Analysis of the first magnetic results of the PSI APPLE X undulators in elliptical polarisation

The new APPLE X undulators are novel elliptically-polarised undulators, which have a highly symmetric geometry. Theoretically, all the elliptical polarisation states expected to same deflection parameter if magnetic and geometrical errors negligible. Nonetheless, measurements of first ten performed at Paul Scherrer Institut (PSI) contradicted this simple statement numerical simulations were required understand validate results. After valuation magnetic-force induced mechanical deformations undulator frame, central role played by susceptibility χ was investigated. simulation results indicate that impact different magnet types in Halbach array, anisotropic properties, choice shaped field radial magnets critical for achieved field. compared with measurement data.