PHOTON SCIENCE

The magnetic structure of a variable period undulator (VPU) consists of cylindrical magnets. Gap and position of the magnets are fixed, but each magnet can be rotated by a small stepper motor in order to adjust the magnetic field and form an arbitrarily shaped magnetic field profile on-axis. In particular, this arrangement of magnets creates a Halbach structure with all parameters of the sinusoidal magnetic field B(s) = B₀∙sin(2πs/λ_U+φ) freely adjustable: amplitude B₀, period λ_U, and phase φ (longitudinal shift). This allows to change the wavelength and results in significantly larger tuning range compared to ordinary fixed-period variable-gap undulators.

We have built a short prototype of the proposed VPU that consists of 32 individually controlled motor axes, 8 in each of 4 rows around a 10 x 10 mm² aperture for the vacuum chamber. Cylindrical magnets with a diameter of 10 mm and 11 mm pitch are fixed at the ends of shafts, which are supported with two ball bearings and driven by NEMA 8 sized (~21 mm) staggered stepper motors. The motors are reinforced by planetary gearboxes to get sufficient torque to overcome magnetic forces between neighboring magnets.

After characterization of the magnetic errors of individual magnets in order to control and predict the actual magnetic field on-axis for a given magnets rotation angles and proven by magnetic measurement of the whole device, this prototype will soon be ready for test installation in the accelerator tunnel.

Further literature: P. Vagin, A. Schöps, and M. Tischer, Variable Period Undulator with Tunable Polarization, Synchrotron Radiation News Vol. 31, No. 3 (2018) and AIP Conference Proceedings 2054, 030024 (2019), https://doi.org/10.1063/1.50845

Contact: P. Vagin, M. Tischer.