SIZE EFFECT IN AC LOSSES OF SUPERCONDUCTING CABLES

We calculate the dependence of the transverse coupling losses in cables, the most important contribution to ac losses in cables without central insulating layer, Two effects cause differences with respect to the infinite samples: 1) changed area of the loops between the strands, and 2) increased resistivity between them, At low frequencies, the transverse losses P for finite samples of length l are well-described by the formula P/P-infinity = 1 - C(0)l(0)/l, where C-0 depends on the ratio b/c (b-cable width, c-thickness of normal layer between strands), l(0) is the cabling length and P-infinity the losses for corresponding infinite sample, We obtain a = 1/C-0 approximate to 3 for b/c approximate to 10 and a approximate to 2 for b/c > 50. The same formula applies for higher frequencies, with frequency dependent correction factor C(omega). This correction factor decreases and becomes even negative at higher frequencies, Thus, the losses in finite samples are higher than in the corresponding infinite cables, This effect could be therefore called the inverse size effect, appearing above omega tau > 0.9 for b/c = 10 and omega tau > 1.53 for b/c = 50. It may explain some experimental results where size effect was expected but not found in the loss measurements.