Distributed among the superconducting strands in ITER conductors are non-superconducting strands like copper. During a quench, as resistance increases in the superconducting elements, the current can "jump" to the copper before the fast discharge units move into action. Photo: US ITER/ORNL.
"When cooled to the temperature of 4.5 Kelvin (around minus 269 degrees Celsius), ITER's magnets will become powerful superconductors. The electrical current surging through a superconductor encounters no electrical resistance, allowing superconducting magnets to carry the high current and produce the strong magnetic fields that are essential for ITER experiments."Superconductivity can be maintained as long as certain thresholds conditions are respected (cryogenic temperatures, current density, magnetic field). Outside of these boundary conditions a magnet will return to its normal resistive state and the high current will produce high heat and voltage. This transition from superconducting to resistive is referred to as a quench.
"During a quench, temperature, voltage and mechanical stresses increase—not only on the coil itself, but also in the magnet feeders and the magnet structures. A quench that begins in one part of a superconducting coil can propagate, causing other areas to lose their superconductivity. As this phenomenon builds, it is essential to discharge the huge energy accumulated in the magnet to the exterior of the Tokamak Building."
ITER is developing a two stage quench detection system, one that connects to the process control system, and the other, a safety instrumented system that automatically initiates the quench cycle.
Read the rest of this very interesting automation article here: http://www.iter.org/newsline/278/1652
