Small-Diameter Rotating Coils For Field Quality Measurements in Quadrupole Magnets

Two rotating coil shafts with a challenging small diameter of 8 and 19 mm, originally developed at CERN for the measurement of linear accelerator magnets, have been used to quantify the magnetic field strength, the harmonic content and the field direction of series quadrupoles for the Swiss Free Electron Laser in construction at the Paul Scherrer Institute. In this paper, we describe the design and construction of these coils, the procedure used for their calibration, and the results obtained on the magnets tested so far. We focus especially on the manufacturing and calibration difficulties related to the very small diameter of these coils, which makes it challenging to obtain relative errors at the level of a few 10 -4 as it is commonly required by accelerator beam optics. The last part of the paper is dedicated to a selection of measurement results obtained on Swiss Free Electron Laser quadrupoles to illustrate the performance of the two types of coils on series measurements.