Dilution refrigerator

Dilution refrigerator, also known as ( un) dilution cryostat, are refrigerators ( cryostat ) that reach temperatures are extremely low. The underlying cooling dilution (English dilution refrigeration ) is the most common non-magnetic engineering to achieve continuous temperature range of a few milli- Kelvin. The underlying cooling mechanism was proposed in 1951 by Heinz London and was first implemented about ten years later, technically. Temperatures of less than one Kelvin are required for example in the semiconductor basic research.

Operation

See also: 3He - 4He mixture cooling.

Cooling is achieved by a phase transition in a mixture of liquid helium isotopes 3He and 4He. Below a critical temperature of about 0.86 K, the helium mixture forms two phases, which arrange themselves due to their different density in horizontal layers: above the 3He - rich, concentrated, quasi-liquid phase below the 3He - poor, dilute quasi gaseous phase.

Due to the shape of the phase diagram a 3He share of 6.5 % is not exceeded even in the dilute phase. There is a thermodynamic equilibrium between the two phases. 3He is continuously further removed from the dilute phase, and fed to the concentrated phase, the equilibrium is disturbed. Once the concentration of 3He in the dilute phase falls below the critical value of 6.5% through 3He atoms from the concentrated phase undergoes a phase transition and go over into the dilute phase. This process corresponds to an evaporation. The energy required for this is the area deprived of thermally: the system is colder.

In principle, this cooling mechanism allows the generation of arbitrarily low temperatures, since even for a T0 K 3He share of 6.5 % is never reached. Practically, however, limited the incompletely suppressible heat input from the surroundings into the cryostat, the minimum achievable temperature to typically a few milli- Kelvin.

Construction

In the dilution refrigerator is the boundary between two phases in the mixing chamber. In order to maintain the cooling dynamic equilibrium, the lower phase 3He must be removed continuously. To the diluted solution is pumped to an evaporation chamber (English Still ) and heated up to about 600 mK. Because of the different vapor pressures of the two isotopes thereby evaporates mainly 3He. Heat exchanger this 3He gas is heated to room temperature, passes through the pump, and is cleaned in cold traps. Then it is cooled by conventional techniques (evaporation cooling in the so-called pot - 1K ) and above the heat exchanger again, until it is liquefied in the condenser and then supplied to the upper 3He - rich phase. The cooling capacity is determined by the flow of 3He and is usually several hundred microwatts.

In order to obtain the lowest possible temperatures of this limited performance, the flowing of external heat energy must be minimized. Therefore, the cryostat is in a Dewar flask, the thermally decoupled it from the laboratory environment. In its layered structure of the Dewar resembles a conventional thermos. Typically come from outside to inside an outer vacuum chamber, a bath of liquid nitrogen (77 K ), a bath of liquid helium ( 4.2 K) and an inner vacuum chamber. In the internal vacuum chamber is the mixing chamber. There, there is the lowest temperature. The test sample is either in the inner vacuum with good thermal contact with the mixing chamber, or directly in the helium mixture within the mixing chamber.

Another source of heat is the leading from the outside inwards electrical wiring. Heat flows from the on to laboratory temperature side of the cable into the cryostat inside. In order to efficiently dissipate this heat, without heating the sample to be wound coming from the outside lines at each stage of the cryostat to metallic cold finger and thereby thermally coupled as well as possible: in the helium bath at 4.2 K, the 1K - pot at about 1.2 K, to the evaporation chamber at approximately 700 mK, and finally to the mixing chamber at a few milli- Kelvin. It must be ensured that the cooling steps are further thermally isolated from each other. Depending on the experimental requirements, this can be either by high-resistance wiring ( Wiedemann - Franz law ) or superconducting materials can be achieved.

In the illustrated Verdünnungskryostat 24 lines and 24 lines of constantan NbTi were transferred, which are each connected to a sample holder. For NbTi is a type -2 superconductor (), which is why the superconducting properties are retained even at high magnetic fields. The doubling of the number of measured lines from 24 to 48 resulted in this cryostat to an increase in the lowest achievable mixing chamber temperature from 13 to 18 mK.