AC losses in superconducting cables for di-pole magnets of pulsed operation

Since the di-pole magnets for synchrotrons are essentially subjected to pulsed operation, the problem of their AC losses is very important. The AC losses in superconducting cables occupy more than half of the refrigerator load for actual synchrotrons, which is estimated to be about 100 W/m at 4.2 K. The cables are made of twisted multi-strand conductors of from several tens to a hundred wires. According to Wilson, the AC losses per cycle in a metal impregnated cable are represented by four losses, namely the magnetization loss of superconducting filaments, the AC loss of strands, the AC loss due to the inductive coupling between strands in a cable, and the eddy current loss due to the impregnated metal in the cable, in relation to the vertical magnetic field change on the cable surface. Magnetization Mo in this expression does not differ much if the current capacity of cables is same. Accordingly, as an index of AC losses, the rising speed of magnetic field can be defined, at which the AC losses become twice as much as the inherent magnetization loss of the superconducting body in a cable. About the cables used in a few laboratories including braided and stranded ones, the measured rising speed of magnetic field and the typical parameters of AC losses are summarized. It is concluded that in order to develop the superconducting cables for synchrotrons or storage rings, it is required to select the cabling method to meet the purpose of the accelerators as well as to scrutinize thoroughly the kind and quantity of impregnated and coated metals. (Wakatsuki, Y.)