Critical currents and AC losses in YBCO coils

Measurements of V-I characteristics and AC losses of YBCO single pancakes and multipancake coils have been performed. YBCO pancakes and coils were wet-wound using non-insulated YBCO tapes. Layer-to-layer insulation was achieved by using epoxy filled with alumina particles that ensures higher effective current density of the coil in comparison with coils of standard insulated tapes. This procedure also improves the coil's thermal conductivity in the direction perpendicular to the tape plane. Static and alternating magnetic fields have been applied to the AC current carrying coils during measurements. DC magnetic fields up to 50 mT affect the V-I curves only slightly. On the other hand, AC magnetic fields with amplitudes up to 50 mT and frequencies 4-100 Hz change the form of the V-I curves and strongly reduce the critical current. AC losses and their distribution along the coil were measured by electrical and calorimetric methods. It was found that AC losses peak at the central region of the coil and depend strongly on distance between adjacent pancakes. Surprisingly, adjacent pancakes, even if kept in an open circuit state, increase the losses of a current carrying pancake. It is suggested that introduction of gaps between pancakes/double pancakes can be used as means for decreasing AC losses in multi-pancakes YBCO coils.