ac losses in superconducting magnets at low excitation levels

Levitation of high-speed transportation vehicles carrying superconducting magnets may be achieved by repulsion from the currents induced in a conducting guideway. The vertical accelerations of such vehicles moving over a guideway of finite roughness will lead to alternating currents in the superconducting magnets and thus to power dissipation in the cryogenic system. Previous investigators have concluded that the resulting contribution to the cryogenic heat load may be as great as that from all other sources and thus impose serious weight and space penalties on the over-all system design. These investigators have quite generally assumed that the alternating fields will completely penetrate the superconducting wire at all field levels and that the use of twisted multifilament composite wire is thus essential if ac losses are to be minimized. We show that this assumption is incorrect and that it results in a significant overestimate of the loss in multicore wire at the low alternating field levels appropriate for the full-scale vehicle-guideway system. Our analysis further predicts, and our experiments confirm, that an order-of-magnitude improvement in low-level ac loss performance can be achieved merely by the substitution of single-core superconducting wire for the multicore wire ususally proposed. This should reduce the ac loss to about 1.3 W/vehicle from the 14 W for a typical multicore wire. We find a cubic dependence of the loss rate on the alternating field level, and this implies that an additional loss reduction would accrue if the coil bundle could be expanded to reduce the average current density while preserving the total current and lift force. By these means the ac loss contribution to the cryogenic heat load of a levitated vehicle could be made quite negligible.