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| MR- System |
Some metals like mercury or Niobium-Titanium alloy lose their electric resistance at very low
resistance decreases current increases and as increases, the magnetic field strength is achieved
by Completely eliminating the resistance to the flow of current. Moreover, once
superconductor wires or coils are energized the current continues in the loop as long as the
superconducting wire is Maintained below the critical temperature. There is no power loss
and a continuous power supply is not required to maintain the magnetic field.
Important Components
The magnetic field is produced when current is passed through the
conducting wires. These wires are Made up of Nb/ti alloy. This alloy becomes
Superconducting at 10K (kelvin). A wire containing filaments of Nb/Ti alloy embedded in a
copper matrix is wound tightly and precisely on an insulated aluminum bore tube. It is fixed in
place with a viscous, high thermal conductivity epoxy binder. The superconducting wire could
be as long as 30 Kilometers. Hence there are thousands of turns of the wire. Since it is not
possible to wound the coil with a single continuous strand, the coil has several interconnecting
joints.
The coil made of superconducting material is cooled to 4K(-269 degrees Celsius)
by cryogens like liquid helium, which surrounds the coil all around. Because of smaller heat
leaks into the system, helium steadily boils off. This boil-off is reduced by much cheaper liquid
nitrogen. However, helium needs to be replenished on regular basis, usually every six
months.
The can of liquid helium is surrounded by cooled
liquid nitrogen and a radiation shield. This Prevents any heat exchange between helium and
the surrounding. Nitrogen boils at 80 degrees K and is much cheaper than helium. The
liquid nitrogen and radiation shields reduce the evaporation of liquid helium to approximately 0.3
liter per hour.
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