AC Loss and Shielding-Current-Induced Field in a Coated-Conductor Test Magnet for Accelerator Applications under Repeated Excitations

We carried out repeated excitation experiments of a test magnet for accelerator applications of RE-123 coated conductors in order to study its tolerance against ac loss and the stability as well as the reproducibility of the generated magnetic field. The magnet consists of a pair of 4-stacked racetrack coils with an iron return yoke in the low temperature region. It can generate 2.4 T of dipole magnetic field at its room temperature beam duct with its rated current of 200 A, and the maximum magnetic field to which the coated conductor is exposed is about 4 T. At first, the magnet was excited repeatedly with a saw-tooth waveform to examine the tolerance against ac loss: the magnetic field was ramped up to 4 T at the conductor in 120 s and down to 0 T in 120 s. We also carried out numerical electromagnetic field analyses to estimate the ac loss in the magnet. Next, we examined the influence of shielding currents on the field stability and reproducibility at the steps during ramping down, where the current was held constant. This simulated the excitation pattern of magnets for carbon gantries.