Stray Field of SMES Coil Configurations

A stray field from a superconducting coil restricts the use of SMES at a demand site, though SMES has an attractive potential for power management and quality control. The stray field outside a solenoid is analyzed by a series of Legendre polynomials, and the results are then applied to various coil configurations. The summation of magnetic moments from all coils must be zero, which leads the stray field decreasing as Rp-3 to vanish, where Rp is distance from the coil. The higher order of the stray field can be cancelled out if the coil arrangement is optimized. In this paper, we treat a single solenoid as a reference, and active shield coil, axially displaced coil, multipole coil and toroidal coil configurations to reduce the stray field. It is shown that the stray field from the active shield coil configuration decreases as Rp-7. On the other hand, the stray field from an axially displaced coil drops as a function of Rp-5. The multipole coil configuration has high potential to reduce the stray field because the stray field behaves like Rp-(n/2+3), where n is the coil number. The toroidal coil configuration has the stray field decreasing as Rp-(n+2), though it has alarge volume facility.