While liquid nitrogen (77 K) is purchased via an external supplier, we liquify Helium (4.2 K) mainly directly in our institute. Helium gas, which evaporates during experiments, is collected via a closed line system, stored and delivered to the Helium liquifier.
The Helium liquifier is of the type Sulzer TCF20, which is a turbine liquifier. This is powered by a 90 kW compressor and liquifies approx. 17-25 liter/h. The liquified Helium is stored in a superinsulated tank with 2000 liters storage capacity. From there its is transfered to 100 liter transport vessels, which are brought to the labs for further experiments..
We also supply various groups in physics and chemistry, as well as the MEG-center and part of the Max-Planck-Insitute for biological cybernetics with liquid Helium. Per year we deliver in total approximately 60,000 liter liquid Helium.
The liquified Helium is 4He. This has at ambient pressure a temperature of 4.2 K. By pumping on the gas above the Helium bath one can cool down to approx. 1 K, while the Heliumbath undergoes a transition to the superfluid state.
If temperatures below 1K are required, one can use the (rare) Helium isotope 3He. Due to its higher vapor pressure, with 3He one can obtain via evaporative cooling temperatures down to 300mK. Two compact „single shot“ 3He cryostats are availble in our group. One of those is suited for large magnetic fields (12 Tesla magnet).
Even lower temperatures can be obtained with 3He/4He dilution refrigerators. Those use a mixture of both Helium isotopes and use the principle of dilution refrigeration of 3He in suprafluid 4He in order to obtain temperatures down to 10mK. At the moment our group has two of such continously working cryostats available.