What is quench in nmr?

Answer 1

First, some background on how an NMR is built.

NMR's have a superconducting magnet in them. A superconducting magnet is an electromagnet that has no resistance in the current flow...not losing the current allows us to generate massively large magnetic fields. Keep in mind that resistance generates heat.

In order for a superconducting magnet to BE superconductive, it needs to be kept very, very cold. The basic definition of a quench is for this magnet to raise in temperature to the point that it is no longer superconducting. To keep the magnet cold enough, it is bathed in liquid helium, which is itself insulated by liquid nitrogen. Liquid helium is TOUCHY...helium boils at -452 degrees Fahrenheit.

If the helium is for some reason heated up, it tries to evaporate, which increases the pressure inside the container. The container is only meant to withstand so much pressure, so eventually it will burst (actually, there's a specifically engineered rupture disk that allows this to happen without blowing up the entire instrument).

Two most common ways for quenches to occur are due, frankly, to user error. First, the liquid helium needs to be refilled on a periodic basis (anywhere from every 2 to 4 months). This is a very sophisticated technique, and doing it wrong can result in allowing room-temperature air go into the helium area, which will rapidly boil the liquid helium. This can build up enough pressure to initiate a quench. The second is the result of not maintaining the liquid nitrogen, which typically needs to be refilled every week. This just happened this past weekend...someone let their nitrogen run out, and the helium evaporation increased to the point that the magnet quenched.

There are also some other causes of quenches, but these are mainly issues that people don't really have a whole lot of control over - basically it comes down to faulty parts.