Novel HTS coil protection method using secondary windings as a quench heater

In NIFS, research and development of HTS magnets have been begun aiming at the application to a next experimental device of Large Helical Device which is a heliotron-type fusion plasma experimental device. The next experimental device needs large coil current density such as 80 A/mm2, therefore it is important to develop a coil protection method. In this study, we have proposed a novel coil protection technology of HTS magnets using secondary windings. By embedding secondary windings which are electrically isolated in a part of the conductor and using the secondary windings as a quench heater, the occurrence of normal conduction is quickly diffused, then prevented local temperature rise in the HTS coil. We applied this method to the conductor and coil under development, and analysed the thermal behaviour of the secondary windings. As a result, we found that the temperature rise of the entire coil can be suppressed and the hotspot region can be diffused. This shows the applicability of coil protection by using the secondary windings, therefore optimal coil protection is possible by changing the ratio of magnetic stored energy between internal consumption and external recovery depending on the value of external resistance.