The Application of High Temperature Superconducting Materials to Power Switches

Superconducting switches may find application in superconducting magnet systems that require energy extraction. Such superconducting switches could be bypass-switches that are operated in conjunction with a parallel resistor or dump-switches where all of the energy is dissipated in the switch itself. Bypass-switches are more suited to higher energy circuits as a portion of the energy can be dissipated in the external dump resistor. Dump- switches require less material and triggering energy as a lower switch resistance is needed to achieve the required total dump resistance. Both superconducting bypass-switches and superconducting dump-switches can be ther- mally activated. Switching times that are comparable to those obtained with mechanical bypass-switch systems can be achieved using a co-wound heater that is powered by a ca- pacitor discharge. Switches that have fast thermal diffusion times through the insulation can be modelled as a lumped system whereas those with slow thermal diffusion times were modelled with the full heat diffusion equation. Superconducting switches can be formed of either high temperature superconductors (HTS) or low temperature superconductors (LTS). Switches based on HTS materials allow operation at higher temperatures where the cost of cooling is less. Extracting the magnet energy and depositing the heater energy at higher temperatures will also reduce the load on the overall cryogen ic system during switching and energy extraction. For magnet circuits that are based on high temperature superconductors the switch must also be formed of HTS material. Due to the approximately T^3 dependence of specific heat capacity, switches that operate at higher temperatures have slower heat diffusion times and require higher triggering energies than those operating at low temperature. HTS based dump-switches and HTS based bypass-switches were tested in liquid nitrogen to show that the required switching time could be achieved at these high temperatures. The design and optimisation of superconducting switches that were formed of various superconducting materials were performed for example magnet circuits to provide reference designs of switches. These example circuits were based on selected Large Hadron Collider 600 A circuits that had a stored energy of 5.5 kJ. Superconducting switches may also find application in magnet circuits with higher transport currents and higher energies. The scaling and suitability of the reference designs to higher energy circuits was also described.