Superconducting devices, such as superconducting magnets and superconducting power transmission cables, must be cooled below their critical operating temperature. In fact, only below this critical temperature are the electrical conductors of these devices superconductive. These devices are cooled with low temperature coolant fluid, which may be a gas or liquid, such as helium, hydrogen, neon, and nitrogen, and others.
When the superconducting material is cooled, the normal resistive state of the material becomes a superconductive state, and the electric resistivity of the material becomes very close to zero. However, during operation, the superconducting material can abruptly change its state change from superconductive to normal. This event is referred to as “quench”. When a quench occurs, it is important to quickly discard the external applied electric power and safely dissipate the electromagnetic energy of the superconducting devices (magnet and/or power transmission cable) into a protection circuit. In order to do this, it is essential to detect the quench as quickly as possible, otherwise the superconducting conductor may be seriously degraded or potentially destroyed.
Recently, high temperature superconductors (HTS), such as Bismuth Strontium Calcium Copper Oxide (BSCCO) and Rare Earth Barium Copper Oxide (REBCO) conductors, have been developed and are commercially available. The devices made with those conductors are operated in a coolant, such as liquid nitrogen, liquid helium, liquid hydrogen or helium gas. HTS conductors have excellent mechanical properties with respect to axial and torsional strains, as well as high current density at high magnetic fields. The second generation (2G) REBCO HTS tapes will be very attractive for high field superconducting magnet applications, such as various industrial magnet devices for compact synchrocyclotrons, MRI, NMR, SMES, transformers, fault current limiters and generators, and accelerator magnets including dipoles, quadrupoles, and corrector magnets.
However, detecting a quench in these new high temperature superconductors, especially REBCO conductors, is very difficult. This is because the normal zone propagation velocities are very slow compared with other type of superconductors known as low-temperature-superconductor (LTS), such as NbTi and Nb3Sn. Furthermore, the quench zone is confined to a very small area. For those reasons, it is difficult to detect a quench of HTS devices using conventional electric voltage methods. To quickly detect a quench for an HTS device, it is essential to implement a very sensitive quench detection method that can quickly detect the quench.
Various quench detection methods have been developed:
(1) Voltage detection of a normal zone,
(2) Hydraulic coolant pressure detection,
(3) Detection of acoustic emission (AE) due to cracking, delamination and rapid temperature changes of a quench, and
(4) Temperature detection by optical fibers.
To date, the voltage detection method using voltage taps has been most commonly used. The voltage taps must be mounted, such as by soldering, on a superconducting conductor and the electric wires are co-wound along the superconducting conductor in order to cancel undesired induced inductive voltages. The electric wires used for voltage taps complicate the fabrication of a magnet and its operation due to high induced voltage. Therefore, it is not desirable to install many voltage tap wires on a magnet. Other quench detections are still under development, and their usage is very limited at present.
Therefore, it would be beneficial if there were a system and method that could readily identify quench conditions without the drawbacks of the current technologies.