Comparison of solenoidal and pancake model windings for a superconducting transformer

We prepared 2 model windings from multifilamentary Bi-2223/Ag tape for a small experimental transformer and studied their electromagnetic behavior. The first winding was a solenoid consisting from 4 helically wound layers, the second one consisted of 3 double pancake coils. The outer dimensions of both types of windings were identical. We calculated the magnetic field in the space occupied by the winding and in its vicinity neglecting the magnetic properties of the tape. To study the influence of winding magnetization on the magnetic field distribution we measured the field in the vicinity of the coil and compared it with the calculated one. The relation between the electric field and the transport current in the turns of the windings was measured using several potential taps. By comparing these results with those obtained on short samples we find a way to estimate the critical current of a tape in the inhomogeneous magnetic field. AC loss of the coils was also measured and compared. Recommendations for improvements of the coil performance are presented.