A cooling agent, typically liquid helium or liquid nitrogen used to reduce the temperature of the magnet windings in a superconducting magnet. All cryogenic liquids are gases at normal temperatures and pressures. Different cryogens become liquids under different conditions of temperature and pressure, but all have two properties in common: they are extremely cold, and small amounts of liquid can expand into very large volumes of gas. The boiling points of cryogens are commonly below -150°C(- 238°F).

See also Helium.

Sometimes called helium meter or nitrogen meter is used to monitor the cryogen level of liquid He or N2 used to cool the magnet.

Rate of cryogen (liquid helium) evaporation in superconducting magnets, usually measured in liters of liquid per hour. The boil off rate of cryogen increases during ramping of the magnet and with eddy currents induced in the cryoshields by pulsed field gradients. In calculating cryogen consumption additional transfer and filling losses have to be considered.

See also Cryoshielding and Helium.

A large vacuum jacketed double walled container generally made out of stainless steel that is used to transport cryogens. Cryogenic liquids are shipped and used non-pressurized in this thermally insulated containers. Dewar flasks are specifically designed to withstand rapid temperature changes and extreme differences in temperature. Flasks containing helium (used in MRI for cooling of superconducting magnets), hydrogen and other low-boiling liquid cryogens have an outer vessel of liquid nitrogen for insulation.
Dewars are also called cryostats.

The element helium (He) was discovered 1868 when P.J.C. Janssen and N. Lockyer detected a new line in the solar spectrum during the solar eclipse. Lockyer and E. Frankland suggested the name helium (Gr. Helios, the sun) for the new element. In 1895, helium was discovered in the uranium mineral cleveite and in 1907 it was found out that alpha particles are helium nuclei.
Properties: Helium belongs to the noble gases, is colorless, odorless, and occurs in two naturally isotopes, helium 3 and helium 4. As an inert gas, helium does not react chemically largely and don't burns. Helium 4 makes up over 99% of naturally occurring helium atoms. Helium is extracted from natural gas e.g. present in various radioactive minerals as a decay product. Deposits and sources are in the USA, Poland, the USSR, and a few in India. The rare deposits and increased consumption lead to a shortage of this gas.
K. Onnes worked for many years to liquefy helium, which persisted as a gas to the lowest temperature. Helium does not freeze at atmospheric pressure. The density of helium vapor at his boiling point of 4.2 Kelvin is very high, with the vapor expanding greatly when heated to room temperature. Nb, Tc, Pb, La, V, and Ta are superconductors at liquid helium temperature. Liquid helium is commonly used as a cryogen for superconducting magnets. A rapid evaporation of the cryogen is named Quench. See also Quenching.

Cryogenic liquids and their associated cold vapors can produce effects on the skin similar to a thermal burn and can cause frostbite. Prolonged breathing of extremely cold gases may damage the lungs and in absence of enough air or oxygen, asphyxiation and death can occur. Unprotected skin can stick to very cold metal (e.g. cooled by liquid helium) and then tear when pulled away.