Scaling of Superconducting Switches for Extraction of Magnetic Energy

Abstract

Abstract-In certain cases it is necessary to extract the energy from a superconducting magnet when it quenches, in order to limit the heat generated by the event and thus prevent irreversible damage. This is usually achieved by opening a contact breaker across a resistor in the circuit feeding the magnet. For the heavy currents used to excite large magnets such switches incorporate sophisticated devices to limit arcing during the operation; the devices are reliable but are also bulky and expensive. It is interesting to consider the use of superconducting switches to perform this function, an important advantage being that they would be housed in the cryogenic environment of the magnet, and thus avoid permanent diversion of the current in and out of that environment to the mechanical switch (which operates at room temperature). However, practical switches for such an application are made up of superconductor in a metal matrix, and it is convenient to work with a relatively low resistance to approximate to the open circuit. This leads to scaling laws for superconducting switches for this application which relate the operating current and stored energy of the magnet system, the type of superconductor, and the necessary size of the device.

Superconductors for fusion: a roadmap

“…As an alternative to the use of magnet feeders at ambient temperature, 'cold' switches were proposed for the coil sectioning [127]. Their basic properties have been evaluated [128] and some promising materials identified [129]; thus, experimental demonstration should finally be conducted. The switches are used essentially as follows; see figure 23: • They are closed in regular operation.…”

CLIQ : a new quench protection technology for superconducting magnets

“…In fact, the superconductor critical current-density significantly increases at low applied magnetic fields, hence when the magnet is operated at low current a very high over-current is required to transfer the coil to the normal state. Whilst this strategy was recently tested to protect a small-scale high-temperature superconducting coil [48], usually its implementation is limited to fault-limiter or power switch applications [49][50][51].…”