= Runaway evaporative cooling
<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 $\Gamma_{\rm el}\sim n\sigma_{\rm el}\bar v$.
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.
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