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.
Michael Faraday's 1845 experiment combined solid carbon dioxide, diethyl ether and reduced pressure. Removal of vapour cooled the bath further. He reported a low alcohol-thermometer indication near , while explicitly discussing lag and uncertainty in the graduation at low temperature.[ref]
Repeated removal of high-energy trapped atoms while lowering the energy cutoff as the sample cools. Elastic rethermalization replenishes the tail. The useful outcome is increased phase-space density of an atomic gas, not temperature reduction alone.
After energy-selective loss, elastic collisions redistribute energy and refill the escaping tail. This restores a nearly thermal distribution when redistribution is fast compared with the changing cutoff and unwanted atom loss. The collision rate depends on number density, cross section, and relative speed; slowing atoms alone does not guarantee continued equilibration.
Runaway evaporative cooling is an evaporation regime in which cooling makes subsequent elastic rethermalization of a trapped atomic gas faster rather than slower. In a sufficiently confining trap, the cooling sample can become denser enough that the increasing number density outweighs the decreasing collision speed in .
The growing collision rate helps replenish the high-energy tail removed by forced evaporative cooling. It is not automatic: trap weakening, unfavorable cross section changes or unwanted particle loss can instead reduce the rate. Measuring atom number, cloud size and temperature during the ramp tests whether the favorable regime is actually maintained.
A radiofrequency field induces transitions from trapped weak-field-seeking magnetic sublevels to untrapped states at a resonance surface. Atoms with sufficient energy reach that surface and leave. Sweeping the frequency changes the energy cutoff without equating every lost atom with the sample-average energy.
For a classical three-dimensional harmonic trap of fixed frequencies, . If each lost atom removes mean energy , differential energy balance gives . The result assumes rapid elastic equilibration and excludes separate work from changing trap frequencies.
For a classical three-dimensional harmonic trap of fixed frequencies, and . Combining with these equations gives . Since evaporation decreases , phase-space density of an atomic gas increases only when the removed mean energy exceeds . Merely lowering temperature requires only . Changing confinement adds trap work and changes these formulas.
The ratio of an evaporation cutoff to thermal energy. Keeping the cutoff fixed while lowering temperature increases this ratio and reduces population in the escaping tail.
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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.