Design of a Magnet for the Spin-Rotator Device for the High Magnetic Field $\mu{\rm SR}$ Instrument at Paul Scherrer Institute

In this work we present the design, the construction and the measurements of the magnet for a so-called spin-rotator (Wien filter), a beam line device used to rotate the spin direction (and the associated magnetic moment) of muons in a beam used for condensed matter research at the Swiss Muon Source at the Paul Scherrer Institute. The design parameters-originating both from the properties of the preferred particle beam as well as the technological constraints for the high-voltage components generating the necessary electric field-were optimized for device compactness, cost and high beam transmission.