Harmonic Coil Design, Fabrication and Commissioning for European XFEL Linear Accelerator Superconducting Magnets Field Measurements

A harmonic coil assembly for the European XFEL superconducting magnet measurements was designed and fabricated in IHEP. Bare coil without protection tube was selected in order to save space and cover a larger measurement radius. The coil assembly can be moved along the anticryostat pipe and can be fixed at any position by rubber brakes touching on the inner surface of the pipe, which is a part of the magnet measurement dewar. The magnetic field can be measured by rotating the coil inside the anticryostat pipe at a selected position. The superconducting magnet package consists of a quadrupole, a horizontal and a vertical dipole. The harmonic coil includes a main coil for the quadrupole and for the dipole field measurement when the single coil mode is used to measure the main field component. Two bucking coils are available to compensate the quadrupole and dipole field respectively when the two-coil mode is used to measure the high order field harmonics. A skew coil with only 2 turns and open angle of 90 degrees can be used to cancel effects due to the angular differences between the main coil plane and the bucking coil plane. The harmonic coil was tested on quadrupoles for Korea Pohang Accelerator Laboratory (PAL), a high bucking ratio of 182 was reached. The coil was shipped and used in DESY to measure the XFEL superconducting magnets. The measurement precision and reproducibility were excellent and the driving and brake system worked well.