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Superconducting Undulators (SCUs) can produce higher photon flux and cover a wider photon energy range compared to permanent magnet undulators (PMUs) with the same vacuum gap and period length. To build the know-how to implement superconducting undulators for future upgrades of the European XFEL facility, two magnetic measurement test stands named SUNDAE 1 and 2 (Superconducting UNDulAtor Experiment) are being developed. SUNDAE1 will facilitate research and development on magnet design thanks to the possibility of training new SCU coils and characterizing their magnetic field. The experimental setup will allow the characterization of magnets up to 2m in length. These magnets will be immersed in a Helium bath at 2K or 4K temperature. In this article, we describe the experimental setup and highlight its expected performances., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

Eight SRF cavities assembled in an accelerating module represent a building block of the particle linear accelerator based on TESLA SRF technology. DESY has two machines, European XFEL and FLASH. Both use almost same module and cavity types. During the module assembly many factors can deteriorate the cavity performance and cause a need for a repair action. Currently two European XFEL modules and two FLASH ones underwent reassembly procedures. The repair was not immediately successful on every of these modules and re-iterations did follow. The degradation causes were investigated. SRF modules were tested on both test-stands at DESY: AMTF and CMTB. The results of the described actions are presented and discussed., Proceedings of the 20th International Conference on RF Superconductivity, SRF2021, East Lansing, MI, USA

EPJ Techniques and Instrumentation 8:5(1), 1-20 (2021). doi:10.1140/epjti/s40485-020-00060-5, At DESY the ALPS II experiment is being installed in the HERA tunnel to search for axion like particles. A laser beam will be injected into the magnetic field of a string of superconducting (s.c.) dipole magnets, available from the HERA proton storage ring, to produce axion like particles. After passing a light tight wall, the ALPs can reconvert into photons in a second string of HERA s.c. dipoles. The sensitivity of the experiment will be increased by two mode-matched optical cavities before and behind the wall. The dipoles for the HERA storage ring are curved, suited for stored proton beam. However, the curvature of the magnets limits the aperture and hence the performance of theoptical resonators beyond a certain length. As the sensitivity of the search scales with the length of the magnetic field, the aperture for the optical resonators inside the HERA dipoles was increased by straightening the curved magnet yoke. The procedure of straightening the s.c. HERA dipoles is described in this report., Published by Springer Open, Heidelberg [u.a.]

At DESY, the development of a 1.6-cell, 1.3 GHz all superconducting gun cavity with a lead cathode attached to its back wall is ongoing. The special features of the structure like the back wall of the half-cell and cathode hole require adaptations of the procedures used for the treatment of nine-cell TESLA cavities. Unsatisfactory test results of two prototype cavities motivated us to re-consider the back-wall design and production steps. In this contribution we present the status of the modified cavity design including accessories causing accelerating field asymmetries, like a pick up antenna located at the back wall and fundamental power- and HOM couplers. Additionally, we discuss preliminary considerations for the compensation of kicks caused by these components., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

The European XFEL is in operation since 2017. The design projected energy of 17.5 GeV was reached, even with the last 4 main linac accelerating modules not yet installed. 2 out of 4 not installed modules did suffer from strong cavity performance degradation, namely increased field emission, and required surface processing. The first of two modules is reassembled and tested. The module test results confirm a successful repair action. The module repair and test steps are described together with cavities performance evolution., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany, 30 Jun 2019 - 5 Jul 2019; JACoW Publishing, Geneva, Switzerland 5 pp. (2019). doi:10.18429/JACOW-SRF2019-MOP082, The coupler is one of the main and most sensitive components of the European X-ray Free Electron Laser (EXFEL) superconducting cryomodule. More than 800 couplers were transported for more than 800 km assembled in a cryomodule during the assembly phase of the EXFEL without any visible damage. However, in a different project, a very similar coupler design showed a week point in one of the bellows when transported over a similar distance with a comparable transport set up. Therefore we decided to further study the coupler behaviour: we investigated the frequency response of the coupler on a vibration table in a controlled environment for different road and loading conditions and compared the data with simulated ones. This paper present the work performed so far and our conclusions., Published by JACoW Publishing, Geneva, Switzerland

