Quench (superconductivity)

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Under Quench ( English to quench for intercept, delete, wipe) is defined as the sudden transition of a superconductor from the superconducting to the normally conducting state due to exceeding the critical temperature . Due to the now finite resistance, high voltages and a great deal of heat arise, which can lead to flashovers through the electrical insulation and, in rare cases, to destruction of the superconductor. The coolant that evaporates in the process can burst the cryostat vessel due to the resulting overpressure .

Quenching is particularly dangerous with superconducting coils , since there the entire field energy is suddenly converted into heat when the superconductivity breaks down.

A quench can have several causes:

  1. If entire bundles of flux of a type II superconductor become detached from their centers of attachment due to temperature fluctuations or vibrations, they migrate through the superconductor at high speed due to the Lorentz force . This movement and the associated transition to normal conduction within the river bundle leads to a large amount of heat. This in turn leads to an expansion of the normally conductive zone, which leads to further heating. The coolant evaporates and the coil changes completely to normal conduction very quickly.
  2. A failure of the cooling also leads to quenching as soon as the critical temperature is exceeded. This fact is used, for example, in the event of an emergency shutdown of an MRT device, in which the liquid helium is purposely blown off in a short time.
  3. Particularly when charging superconducting magnets, large forces are exerted on the coil. This leads to a barrel-shaped bending of the entire coil, in which individual wires sometimes move abruptly. With this movement in the magnetic field, a current flow and thus heat generation is induced in the normally conductive copper.

By coating the superconductor with a low-resistance normal conductor layer or appropriately designed surrounding copper wires, negative consequences can be prevented or mitigated.

If normal conduction occurs in certain areas, a short circuit is generated by the low-resistance layer, which means that heating takes place more slowly.

With continuous voltage measurement, the superconductor can be automatically connected to an external load resistor when a critical value is exceeded , in which the excess electrical energy is converted into heat.

literature

  • Yukikazu Iwasa: Case Studies in Superconducting Magnets: Design and Operational Issues . 2nd Edition. Springer, 2009, ISBN 978-0-387-09799-2 .