Design and Magnetic Performance of Small Aperture Prototype Quadrupoles for the SwissFEL at the Paul Scherrer Institut

The Paul Scherrer Institut is building an X-ray free electron laser (SwissFEL) providing a source of intense, ultra-short pulses of coherent radiation in the wavelength range of 0.1 to 0.7 nm. In this facility, there will be 162 small aperture quadrupoles of two types (22- and 12-mm aperture), which both include horizontal and vertical steering functions, under preparation for the linac and the undulator line. Before the series production, eight full-scale prototypes of the final design were built in collaboration between Industry and the Paul Scherrer Institut. This paper describes the features of the quadrupole design and the measurement challenges and reports the main results obtained during the magnetic measurements. The integrated field gradient, the field profile, the harmonic content, and the magnetic axis are measured using four independent systems well suited for small bore apertures: an 8 mm and a 19-mm diameter rotating coil, a Hall probe, and a vibrating wire system. The results of the integrated transfer function and of the field quality are given and discussed.