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Here is a diagram of the internal parts of a modern cryomagnet. Older electromagnets and permanent magnet nmr spectrometers have all but disappeared because cryomagnets are able to reach much higher fields, are more stable and give much improved field homogeneity compared to the older systems. |
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There is a primary coil made up of Nb/Sn superconducting wire. Of course, to become a superconductor it must be immersed in liquid helium to reach the appropriate temperature. The coil can then be charged to give a very high field strength ... much higher than can be practically obtained with a conventional room-temperature electromagnet. Higher is usually better in the nmr world for sensitivity and resolution reasons (although not always). Of course, the liquid helium and (liquid nitrogen) must be replenished periodically:   The helium dewar is shielded from external infrared radiation and surrounded by a liquid nitrogen dewar. The liquid nitrogen acts as a heat sink, soaking up heat from outside the magnet and constantly boiling off. Thus the boiloff rate for the liquid helium is much lower than it would otherwise be. This is good because liquid helium is quite expensive and not likely to become any cheaper in the future. Liquid nitrogen, on the other hand, is pretty cheap. We usually top up the liquid nitrogen on a weekly basis and the liquid helium about once every three or four months, depending on the magnet (they all seem to have their own personalities and quirks). Down the center of the magnet is the 'bore', a long tube that is at room temperature and, obviously, insulated from the very cold temperatures in the magnet. At the bottom of the bore tube are the room temperature shim coils for making fine adjustments to the magnetic field. Inside the RT shims sits the nmr probe, the heart of the spectrometer. This is where the signals from the sample are received and is a pretty high-tech and sensitive gadget (but conceptually simple, as we will see below). The sample is inserted into the magnet from the top of the bore tube on a cushion of air or nitrogen. When the air flow is turned off the sample sinks down into the bore tube until it is sitting in the probe and positioned in the 'sweet spot' of the magnet. Modern magnets (from Bruker at least) are 'shielded' which means that there is an extra coil wound in the opposite direction to the main coils to counteract the stray field outside of the magnet. This means that the field outside of the magnet can is much lower than it would be otherwise. I can attest to this, having worked with both shielded and unshielded 500 MHz magnets. You couldn't put a crt monitor within about 3 meters of an unshielded magnet but with a shielded magnet ... The magnet field strength is given in Tesla (or sometimes gauss). More often, however, the field strength is obtusely referred to by specifying the 1H Larmor frequency for the magnet. Hence the use of the phrase "500 MHz magnet". This would correspond to a field of 11.7 Tesla (at the sample). Everyone is always curious about what it actually looks like inside a cryomagnet but very few ever actually get a chance to see for themselves. Well, thanks to some folks out there in internet land, we have a picture of the inside of a Bruker magnet (presumably dead!). Notice the attention given to shielding to cut the loss of expensive liquid helium. The various functional parts are labelled and I will try to identify them properly (but some I can't identify):
You do NOT want the helium level in the helium dewar to drop to the top of the superconducting coils because if the top of the coils are not immersed in the liquid they cease to be superconducting. If this happens, the extremely large current in the coil causes it to heat up and the heat boils the remaining liquid helium ... quickly. This is called a quench. Not nice. |