19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany, 30 Jun 2019 - 5 Jul 2019; JACoW Publishing, Geneva, Switzerland 5 pp. (2019). doi:10.18429/JACOW-SRF2019-TUP033, Future upgrades of the European X-ray Free Electron Laser (EXFEL) may require driving the linac at higher duty factor, possibly extending to Continuous Wave (CW) mode. An R program has started at DESY, to prepare for a CW upgrade. Cryomodules are being tested in CW mode in our CryoModule Test Bench (CMTB) to study the performance and main issues for such an operation mode. Sensitivity to vibration causing microphonics is one of the main concerns for the CW operation in mode. Therefore a detailed analysis is being performed to evaluate the frequency spectrum of the EXFEL cryomodule main components: the cavity itself, the cavity string, the cold mass and the vacuum vessel. Finite Element Modal Analyses have been performed and the results compared with data measured at the CMTB. This paper summarizes the main results and conclusions of such a study., Published by JACoW Publishing, Geneva, Switzerland

29th Linear Accelerator Cnference, LINAC18, Beijing, China, 17 Sep 2018 - 21 Sep 2018; Geneva : JACoW 20-24 (2018)., A future upgrade of the European XFEL (E-XFEL) foresees an additional CW operation mode, which will increase the flexibility in the photon beam time structure [1–3]. One of the challenges of this operational mode is the need fora CW operating photo injector. We believe that using an SRF gun is the preferred approach as the beam parameters of normal conducting pulsed guns can be potentially met by SRF guns operating CW. For more than a decade DESY, incollaboration with TJNAF, NCBJ, BNL, HZB and HZDR, has performed R&D to develop an all superconducting RFgun with a lead cathode. In the frame of E-XFEL CW upgrade feasibility studies, the SRF-gun R&D program gained more attention and support. Within the next few years we would like to demonstrate the performance of the all superconducting injector required for the E-XFEL upgrade. The selected approach offers advantages w.r.t. the cleanliness of the superconducting surface, but requires a complete disassembly of a cryostat and stripping the gun cavity in a cleanroom to exchange the cathode. Thus it is practical only when the life time of the cathode is at least several months. In this paper we present the actual status of the R&D program, next steps and the longer term plans., Published by JACoW, Geneva

Repair actions of different extent have been performed at 61 modules of the 100 accelerating series modules for the European XFEL to qualify them for the tunnel installation. Four modules could not be repaired in time. CEA Saclay managed to perform three major repairs in parallel to the series module integration, the residual repair actions took place at DESY Hamburg. In this paper we will give an overview on the various technical problems which required being fixed before the tunnel installation and on the repair actions performed., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

In the European-XFEL superconducting linac, mechanical frequency tuners equipped with stepper motors and piezoelectric actuators provide cold tuning of each of the 768 1.3 GHz cavities. More than 820 complete tuning systems were fabricated and pre-assembled in industry, tested at several stages before and after assembly and successfully commissioned during cryo-module cold tests at AMTF (DESY). Quality control strategy adopted to preserve the well-assessed tuner reliability through such a large-scale industrial production is critically reviewed and the lessons learned are presented in this paper. The statistical analysis of the large set of data acquired up to the recent commissioning of the entire linac is then summarized., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

The European X-ray Free Electron Laser (EU-XFEL) cold linac consists of 100 assembled cryomodules, 6 feed-/end-boxes and 6 string connection boxes fixed to the ceiling of the accelerator tunnel; the injector consists of a radio frequency gun, one 1.3 GHz and one 3.9 GHz cryomodule, one feed- and one end-cap lying on ground supports. The components are connected together in the tunnel, after cold testing, transport, final positioning and alignment. The cold linac is a pressure equipment and is therefore subjected to the requirements of the Pressure Equipment Directive (PED). This paper describes the welding and subsequent non-destructive testing of the cryo-pipes (with a deeper look at the technical solutions adopted to satisfy the PED requirements), the assembly of the beam line absorbers and the final steps before closing the connection with a DN1000 bellows. A special paragraph will be dedicated to the connection of the injector components, where the lack of space makes this installation a particularly challenging task., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

