Cryocooler
A cryocooler is a refrigerator that is built to reach cryogenic temperatures. These are temperatures below . These are most often smaller systems. They are normally small enough to sit on a table. Most cryocoolers have input powers less than about 20 kW. Some can have input powers as low as 2–3 W.  Most cryocoolers use a cryogenic fluid and moving parts to move the fluid around a thermodynamic cycle. The fluid is normally compressed at room temperature. It is precooled in a heat exchanger. The fluid is then expanded at some low temperature. When the low-pressure fluid returns, it passes through the heat exchanger to precool the high-pressure fluid before entering the compressor intake. The cycle is then repeated.