Mixture cryostat
Dilution cryostats , also known as (de) mixing cryostats , are cooling devices ( cryostats ) that reach particularly low temperatures . The underlying dilution cooling (engl. Dilution refrigeration ) is continuous to temperature ranges of a few milli the most common non-magnetic technique kelvin to achieve. The underlying cooling mechanism was proposed by Heinz London in 1951 and implemented for the first time about ten years later. Temperatures below one Kelvin are required in basic semiconductor research , for example .
functionality
The cooling is effected by a phase transition in a mixture of the liquid helium - Isotope 3 He and 4 He. Below a critical temperature of about 0.86 K, the helium mixture forms two phases which, due to their different densities, are arranged in horizontal layers: above the 3 He-rich, concentrated, quasi-liquid phase, below the 3 He-poor, dilute, quasi-gaseous phase Phase.
Due to the form of the phase diagram , the 3 He content never falls below 6.5% , even in the diluted phase . There is a thermodynamic equilibrium between the two phases . If further 3 He is continuously removed from the dilute phase and added to the concentrated phase, the equilibrium is disturbed. As soon as the concentration of 3 He in the dilute phase falls below the critical value of 6.5%, 3 He atoms from the concentrated phase undergo a phase transition and pass into the dilute phase. This process corresponds to evaporation . The energy required for this is thermally extracted from the environment: the system becomes colder.
In principle, this cooling mechanism allows the generation of any lower temperature, since even for a 3 He content of 6.5%, the temperature never falls below 6.5%. In practice, however, the heat input from the environment in the cryostat, which cannot be completely suppressed, limits the minimum attainable temperature to typically a few millikelvin.
construction
In the dilution cryostat, the boundary between the two phases is in the mixing chamber. In order to maintain the cooling dynamic equilibrium, 3 He must be continuously withdrawn from the lower phase . For this purpose, the diluted solution is pumped into an evaporation chamber ( still ) and heated there to about 600 mK. Because of the different vapor pressure of the two isotopes, mainly 3 He evaporates . This 3 He gas is heated up to room temperature via a heat exchanger , passes through the pump and is cleaned in cold traps . It is then cooled again using conventional techniques ( evaporative cooling in the so-called 1K pot) and the above-mentioned heat exchanger until it liquefies in the condenser and is finally fed to the upper, 3 He-rich phase. The cooling capacity is determined by the flow of 3 He and is usually a few hundred micro watts .
In order to achieve the lowest possible temperatures with this limited power, the heat energy flowing in from the outside must be minimized. The cryostat is therefore located in a dewar which thermally decouples it from the laboratory environment. In its layered structure, the Dewar resembles a standard thermos flask . Typically from the outside to the inside come an outer vacuum chamber, a bath with liquid nitrogen (77 K), a bath with liquid helium (4.2 K) and an inner vacuum chamber. The mixing chamber is located in the inner vacuum chamber. That is where the lowest temperature prevails. The sample to be examined is either in the internal vacuum with good thermal contact to the mixing chamber, or directly in the helium mixture within the mixing chamber.
Another source of heat is the electrical wiring that runs from the outside to the inside . Heat flows into the cryostat from the laboratory side of the cables. In order to efficiently dissipate this heat without heating the sample, lines coming from the outside are wrapped around metallic cold fingers at every stage of the cryostat and thus thermally coupled as well as possible: in the helium bath at 4.2 K, on the 1K pot at around 1.2 K, at the evaporation chamber at around 700 mK and finally at the mixing chamber at a few millikelvin. It must be ensured that the cooling stages are still thermally isolated from one another. Depending on the experimental requirements, this can be achieved either through high-resistance cabling ( Wiedemann-Franz law ) or superconducting materials .
In the dilution cryostat shown, 24 lines made of constantan and 24 lines made of NbTi were laid, each connected to a sample holder. NbTi is a type 2 superconductor ( ), which is why the superconducting properties are retained even with high magnetic fields. The doubling of the number of measuring lines from 24 to 48 led to an increase in the lowest achievable mixing chamber temperature from 13 mK to 18 mK in this cryostat .