In order to validate the cavity package concept before the module preparation for the European XFEL Injector, one 3.9 GHz cavity, complete with magnetic shielding, power coupler and frequency tuner was tested in a specially designed single cavity cryomodule in one of the caves of the DESY Accelerator Module Test Facility (AMTF). The cavity was tested in high power pulsed operation up to the quench limit of 24 MV/m, above the vertical test qualifications and all subsystems under test (coupler, tuner, waveguide tuners, LLRF system) were qualified to design performances., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

Longitudinally-welded 72 mm ID service pipes (HSP) made from titanium grade 2 is used by the two suppliers of the helium tanks for the EU-XFEL accelerator. From the perspective of the PED DESY is legally designated as the manufacturer and is responsible for conformity to all relevant codes. During module assemblies at CEA Saclay the orbital welds of the interconnection bellows between cavities showed pores with dimensions outside the specifications set by DESY. These welds needed to be redone which caused a project delay of several months. The X-ray examination of the HSP showed that the pipes already exhibited many out-of- DESY spec pores in the longitudinal welds and were most likely the main cause of the problems in the orbital welds. It was decided to replace the extremities of the service pipes with seamless titanium tubes both on “naked” helium tanks as well as on tanks with cavities already welded in. At DESY more than 750 service pipes were exchanged over a period of 2 years. The qualification of the repair line according to PED regulation and the prove with RF test at 2 K that the repairs do not influence the high performance of the s.c. cavities were done., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

The European X-ray Free Electron Laser (EU-XFEL) [1] cold linac [2] consists of 100 assembled cryomodules, 6 feed/end boxes and 6 string connection boxes fixed to the ceiling of the accelerator tunnel; the injector consists of a radio frequency gun, one 1.3 GHz and one 3.9 GHz cryomodule, one feed and one end cap lying on ground supports. The components are connected together in the tunnel, after cold testing, transport, final positioning and alignment. The cold linac is a pressure equipment and is therefore subjected to the requirements of the Pressure Equipment Directive (PED). This paper describes the welding and subsequent Non-Destructive Testing (NDT) of the cryo-pipes (with a deeper look at the technical solutions adopted to satisfy the PED requirements), the assembly of the beam line absorbers and the final steps before closing the connection with a DN1000 bellows. A special paragraph will be dedicated to the connection of the injector components, where the lack of space makes this installation a particularly challenging task.

The 3.9 GHz cryomodule and RF system for the XFEL Injector is being assembled and delivered to the underground building in summer 2015, for the injector commissioning in Fall 2015. This contribution outlines the status of the activity and reports the preparation stages of the technical commissioning of the system., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

The CE stamping of the one hundred 1.3 GHz cryomodules for the European XFEL Linac is a main step in the process of the certification of the entire Linac as pressure equipment. Stringent requirements on materials and the quality of the welds of the pressurised components need to be satisfied to obtain the stamp. This paper summarizes these requirements, describes the process developed to qualify each module and summarises the rework campaign on the cavity helium vessels made necessary to obtain the required quality for a reliable and safe accelerator.

The series production of the superconducting accelerator modules for the European XFEL requires a production rate of one module per week. For this, assembly procedures have to be well-defined and repeatable, and the punctual supply of parts from the contributing institutes has to be assured. Configuration management (CM) has been introduced for clarification of responsibilities and establishing procedures. CM provides unique identification of parts, part status and location tracking, versioning of documentation, and procedures for change control, auditing and handling non-conformities. The configuration database, which is based on DESY’s Engineering Data Management System, contains the entire information which is necessary for assembling the accelerator modules. The content ranges from work instructions how to build a cryomodule up to individual records of all produced parts. Workflow and reports help tracking production progress and establishing production quality. The presentation gives an overview of the CM solution which is in place for the assembly of the XFEL accelerator modules, and reports experience and lessons learned from series production of the first modules., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany