure in the still is kept low (about 10 Pa) by the pumps at room temperature. The vapor in the still is practically pure 3He. Heat is supplied to the still to maintain a steady flow of 3He. The pumps compress the 3He to a pressure of a few hundred millibar hence closing the cycle.

Modern dilution refrigerators can be precooled not by liquid nitrogen, liquid helium, and a 1K bath, but by a cryocooler as shown in Fig.4. As resulting advantage no external supply of cryogenic liquids is needed in such "dry cryostats" and operation can therefore be automatized better. Disadvantages include high energy requirements of the cryocooler and mechanical vibrations as e.g. intrinsic to the pulse tube refrigerator.