A 1-K pot
is a cryogenic 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
Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
. Usually it is a few cubic centimeters in size with a pickup-tube extending into the primary liquid helium bath of the dewar
A Dewar flask is a vessel designed to provide very good thermal insulation. For instance, when filled with a hot liquid, the vessel will not allow the heat to easily escape, and the liquid will stay hot for far longer than in a typical container...
. 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.
At atmospheric pressure, 4He
Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
(the more abundant isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
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
A helium-3 refrigerator is a simple device used in experimental physics for obtaining temperatures down to about 0.2 kelvins. By evaporative cooling of helium-4 , a 1-K pot liquefies a small amount of helium-3 in a small vessel called a helium-3 pot...
, condensed 3He
Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
(a rare isotope of helium) is evaporatively cooled and can attain temperatures as low as 200 mK. But the 3
He 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 dilution refrigerator is a cryogenic device first proposed by Heinz London. Its refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4...
, where a mixture of 3
He and 4
He forms a phase boundary
The behavior of phase boundaries has been a developing subject of interest and an active research field in physics and mathematics for almost two centuries. One reason behind this is that phase boundaries naturally arise in many physical processes due to immiscibility of two or more substances with...
in a mixing chamber and can cool down to a few millikelvins. Dilution refrigerators typically use a 1-K pot to condense the 3