Evaporative cooling is a process that occurs in atomic physics, particularly in the context of ultracold gases. It refers to the technique used to achieve and maintain very low temperatures in a system of atoms or particles. Here's how it works: 1. **Basic Concept**: In a system of particles, the temperature is related to the average kinetic energy of the particles. Higher energy particles move faster, while lower energy particles move slower. Evaporative cooling takes advantage of this distribution of energies.
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Evaporative cooling of atoms lowers the mean motional energy of a trapped gas by preferentially removing its more energetic atoms. Subsequent elastic collisions redistribute the remaining energy, producing a cooler approximately thermal sample.
In forced evaporative cooling, a decreasing trap-depth threshold or a radiofrequency evaporation knife continuously removes the high-energy tail. Losing atoms alone is insufficient: losses that sample all energies equally do not provide the same cooling mechanism. Useful evaporation increases phase-space density of an atomic gas while retaining enough collisions for rethermalization. An expansion measurement before and after evaporation tests the velocity distribution as well as the retained atom number.