Speaker
Description
Superconducting magnets include various electrically-insulated metallic parts. The stray capacitance between these elements can be monitored and utilized to acquire information about the magnet condition and behaviour.
It is well known that dedicated capacitive sensors can be used to monitor the local strain. Furthermore, it was recently proposed to use a stray-capacitance monitoring system as a quench detection system for high-temperature superconducting magnets. The latter is based on the observation that part of the helium impregnating the insulation layers boils off when local heating occurs. Since the electrical permittivity of helium drops after its transition from the liquid to the gaseous phase, a stray-capacitance reduction is an indication of local heating of the conductor.
In this paper, we describe the application of a stray-capacitance monitoring system on a 2 m long, Nb-Ti, canted-cosine-theta (CCT) magnet tested in liquid helium at 4.2 K. The capacitance between one coil former and the magnet iron yoke was continuously monitored during powering and energy-extraction tests.
The effect of electro-magnetic forces on the measured capacitance was observed with good reproducibility. Furthermore, relatively fast capacitance reduction was observed after the energy-extraction was triggered. This result is an indication of local heating due to eddy currents in the former, coupling currents in the superconductor, and ohmic loss after a quench occurs.
