Editing Fusion power (section)

=== Magnet quench ===
A [[magnet quench]] is an abnormal termination of magnet operation that occurs when part of the superconducting coil exits the superconducting state (becomes normal). This can occur because the field inside the magnet is too large, the rate of change of field is too large (causing [[eddy current]]s and resultant [[Joule heating|heating]] in the copper support matrix), or a combination of the two.

More rarely a magnet defect can cause a quench. When this happens, that particular spot is subject to rapid [[Joule heating]] from the current, which raises the [[temperature]] of the surrounding regions. This pushes those regions into the normal state as well, which leads to more heating in a chain reaction. The entire magnet rapidly becomes normal over several seconds, depending on the size of the superconducting coil. This is accompanied by a loud bang as the energy in the magnetic field is converted to heat, and the [[cryogenics|cryogenic]] fluid boils away. The abrupt decrease of current can result in [[kilovolt]] inductive voltage spikes and arcing. Permanent damage to the magnet is rare, but components can be damaged by localized heating, high voltages, or large mechanical forces.

In practice, magnets usually have safety devices to stop or limit the current when a quench is detected. If a large magnet undergoes a quench, the inert vapor formed by the evaporating cryogenic fluid can present a significant [[asphyxiation]] hazard to operators by displacing breathable air.

A large section of the superconducting magnets in [[CERN]]'s [[Large Hadron Collider]] unexpectedly quenched during start-up operations in 2008, destroying multiple magnets.<ref>{{Cite book|url=https://edms.cern.ch/ui/file/973073/1/Report_on_080919_incident_at_LHC__2_.pdf|title=Interim Summary Report on the Analysis of the 19 September 2008 Incident at the LHC|publisher=CERN|year=2008}}</ref> In order to prevent a recurrence, the LHC's superconducting magnets are equipped with fast-ramping heaters that are activated when a quench event is detected. The dipole bending magnets are connected in series. Each power circuit includes 154 individual magnets, and should a quench event occur, the entire combined stored energy of these magnets must be dumped at once. This energy is transferred into massive blocks of metal that heat up to several hundred degrees Celsius—because of resistive heating—in seconds. A magnet quench is a "fairly routine event" during the operation of a particle accelerator.<ref>{{cite web|last=Peterson|first=Tom|title=Explain it in 60 seconds: Magnet Quench|url=http://www.symmetrymagazine.org/article/november-2008/explain-it-in-60-seconds-magnet-quench|website=Symmetry Magazine|date=November 2008 |publisher=[[Fermilab]]/[[SLAC]]|access-date=February 15, 2013}}</ref>