Measurements of Energy Losses in Pulsed Superconducting Magnets

Abstract

Measurements have been made of ac power losses in a number of small superconducting magnets. The losses were determined by the volume of helium vaporized. The magnets had inductances from 1.4 to 47 mH and were wound with commercially available, half-inch wide, partially stabilized Nb3Sn ribbon. They were powered by sawtooth current pulses of 1 to 1/20 Hz, similar to those of a proton synchrotron and also by sinusoidal current waveforms of 60 Hz. Peak currents ranged from 1–1100 A and the corresponding peak fields from about 500–50 000 G. It is found that at low frequencies, loss per cycle is independent of frequency. It is also found that at low frequencies and moderate fields, the loss depends only on the total field change and is largely independent of current or of steady components of field. Losses measured at 60 Hz and low fields are in agreement with these low-frequency results where the data overlap. Losses obtained from magnets of different geometries can be correlated on the basis of the average component of peak field normal to the ribbon surface. Losses at high fields are quite dependent on winding density, which is attributed to a diamagnetic or field-exclusion effect. They increase with Nb3Sb content at high fields, but tend to be independent of Nb3Sb content at low fields. Losses vary approximately as the 2.5 power of the change in field. The ratio of energy stored to energy dissipated is about the same in all magnets in moderate to high fields. The results are compared with theory and with those of other experimenters.