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Description
High temperature superconducting (HTS) accelerator magnets can achieve higher magnetic field with higher current capacity. However, with the shielding current effect of the HTS materials, the current distribution inside REBCO tape will deviate from uniform current assumption, which results in a higher sextupole field harmonics. To compensate sextupole harmonic field in the accelerator magnet due to the shielding current effect, an active shimming coil was designed and tested. The shimming magnet had three racetrack coils wounded with 2 mm wide REBCO tapes. The top and bottom coils were normal racetrack coils with 300 mm long straight sections,
and the middle racetrack coil had hard-bending sections at the coil end with a radius of 800 mm. According to simulation, the shimming magnet could generate a sextupole field under self-field ramping. As for the situation when the shimming magnet was inserted into a dipole magnet, the shimming magnet could compensate the sextupole error harmonic field with small effects on other higher harmonics. Under 77 K self-field test, the active shimming magnet could ramp up to 20 A without significant voltage rise. Then the active shimming magnet was inserted into a dipole accelerator magnet and tested under 4.2 K with a dipole magnetic field of 1 T and 3 T. The field quality was measured with a rotating PCB pick-up coil with a reference radius of 4 mm. The test result showed that the sextupole harmonic field decreased with the ramping of the shimming magnet linearly, and the sextupole field quality reached below 1 unit with a shimming current of 35 A under 1 T and 40 A under 3 T, which proved the feasibility of the active shimming magnet. This work proved the possibility of reaching good field quality below 1 unit using active shimming technology with HTS magnet, and will contribute to the development of HTS accelerator magnets.
