Design and use of an electronic loss measurement apparatus for superconducting coils

A description is given of an apparatus that makes use of an electronic technique for measuring losses in rapidly pulsed superconducting coils. Data taken with this apparatus on a coil having an energy storage capability of approximately 7 kJ is presented. The apparatus described is essentially an electronic wattmeter. It makes use of a magnetically coupled coil to cancel out as nearly as possible the inductive portion of the coil voltage during charge and discharge, thus leaving a voltage which is due almost entirely to the coil energy losses. This voltage is then integrated and displayed along the vertical axis of an X-Y oscilloscope, the horizontal axis being the coil current. The area of the figure so formed is proportional to the energy dissipated during the cycle. Two different techniques for sensing the inductive component of the coil voltage are described together with data taken by each technique. Data are presented for full cycle hysteresis loops at frequencies of 11.7, 16.2, and 26.2 Hz and for half cycle hysteresis loop at discharge rates of 54 and 348 amperes per second and several charge rates. These data show the coil losses to be higher than those predicted by a simplified theory taking into account magnetization losses only.