The Coldest place in Kazakhstan
We have different type of refrigerators that can be classified to the Passive coolers and active coolers. When we don’t need input power for a refrigerator it is called passive coolers and if we need input power we will call them the type of active coolers. Active coolers use closed thermodynamic cycles to transport heat up a temperature gradient to achieve lower cold-end temperatures at the cost of electrical input power.
ECL cryostat consists of Pulse tube and ADR (Model 106 Shatsa from HPD company). Figure 1 shows internal view of this refrigerator. It has 4 different temperature levels for doing experiments.
Fig. 1. Inside of the ECL refrigerator
Pulse tubes are based on causing a working gas to undergo a Stirling cycle which consists of a compressor and a fixed regenerator. The reliability and efficiency of pulsed tubes are higher than Stirling cycles machines theoretically and The process in Adiabatic Demagnetization Refrigeration (ADR) utilizes the magneto-caloric effect with a paramagnetic salt. (Gadolinium Gallium Garnet (GGG) paramagnetic salt pill, generating a 500 mK stage and a Ferric Ammonium Alum (FAA) paramagnetic salt pill, generating a 30 mK stage). The basic scheme of ADR process is shown below:
Fig. 2. Analogy between magnetic refrigeration and vapor cycle or conventional refrigeration. H = externally applied magnetic field; Q = heat quantity; P = pressure; ΔTad = adiabatic temperature variation.
Figure 3 shows a series of entropy over R curves as a function of time and magnetic field for a paramagnetic salt pill (in this case FAA). The cycle progresses in three steps. At the start of step one, the salt pill is thermally sunk to the 4 K stage with the ADR magnet at zero field (labeled “Before Mag Up” above). The first step involves isothermally magnetizing the pill from zero field to 4 T ending at the point labeled “At Field After Dwell”. During the second step, after thermally isolating the pill from the 3 K stage by opening the heat switch, the pill is adiabatically demagnetized to zero field reaching the base temperature of the pill (in this example 50 mK). During the demagnetization, heat is absorbed from the surrounding ballast mass spending the energy represented by the area shaded blue. The third step involves either allowing the salt pill to warm up naturally along the line labeled B = 0 T or regulating the temperature of the pill (in this example at 100 mK) by regulating the magnetic field. During natural warm up, the cooling power of the pill is given by the area shaded red. During a regulated warm up, the cooling power of the pill is given by the area shaded in red plus the area shaded in green. The green area is the “bonus” entropy gained by regulating at a temperature instead of allowing the system to warm-up naturally.
Fig .3. Paramagnetic Salt Pill Cooling Cycle Schematic
Fig. 4. ECL and HPD people celebrating the refrigerator installation successfully at Nazarbayev University. (from right to left: Mehdi Shafiee, Marzhan Bekbalanova, Orri Jonsson, Charlie Danaher, Aliya Nurmukhanbetova, Mikhail Denissenya, Baurzhan Alzhanov, Ainur Koshkinbayeva)