Field Quality Measurement of an HTS Magnet for a Rotating Gantry

We report a measurement of the field quality of the dipole magnet whose coils were wound by YBCO coated conductor. The magnet was made to study feasibility of application of the high T
_c
superconducting magnet to a rotating-gantry system for heavy-ion therapy. The magnet was cooled down to 3.4 K, and the maximum dipole field of 1.2 T was achieved at the operating current of 153 A. The field quality was evaluated using a harmonic coil system. As a result, we observed larger contribution to the dipole field especially from bε and b
₃
as compared to the result from the numerical calculation, reason of which can partially be explained by mis-alignments of the coils. The effective length of the magnet was measured and the difference from the numerical calculation was 2% level. Finally, we give consideration on spikes of the magnet coil voltage, which were observed during the ramp-up test.