PHOTON SCIENCE

Before building the afterburner undulator in the course of the FLASH2020+ project, a short prototype structure with a period length of 16 mm was designed and constructed. It followed the design shown on the right: 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. The outer arrays form the force compensation scheme and need to be implemented between both, top and bottom girder and also between rows on each girder. Our calculations show that a reduction of magnetic forces by roughly a factor of eight is obtained in this design. Further, a rotatable correction magnet located in each keeper behind the function magnet (red and blue in the right figure) and magnetised across the diameter was intended for tuning of the magnet structure. The force compensation scheme was proven to work successfully as all the sub-girders were moving easily and smoothly in different configurations during manual test performances. However, magnetic measurements of the prototype revealed that a period length of 16 mm did not allow for the desired maximum field when also accounting for errors and tuning. In consequence, the required wavelength range would not be reached. We have, therefore, decided to increase the period length to 17.5 mm for the final version of the FLASH2 afterburner.

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

Further literature: Bahrdt J, Frentrup W, Gaupp A, Kuske B, Meseck A, and Scheer M 2004 Proc. SRI Conference 2003 AIP CP 705 215‒18
Liang X, Calvi M, Kittel C, Sammut N J, and Schmidt T 2019 Proc. FEL'19 541‒544
Bahrdt J and Grimmer S 2019 Proc. SRI Conference 2018, AIP CP 2054 030031-1‒6