You make an NMR
magnet in the following general way. Wind an
electromagnet from
niobium tin alloy wire. This
wire should be very thin and very long, as you want to make a very strong magnet. The
energy of the
magnet comes from the
electrons held within the
coil of
wire, in NMR magnets you have a lot of electrons, 100
amperes of more. (Also see
coulombs.) The magnet does not have many
volts though.
Then cool this electromagnet until it exhibits
superconductivity this will happen at about 4 degrees
kelvin, very near to
absolute zero. You do this by immersing the magnet into a vaccuum flask of
liquid helium, which is at less than 4 kelvin. In order to keep the helium from boiling away too quickly, you need another vacuum vessel around this containing liquid
nitrogen. The vacuum spaces are pumped down to about 10
-5 atmospheres pressure and this coupled with very effiecent insulation (which consists of many layers of molecule thick silvered
mylar; like a '
space-blanket') can enable a magnet to keep it's helium for up to a year or more! This gives the external appearance of a big silver flask about 2.5 metres tall and 1 meter in diameter.
Once the coil of the magnet is cooled to liquid helium temperature, you can then slowly
charge the magnet up to it's full strength. You have to do this carefully as any instabilities can cause a 'hot spot' in the coil, which
boils away the helium, causing that bit of the wire to stop superconducting, which then causes the wire to heat up, boiling more helium off, heating up more wire....And in a matter of seconds all the energy you put into the coil has transferred itself as
heat into the liquid helium, which turns into a
gas expanding about 1000 times as it does so.
The strength of the magnet is usually expressed at the frequency
hydrogen resonates at the magnetic field, for a field strength of 21.1
tesla the resonant frequency is 900 MHz.
Modern designs of magnet have huge forces acting on them. If you consider the magnetic fields running through the magnet will actually work to collapse the magnet in on itself, and the design must take this into account. The wire is made hexagonal in
cross-section so as to prevent movement of the individual strands. (A circular cross-section would have gaps, which could allow the wires to move). The magnet as a whole has to be designed to withstand forces of up to 200 tons and
stresses of 250
MPa, that the current density of 200 Amps per
mm2 can generate. Pecision engineering results in a collosal magnetic field strength,
homogenous to parts per million.
Once you have the correct amount of energy in the magnet coil, and it's stable, you can flick a switch to close the circuit of current flowing through the magnet, and as it's
superconducting the electrons always flow, and the magnet is said to be
persistant! The magnet can stay charged for several years, with no drop in performance as long as the
cryogens are replenished.