Optimization of drive coil for inductive measurement of absolute lambda(T) of superconductor films

The drive coil is designed to nullify the screening current density at the film edge in two-coil apparatus which has the drive coil and receive coil coaxially located on the opposite side of a superconducting film in order to obtain lambda(T). This is achieved by adopting fewer counter winding turns in the top section of drive coil than that of the bottom which is closer to the film. The optimal configuration of drive coil winding is numerically determined for a circular film and corresponding magnetic field at the film edge is parallel to the plane of the film surface. Consequently, the mutual inductance M, which is proportional to the coupled flux to receive coil, is insensitive to small differences in film size and transverse positioning of the film to coil axis. As result, the accuracy of measurement can be greatly improved by enhancing the precision of the film's vertical positioning.