Magnet structure of APPLE-type undulators

Arrangement of magnets in the APPLE prototype. Orange arrows indicate direction of magnetisation of the permanent magnets.
Magnets A and B. Green arrows indicate the direction of magnetisation; the dashed orange line represents the beam axis. Magnets are glued together to form AB- or BA-pairs.
Keeper with magnets for the FLASH1 radiators with 35mm period length.
Bottom magnet structure of the FLASH2 afterburner with 17.5mm period length.

In the APPLE-III configuration, the magnet structure consists of four permanent magnet arrays in Halbach configuration, where the B magnets of the inner arrays around the circular beam pipe are transversely magnetised with their easy axis along the 45° direction as sketched in the figure on the right.

Two types of magnets, commonly named A and B, are used to build the APPLE-III magnetic structure: Type A is magnetised along the short edge, while type B is magnetised along the diagonal of the large plane. In order to improve stability and to simplify the mechanical design, magnets of type A and B are glued together in pairs as shown in the second picture on the right. Both types are made from transversely dye-pressed (TP) material with Br,min=1.32 T and Hcj,min=21 kOe. For magnet type B, first, larger magnets which are magnetised along the long edge are pressed and then wire-cut to the desired shape and dimensions.

The magnet pairs are mounted into aluminium keepers which feature a weak link that allows for virtual shimming of the magnetic field by moving a bronze wedge via differential screws. The outer arrays of the magnet structure form the force compensation scheme and need to be implemented between both, top and bottom girder and also between rows on each girder. Therefore, compensation magnets are included in the keeper. Our calculations show that a reduction of magnetic forces by roughly a factor of eight is obtained in this design.

After building and succesfully testing a prototype, we built an afterburner undulator with 17.5 mm period length for FLASH2. In August 2023, it was installed downstream of the present FLASH2 SASE undulators. It provides circularly polarized light in a wavelength range between 1.33 nm and 1.77 nm (890-700 eV) to widen the spectral range to the L-edges of Fe, Co, and Ni and allow for related research in the field of nanomagnetism.

For the seeding upgrade of FLASH 1, we built a series of six of helical IDs with 35mm period length based on the APPLE-III design.

Contact: M. Tischer, K. Götze, P. Neumann, A. Schöps and P. Vagin

Further literature: M. Tischer, H. Bienert, K. Götze, D. Meissner, P. Neumann, P. N'Gotta, T. Ramm, A. Schöps, P. Talkovski, S. Telawane, P. Vagin, Development of an APPLE-III undulator for FLASH-2, Journal of physics / Conference Series 2380(1), 012017 (2023)