Design considerations for an inductive heater for conductor stability testing

A discussion is presented of the considerations and the lessons learned in designing inductive heaters for stability testing of a 200-kA conductor for superconducting magnetic energy storage (SMES). Two separate tests were run using different inductive heater designs: the first was a long heater about a subcable pitch in length (50 cm) and was wound with inner and outer coils with currents flowing in opposite directions to minimize stray fields; the second was a short coil about 2 cm long without an inner coil. The thermocouples were used in both tests to measure the total energy delivered to the conductor. In order to calculate the stability margin, it is necessary to know the energy delivered per unit volume of conductor. For the long heater with inner and outer coils, the energy distribution has been calculated to be fairly uniform along the length of the heater. However, for a single short heater, the losses are distributed in a very non-uniform fashion; in fact, they are considerably higher at the test conductor ends due to coupling losses induced by the radial field component. Another design consideration in the test was to let the LRC circuit ring, with the hope of increasing the efficiency of energy transfer to the test conductor. The main conclusions of this study are that the induced heating in the test conductor will depend on the cabling geometry, resistance, and applied field distributions and that great care must be taken to correctly interpret stability margin results based on inductive heat input to very large cabled conductors.