Generally, cryomagnets are used for NMR and MRI applications, though they are also used in particle accelerators, some mass spectrometers, and for magnetic separations. The cryomagnet consists of a coil of wire, just like a standard electromagnet, but the wire is made of a material which is superconducting at very low (i.e., cryogenic) temperatures. Below a certain temperature threshold (usually ~10K), the wire begins to conduct current with insignificant resistance, allowing much higher currents, and thus the magnet can attain much higher field strengths, up to 25-30 Tesla. For comparison, a standard refrigerator or bulletin board magnet has a field strength of approximately 5/1000ths of one Tesla, and high-strength rare-earth magnets, such as neodymium magnets, have a field strength at their surface of roughly 1.25 Tesla. This document does not cover the hazards associated with high strength magnetic fields; information on the nature of cryomagnets is provided solely for context.
As the name implies, cryomagnets require cryogenic temperatures to operate. While a few high-temperature superconductors do exist, most superconductors require temperatures below about 10K (-263°C or -440°F) to exhibit superconducting properties; above that temperature, they conduct electricity like any other material, with some resistance and thus some energy loss to heat. This presents a risk because if the cryomagnet is allowed to warm to above this threshold it will immediately begin to produce significant heat. This is accompanied by a “magnet quench”, in which some or all of the cryogenic liquid that was being used to cool the magnet is rapidly boiled off by the heat produced by the coil. Since cryomagnets typically contain large volumes of both liquid helium and liquid nitrogen, this can cause severe and rapid oxygen depletion in the room, reducing oxygen levels to well below concentrations that can be fatal in a short period of time. These systems usually necessitate engineering controls such as oxygen monitoring. See Oxygen Deficiency Hazards and Oxygen Deficiency Hazard Analysis for more information.
Magnet quenches are rare (at least one system at LBNL has gone for over 10 years without a single quench), but they are more likely to occur during maintenance of the system and filling of the cryogenic liquid reservoirs. Magnet quenches can also occur if the level of cryogenic liquid in the system drops too low to sustain superconducting temperatures, so it is important to follow the manufacturer’s instructions for the filling schedule of the cryogenic liquid reservoirs.
Only personnel who are trained, qualified, and authorized should ever attempt to fill the reservoirs or perform maintenance on a cryomagnet system.
