A cryostat (or dewar) is a vessel designed to store or transport liquefied gases (cryogens) at low temperatures. Details of design and construction materials depend on the cryogens, the operating temperature, and the duty cycle. (See Cryogenic Fluids.)
A range of techniques is used to achieve the required thermal insulation between the cryogen and its surroundings. Powder or foam insulation can be used economically for relatively high-temperature cryogens, but Sir James Dewar's invention of the double-walled, vacuum-insulated vessel has been one of the most important contributions to this technology.
The high-vacuum jacket around the cryogen reservoir effectively eliminates thermal conduction and convection. Thermal radiation is reduced by using low-emissivity materials, and more recently by using multilayer superinsulation. A single reservoir without actively-cooled shielding is sufficient for cryogens with normal boiling points above approximately 50 K. However, liquid helium (4.2 K) and liquid hydrogen (20 K) are usually stored in vessels surrounded by a jacket filled with liquid nitrogen (at 77 K), or by multiple radiation shields to reduce heat load from the surroundings. These shields are typically cooled by enthalpy of the exhaust gas or by mechanical coolers.
Cryostats have a wide range of applications. They are often used to contain other equipment that has to run at low temperature, such as:
superconducting magnets designed to produce high magnetic fields for medical imaging, research applications, or energy storage;
superconducting quantum interference device (SQUID) magnetometers;
mechanical coolers which can now reach temperatures below 4 K;
specialized refrigerators working at temperatures below 1 K.