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The European XFEL GmbH is a new X-ray FEL user facility and started lasing in 2017. Three movable gap SASE Undulator Systems based on hybrid NdFeB permanent magnet technology were built. Radiation damage due to machine operation affects the magnetic properties of the segments and the quality of the SASE process. Dosimeters monitor the absorbed doses in every undulator segment and each SASE system is equipped with a diagnostic undulator (DU). We report on the profile of the absorbed doses seen in the first year of the Undulator Systems operation. Magnetic field degradation higher than 3.5% in the DU is found and associated to absorbed doses up to 4.4 kGy. An initial limit of 55 Gy for the 5-m undulator segments is recommended based on the precise K-parameter determination for beam operation. INTRODUCTION AND METHODS The European XFEL (XFEL.EU) uses a superconducting accelerator to operate at energies up to 17.5 GeV with a repetition rate up to 27000 bunches per second and at 1nC [1]. Its accelerator has the highest duty cycle currently in operation worldwide. Three movable gap SASE Undulator Systems produce FEL radiation with tunable wavelength from 0.05 to 5.2 nm and pulse lengths of less than 100 fs [2]. SASE1 achieved the first lasing in May 2017 and is currently in user operation. SASE3 started lasing in February 2018 and SASE2 started commissioning in March 2018. The facility has successfully reached lasing at 14 GeV with 10-3000 bunches per second and at 0.25-0.5 nC [3]. Typical SASE1 photon energy is 9.2-9.3 keV with 1 mJ intensity. A total of 91 5-meter long undulator segments based on hybrid NdFeB (VACODYM 776AP) permanent magnets were tuned respecting tight specifications in order to optimize the SASE effect (table 1). The de-magnetization of the permanent magnets has been observed in other facilities [4-10] as an effect caused by radiation damage. However, the mechanisms behind the magnetic degradation are not yet completely understood or controlled and studies have been proposed [11-13] or are subject for current discussions in the accelerator community. The radiation damage on the XFEL.EU Undulator Systems was expected and dedicated machine protection systems were designed [14,15]. Among them, we have operated an array of dosimeters [16] based on on-line readable Radfets and offline TLD-800 dosimeters. The Radfets permit to monitor the total absorbed doses and beam losses on the upstream side of each undulator segment since first day of beam commissioning. Furthermore, each SASE Undulator System is equipped at its upstream side with a so-called diagnostic undulator (DU). The DU contains the same hybrid NdFeB structure as the 5-meter segments. It has four magnetic periods (table 1) and 12 mm fixed gap. With a total length of about 300 mm and weight of 47 Kg, the DU can be mounted or dismounted in an hour making it suitable for frequent remeasurements of the magnetic properties during maintenance days or weeks. Table 1: XFEL.EU Magnetic Specifications In this manuscript we report on the characteristics of the absorbed doses at SASE1 and SASE3 and on the measurable de-magnetization effects observed on the diagnostic undulators and on one 5-m segment during the first year of beam commissioning/operation. ABSORBED DOSES CHARACTERISTICS At the XFEL.EU facility, the SASE3 tunnel is located downstream of the SASE1 tunnel. The Radfet doses in both systems are therefore monitored since the start of SASE1 commissioning once the electron beam is necessarily travelling through both SASE systems before being dumped. Two types of dose exposure have been observed. The so-called “singular events” are defined by a steep rise of the integrated doses during a short time interval, as shown in Fig. 1A for the upper Radfets installed at the undulators in cell#3 and cell#4 in SASE1. These events are mostly seen due to electron beam steering, change of beam energy or correcting the quadrupoles and/or air coils parame5-m Undulators SASE1 & 2 SASE3 # of Segments 35 21 Max. K-parameter @10mm ≥ 3.9 ≥ 8.0 End kicks By and Bz ≤ │0.15T.mm│ RMS Phase jitter ≤ 8° Diagnostic Undulators Period length 40 mm Peak Field 0.916 T K-parameter 3.28 Number of poles 8  † f.wolff-fabris@xfel.eu Th is is a pr ep ri nt — th e fin al ve rs io n is pu bl ish ed w ith IO P 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEYGBD2

Status of radiation damage on the European XFEL undulator systems