A Drag-Torque Method for Measuring AC Losses in Superconducting Samples

AC losses in high-temperature superconductors are considered a major obstacle in the development of fully superconducting machines. Current methods for measuring ac losses can be difficult to implement for superconducting samples that are large or have complex geometries. This article presents a new method, known as the drag-torque method, for measuring ac losses. The present measurement system involves suspending the superconducting sample holder, which is cooled by liquid nitrogen, in the air using a pair of air-bearing journals. When the sample is placed in a rotating magnetic field generated by a permanent magnet rotor, ac losses are induced in the superconducting material. As a result, the sample holder tends to rotate with the permanent magnet rotor and is stopped by a load cell. By measuring the torque exerted on the load cell, the ac losses can be calculated. This method can be applied to complex superconducting samples, such as coils and stacks of superconducting tapes.