How are materials cooled to cryogenic condition?

Answer 1

1. Air is compressed to very high pressure until it liquifies.
2. This produces hot high pressure liquid air in a pressure vessel.
3. The hot liquid air is allowed to cool back to ambient temperature.
4. This reduces the pressure some, but it still must be stored in a pressure vessel to keep it liquid.
5. The pressure vessel is now carefully vented to permit the gasses in the liquid air with higher boiling points evaporate.
6. This causes the temperature of the remaining liquid air to drop, stopping evaporation.
7. The liquid air can now be transfered to a dewar at atmospheric pressure and ambient temperature, with minor loss to evaporation.
8. By allowing it to continue to evaporate and collecting the gasses that evaporate off and recompressing them as above, you can get colder and colder cryogenic gasses as well as recapturing with little loss the warmer ones that evaporated. This process is called fractional liquefaction.
9. To get to the coldest liquid helium, you must let it cool itself by self evaporation of the warmest atoms in the liquid. After this point is reached laser beam traps can hold the helium and kick out more barely warmer atoms taking the residue down to about 10E-6 Kelvin.