Monday, May 23, 2005

Spectroscopists do it 'til it Hertz

Well, tomorrow is the big day! The big magnet is being delivered at work, and is quite the ordeal. The magnet itself is huge and it has to be lowered into a 6 foot pit. It is being brought in through a hole in the wall and several peices of the ceiling needed to be cut away in order to fit it into the building. Then it is raised by two 40,000 lb hydraulic jacks and lowered gradually into the pit. All this with about half an inch of clearance under the ceiling. To get an idea of the size of this thing check out this install at Wisconsin. It's going to be a long day, but never a dull moment.

Once the magnet is moved in and everything is in place, the magnet will be energized. That probably won't happen for a few weeks yet. It's a superconducting magnet, which means that there is a large coil of superconducting wire in the middle, which is cooled with liquid helium. Once everything is cold enough to sustain superconductivity, the coil of wire is slowly run up to higher and higher current with an external power source. Once the desired field has been reached (in this case 21.1 Tesla, giving a proton resonance frequency of 900 MHz) the switch is closed and the current remains in the coil indefinitely. Because there is no resistance in the wire (superconducting), there is no loss of current once the external source is removed. Well in reality there is a little loss, but it is small enough that the magnet will not need to be recharged for many years. In the case of this magnet a controlled quench is required to settle the coil into place, so this process will have to be carried out at least twice (sometimes the magnet will quench a few times as it is being brought to field). A quench is when there is a sudden loss in superconductivity of the wire. All of a sudden the wire becomes non-superconductive, and the current in the wire generates a huge amount of heat as it runs out of the wire. This causes all of the liquid helium that cools the magnet to boil suddenly and fire out of the magnet as its volume changes. This results in a huge bang and giant plume of helium. In the case of this magnet there is so much helium that it could force all of the oxygen out of the room and suffocate all of its inhabitants, so a large duct is required to get the helium out of the building. All of this makes for a really exciting installation. I hope to be around for as much of it as I can, as I always learn a lot from the folks that build and install these suckers.

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