Study on thermal measurement method of ac loss in a conduction-cooled HTS coil

The miniaturization of a synchrotron accelerator can be realized by applying a conduction-cooled high-temperature superconducting (HTS) bending magnet with a cryocooler. A variation of an exciting current through the HTS magnet generates the ac loss, and the ac loss causes a temperature rise of the HTS magnet. AC loss in HTS coils is typically measured using the electromagnetic method or thermal measurement method. However, the measurement method of the ac loss of the conduction-cooled HTS coil under non-sinusoidal current excited has not been established because of the very large inductive voltage of the coil, the very low power factor, and no refrigerant liquid or gas. This study reports a novel thermal measurement method of ac loss in a conduction-cooled HTS coil using a heater and a cryogenic temperature controller. The validity of the proposed thermal measurement method was verified by measuring the loss in a copper coil which was cooled by conduction cooling with a 4-K Gifford-McMahon cryocooler. The measured electrical loss in the coil was good agreement with analytical one. Moreover, the validity of our method was verified by measuring the ac loss in a conduction-cooled small double-pancake HTS coil wounded by a rare-earth barium copper oxide (REBCO) tape conductor at 20 K. The measurement results of ac losses in the HTS coil show that our proposed measuring method is very useful for evaluation of the ac loss in the conduction-cooled HTS coil.