Quench Detection
A quench is the loss of superconductivity in part of a coil; detecting the resistive voltage it produces fast enough is the core protection problem.
What a quench is
A superconductor carries current with no resistance below its critical surface in temperature, field, and current density. If a spot warms past its current-sharing temperature, it becomes resistive, dissipates power I squared R, heats its neighbours, and the normal zone spreads. Left alone, the coil's stored magnetic energy deposits into a small volume and can melt conductor or crack insulation.
The detection signal
The unambiguous signature is a resistive voltage that grows where a healthy coil would show only inductive voltage. The classic method measures the voltage across a coil section and subtracts the inductive part L dI/dt, often by comparing two nominally identical halves in a bridge so the large inductive term cancels and only the resistive imbalance remains.
Thresholds and validation
A detector declares a quench when the compensated voltage exceeds a threshold for longer than a validation time, typically a few hundred milliseconds to a second. The threshold and time are a trade: too sensitive and ramp transients or noise trigger false dumps; too slow and the hot spot overheats. Multiple redundant channels vote so a single sensor fault does not spuriously fire or mask a real event.
- Voltage-imbalance bridges cancel inductive signal
- Co-wound voltage taps sense local normal zones
- Redundant channels vote to reject noise
- Validation timer rejects brief transients
Beyond voltage
Because voltage detection can be blinded by noise near power supplies, systems add complementary methods: quench antennas sensing the changing flux of a moving normal zone, fibre-optic and co-wound sensors, and acoustic emission. In high-temperature superconductors the normal zone spreads slowly, making early detection harder and raising the value of these secondary channels.
Detection is only half the job; once a quench is confirmed the protection system must remove the stored energy safely, described in the companion protection page.