Evaporative cooling

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Evaporative cooling is a class of cooling processes that are based on removing the most energetic particles from a thermodynamic system . It is related to and is sometimes referred to as evaporative cooling .

If there is an interaction between the particles in the system, they can thermalize, that is, a new thermodynamic equilibrium is established . Since the mean energy of the remaining particles is smaller than in the original system, the temperature of the new state of equilibrium is lower. Repeated use of this process can produce very low temperatures, but in doing so, a large part of the particles is lost.

Evaporative cooling processes are used in quantum optics to generate gases with temperatures close to absolute zero . The gas is in a magnetic or optical trap , the particles are removed from the trap either by themselves or, much more efficiently, by lowering the trap potential or by transitioning the particles to other internal quantum states that are not trapped in the trap .

Evaporative cooling, e.g. B. by radio frequency transitions between different Zeeman states in a magnetic trap, is the decisive step in the production of Bose-Einstein condensates .

This can be clearly compared to cooling a cup of coffee by “blowing” it. Among other things, hot particles near the surface of the liquid are removed so that further hot particles can leave the liquid and change into the vapor phase. The liquid loses some thermal energy (it becomes cooler).

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