A Method for the Submicrometer Accuracy Determination of Quadrupole Magnetic Axis

For the quadrupole magnets of the future free electron laser facility (SwissFEL) at the Paul Scherrer Institute (PSI), the location of the magnetic axis has to be known to be better than 50 μm, to minimize the commissioning time, and facilitate the final positioning of magnets to an accuracy of 1 μm using beam alignment. For this purpose, the PSI Magnet Section has developed a vibrating wire measurement system with a new type of detector capable of measuring large numbers of magnets in a short period of time. The system has been upgraded and improved so that the total accuracy of finding the magnetic axis is better than 10 μm. A new measurement procedure is described and a method is detailed with which a submicrometer reproducibility of a magnetic axis location is achieved. In addition to the axis offset, the proposed method is applicable to the measurements of the magnet roll angle. With temperature sensors attached to magnets, the method allows for valuable studies of the effect of thermal dilation on the position of the magnetic axis which is of a particular interest for the SwissFEL air cooled quadrupole magnets. Results of measurements performed on SwissFEL prototype quadrupoles with 12 mm aperture are shown.