A new measurement method for AC losses in circular disks of superconducting films in perpendicular magnetic fields

An expression P=2R_cV_{L rms}H_{a rms} was theoretically derived for the ac power losses in a circular superconducting disk in a perpendicular ac applied magnetic field when the losses are measured by a planar circular pickup coil surrounding the disk. Here, R_c is the radius of the pickup coil, which is coplanar with the disk, and V_{L rms} and H_{a rms} are rms values of the loss voltage and applied magnetic field. This method of measuring losses has an advantage over a susceptibility technique, since the coil need not be calibrated. However, this expression is valid only for large values of R_c relative to the radius R of the disk, since the dipole approximation was used in the derivation. We experimentally evaluated the minimum value of the diameter 2R_c for which this method provided approximately correct losses. A YBa₂Cu₃O₇ disk with diameter 2R≈5.3 mm was used for the measurements. The electronically determined apparent losses at all field values decreased with increasing coil radius, and asymptotically approached a constant value for R_c/R>∼3.