What is a quench?

Since last month's incident several people have asked, "What is a quench?" To answer this, and give some idea of its causes, we need some understanding of how a superconducting magnet is constructed.

The schematic below shows some of the features of a modern superconducting magnet. The magnet is constructed like a set of nested Russian dolls with several dewars surrounding a coil of superconducting wire. The outer dewar is lined with insulation and is evacuated to produce a high vacuum. Inside this sits a dewar filled with liquid nitrogen. Inside the liquid nitrogen reservoir is a dewar holding liquid helium, which houses the superconducting coils

The coils are immersed in the liquid helium at a temperature below 4.2 K where the coils become superconducting. This means that the electrical resistance of the wire is eliminated and once a current is introduced to the wires it can circulate indefinitely, as long as the coils are kept cold and remain superconducting. The liquid nitrogen and vacuum dewars are used simply to keep the liquid helium cold. Liquid nitrogen is easier to obtain and cheaper than liquid helium. Both the liquid helium and liquid nitrogen reservoirs open through ports at the top of the main dewar. The cryogens evaporate slowly over time through these ports and so have to be topped up at regular intervals. Typically the liquid nitrogen is topped up weekly and the helium every three months.

A quench occurs when the coils warm above the critical superconducting temperature and start to heat the cryogens around them. The liquid helium and nitrogen is rapidly vaporised and occupies a much larger volume, forcing its way out of the magnet. This could be a sudden explosive event, or a slower one, depending on how rapidly the cryogens are heated. In the case of an explosive event the room housing the magnet could become filled with helium and suffocate any individuals inside. For this reason NMR facilities normally have oxygen monitors and large exhaust systems to rapidly remove vaporised helium from the room.

The coils could start to warm up because the helium level in the inner dewar drops too low, or because the vacuum deteriorates and allows greater heat transmission into the dewars. It seems that the loss of vacuum in the outer dewar was what caused the quench of our magnet.