Flux pumping

An electric current flowing in a loop of superconducting wire can persist indefinitely with no power source.

In a normal conductor, an electric current may be visualized as a fluid of electrons moving across a heavy ionic lattice.

The resistance due to this effect is tiny compared with that of non-superconducting materials, but must be taken into account in sensitive experiments.

When the material is cooled below the critical temperature, we would observe the abrupt expulsion of the internal magnetic field, which we would not expect based on Lenz's law.

In the same year, Josephson made the important theoretical prediction that a supercurrent can flow between two pieces of superconductor separated by a thin layer of insulator.

, and thus (coupled with the quantum Hall resistivity) for the Planck constant h. Josephson was awarded the Nobel Prize for this work in 1973.

The E–J power law can be used to describe the phenomenon of flux-creep in which a superconductor gradually loses its magnetisation over time.

It has already been shown that large fields can be obtained in single domain bulk samples at 77 K. A range of possible applications exist in the design of high power density electric motors.

There are several reasons for this: first, if the superconductors should become demagnetised through (i) flux creep, (ii) repeatedly applied perpendicular fields or (iii) by loss of cooling then they may be re-magnetized without the need to disassemble the machine.

Thirdly, ex situ methods would require the machine to be assembled both cold and pre-magnetized and would offer significant design difficulties.

Until room temperature superconductors can be prepared, the most efficient design of machine will therefore be one in which an in situ magnetizing fixture is included.

The field would then be 5 × 108 × 10−2 × (2 × 4π × 10−7) = 10 T. Clearly if the magnetisation fixture is not to occupy more room than the puck itself then a very high activation current would be required and either constraint makes in situ magnetization a very difficult proposition.

They are used in MRI and NMR machines, mass spectrometers, Magnetohydrodynamic Power Generation and beam-steering magnets used in particle accelerators.

Other early markets are arising where the relative efficiency, size and weight advantages of devices based on HTS outweigh the additional costs involved.

Promising future applications include high-performance transformers, power storage devices, electric power transmission, electric motors (e.g. for vehicle propulsion, as in vactrains or maglev trains), magnetic levitation devices, and fault current limiters.