Evaporative cooling

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By pumping the vapor over a liquid, it can be cooled. The vapor pressure on the surface of the liquid is primarily formed by atoms or molecules with higher kinetic energy than the mean value , which can then be released from the liquid. If these particles are pumped out, the mean kinetic energy in the system drops. This is equivalent to a lower temperature.

4 He evaporative cooling

Schematic structure of a cryostat with 4 He evaporative cooling.

In order to generate low temperatures, liquid helium is usually used as a starting point (boiling temperature: 4.21 Kelvin ). A temperature of around 0.8 K can be continuously maintained by pumping in a 4 He bath.

Since a large part (more than 40% of the liquid) would have to be pumped out in order to achieve this type of cooling, in the case of 4 He evaporative cooling, the pump is not pumped directly at the bath, but at a thermally insulated pot that flows out of the Bad of the cryostat is replenished. In an arrangement (a cryostat ) for ultra-low temperatures , this stage is therefore usually referred to as a 1K plate or 1K flange.

See also: low temperature physics

3 He evaporative cooling

By using the rare 3 He isotope, cooling to around 0.3 K can be achieved. The main reason for this is the smaller mass of the atoms, which leads to a significantly higher vapor pressure (at 1 K by a factor of more than 500). In this case, a rare isotope circuit isolated from the helium bath is used.


  • Charles Kittel, Herbert Krömer: Thermodynamics. 6th improved edition, Oldenbourg Verlag, Munich 2013, ISBN 978-3-486-73607-6 .
  • Rudolf Plank (Ed.): Handbook of refrigeration technology. Volume 12, Springer Verlag Berlin Heidelberg GmbH, Berlin Heidelberg 1967.

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