A 1-K pot (i.e. 1-

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...

pot) is a

cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...

device used to attain temperatures down to approximately 1 kelvin. The 1-K pot is a small vessel in a cryogenic system that is filled with

liquid helium Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temp ...

. Usually it is a few cubic centimeters in size with a pickup-tube extending into the primary liquid helium bath of the dewar. When a vessel containing liquid helium is connected to a vacuum pump, the vapor pressure above the liquid surface drops, thereby allowing the more energetic helium molecules to evaporate out of the liquid. As the particles evaporate and are pumped away from the liquid, they carry heat energy with them, so the remaining fluid tends to cool. This technique is known as

evaporative cooling An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning sy ...

. At atmospheric pressure, ⁴He (the more abundant

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

of helium) liquefies at 4.2 K. By employing evaporative cooling, temperatures down to 1 K can be easily produced. While this technique is fairly simple to operate, it is inefficient for large helium baths because about 50% of the liquid helium must evaporate to attain to the lowest temperatures. If only a small volume needs to be cooled to 1 K, the 1-K pot is used. Only the surface of the small 1-K pot is pumped, leaving the rest of the liquid helium bath at atmospheric pressure. While this method is commonly used in simple cryogenic systems to cool objects down to 1 K, it is also fairly popular in more complicated cryogenic systems to bootstrap to lower temperatures. For example, in a ³He refrigerator, condensed ³He (a rare isotope of helium) is evaporatively cooled and can attain temperatures as low as 200 mK. But the ³He must be condensed to a liquid first, and a 1-K pot is typically used for this purpose. Another example is a

dilution refrigerator A 3He/4He dilution refrigerator is a cryogenics, cryogenic device that provides continuous cooling to temperatures as low as 2 Kelvin, mK, with no moving parts in the low-temperature region. The cooling power is provided by the heat o ...

, where a mixture of ³He and ⁴He forms a

phase boundary Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...

in a mixing chamber and can cool down to a few millikelvins. Dilution refrigerators typically use a 1-K pot to condense the ³He/⁴He mixture.

References

Cryogenics {{thermodynamics-stub