What Is Cryogenics?

Cryogenics is the branch of physics and engineering concerned with the production, measurement, and application of temperatures below approximately 120 K (-153 degrees Celsius). At these temperatures, many materials exhibit physical properties that are absent or negligible at room temperature, including superconductivity, superfluidity, and substantially modified mechanical and electrical characteristics. The field spans the science of achieving and sustaining low temperatures, the engineering of the equipment used to do so, and the study of how matter behaves under cryogenic conditions.

The NIST Cryogenic Technologies Group defines cryogenics as beginning below 120 K, the temperature at which common atmospheric gases liquefy, and extending to absolute zero (0 K), which thermodynamic law prohibits reaching but allows approaching arbitrarily closely. Practical cryogenic systems operate with a small set of working fluids, principally liquid nitrogen at 77 K, liquid hydrogen at 20 K, and liquid helium at 4.2 K, each serving different temperature regimes and carrying different safety and handling requirements.

Cryogenic Fluids and Temperature Ranges

The properties of cryogenic fluids underpin the design of every cryogenic system. Liquid nitrogen is the most widely used cryogen because of its low cost, abundance as a byproduct of air separation, and the convenient temperature it provides for many engineering applications including food preservation, materials testing, and biological sample storage. Liquid helium is essential for the lowest temperatures accessible without exotic methods: it boils at 4.2 K at atmospheric pressure and can be pumped to 1 K or below at reduced pressure. Liquid hydrogen at 20 K is used primarily in space propulsion as a high-specific-impulse rocket propellant. Below 1 K, dilution refrigerators exploit the thermodynamic properties of helium-3 and helium-4 mixtures to reach temperatures below 10 millikelvin, the regime required for superconducting qubit operation and many fundamental physics experiments. CERN's Large Hadron Collider uses approximately 96 tonnes of liquid helium to cool its superconducting dipole magnets to 1.9 K, making it one of the largest cryogenic systems in the world, as described in CERN's overview of cryogenics engineering.

Refrigeration Systems and Liquefaction

Producing and maintaining cryogenic temperatures requires refrigeration systems that remove heat from the cold stage against a thermal gradient to a warmer environment. The Joule-Thomson effect, in which a gas cools when it expands through a throttling valve, is the basis for the liquefaction of nitrogen and oxygen in industrial air-separation plants. Stirling, Gifford-McMahon, and pulse-tube cryocoolers use reciprocating gas cycles to reach temperatures from 77 K down to approximately 4 K without requiring consumable cryogens, and are used wherever liquid cryogen refilling is impractical. Dilution refrigerators, which rely on the enthalpy of mixing of helium isotopes rather than gas compression cycles, are the only practical means of reaching temperatures below 300 millikelvin. Thermal insulation in cryogenic systems uses multi-layer superinsulation consisting of alternating layers of reflective film and low-conductivity spacer material in an evacuated jacket, and achieves effective thermal conductivities three to four orders of magnitude lower than conventional insulation materials.

Superconductivity and Quantum Applications

Superconductivity, the complete loss of electrical resistance in certain materials below a critical temperature, is among the most technologically consequential properties accessible only through cryogenics. Niobium-titanium and niobium-tin alloys, cooled to 4 K with liquid helium, produce the high-field superconducting magnets used in MRI scanners and particle accelerators. High-temperature superconductors, such as yttrium barium copper oxide, exhibit superconductivity up to about 92 K and can be cooled with liquid nitrogen, reducing operational complexity. The Cryogenics Society of America's review of helium applications describes how liquid helium serves as the coolant for superconducting magnets and, beyond that function, as the working fluid in quantum computing platforms and fundamental measurement standards.

Applications

Cryogenics has applications in a range of fields, including:

• Magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) spectroscopy
• Space launch vehicle propulsion using liquid hydrogen and liquid oxygen
• Particle accelerator and fusion reactor superconducting magnet systems
• Quantum computing platforms requiring millikelvin qubit operating environments
• Industrial gas separation, semiconductor manufacturing, and food processing