CompactLight is a Design Study funded by the European Union under the Horizon 2020 research and innovation funding programme, with Grant Agreement No. 777431. CompactLight was conducted by an International Collaboration of 23 international laboratories and academic institutions, three private companies, and five third parties. The project, which started in January 2018 with a duration of 48 months, aimed to design an innovative, compact, and cost-effective hard X-ray FEL facility complemented by a soft X-ray source to pave the road for future compact accelerator-based facilities. The result is an accelerator that can be operated at up to 1 kHz pulse repetition rate, beyond today’s state of the art, using the latest concepts for high brightness electron photoinjectors, very high gradient accelerating structures in X-band, and novel short-period undulators. In this report, we summarize the main deliverable of the project: the CompactLight Conceptual Design Report, which overviews the current status of the design and addresses the main technological challenges.

Free electron laser (FEL) facilities provide broadly tunable and highly coherent photon beams. These machines still have an unexplored potential and development. The XLS-CompactLight design aims at a flexible hard plus soft X-ray FEL facility exploiting the latest concepts in terms of short period magnetic undulators, paving the road towards more compact photon sources.

European XFEL is going to provide full polarization control in the soft X-ray SASE line (SASE3). For this purpose, four helical APPLE X undulators with 90 mm period are installed downstream with respect to the planar undulators of the SASE3 undulator line consisting of 21 planar undulators with 68 mm period. In this contribution, the measurement technique, as well as the results of the measurements and tuning of the APPLE X undulators performed at European XFEL are presented.

Synchrotron radiation newS, Vol. 31, No. 3, 2018 35 Introduction: APPLE-type undulators in a nutshell Back in 1993, a first planar undulator was proposed, which produced circular (left and right) and vertical polarization, in addition to the standard linear horizontal polarization [1]. This flexibility is needed for soft X-ray beamlines because of the lack of optics to manipulate the polarization. This undulator was known as APPLE, or Advanced Planar Polarized Light Emitter. In this design, there are two I-beams with two magnet arrays each. In the first undulators, two diagonally opposite magnet arrays were fixed, while the others were shiftable by plus/minus half a period in the same direction. A horizontal field is generated with a phase shift of π/2 with respect to the vertical field, which forces the electrons on a helical trajectory and, hence, circular polarized light is emitted. The energy is changed by a gap variation. Vertical and horizontal fields show a different gap dependence because of the asymmetry in the horizontal slit dimension with respect to the vertical gap. Thus, the correct shift setting for circular light with equal vertical and horizontal fields is gap dependent. Later, Sasaki and Bahrdt found that the APPLE II configuration allowed users to rotate the linear polarization vector continuously by shifting the two diagonal arrays in opposite directions. The horizontal field has the same phase as the vertical field, and so the sum of the vectors defines the new polarization plane. However, in order to use this mode, the undulator support structure needed to be reinforced because of new strong longitudinal forces in this mode. Moreover, to allow the polarization to be changed from 0 to 180°, all four magnet arrays need to be adjustable. These devices have been developed to be used in storage ring facilities where a large horizontal vacuum chamber was needed. While the magnet arrays on the I-beams are separated only by a small slit, on the order of 1 mm or less, the gap typically changed from a typical minimum gap of 16 mm to about 80 mm. For use in single-pass linear accelerator-based facilities like FELs, the APPLE III concept has been proposed by Johannes Bahrdt. Here, the original 45° direction from the APPLE I concept was used to increase the on-axis field in a configuration where the magnet could enclose a small round vacuum pipe [2]. On the way towards optimized undulators for use in FELs, two fixed-gap designs have been realized where the energy is also changed by shifts of magnet arrays; namely, two neighboring magnet arrays versus the two opposite ones. This was first proposed in 1994 by Roger Carr [3]. A fixed-gap APPLE II has been implemented at the Swiss Light Source (SLS) at PSI [4]; a symmetric design with the gap as small as the slit (named DELTA) is in use at CHESS at Cornell [5]; and as an afterburner for the LCLS free electron laser at SLAC [6]. However, the operation at SLS showed an unexpected broadening of the blue edge of the spectrum, which could be explained by a gradient in the vertical field in the horizontal plane. At SLS, this had been solved by changing the energy shift from top versus bottom to left versus right. As a consequence, the gradient is in the vertical plane, and because of the small vertical emittance in storage rings, the gradient is no longer degrading the spectrum. For LCLS with its fixed undulator, the DELTA afterburner does not need an energy shift and the gradients do not occur. But for general applications, a gap drive system is mandatory for APPLEtype undulators to decouple K from its gradient in the circular mode. For the SwissFEL soft X-ray Athos beamline, a new concept has been proposed, which is based on the long development of the APPLEtype undulator family. Because of the gradients in fixed-gap circular mode, a gap variation was mandatory. Second, the small slit affects only the vertical field, not the horizontal one. In order to meet the user demand to cover the entire wavelength range in all polarization modes, the magnet design has to be matched to the weaker horizontal field. This means that the vertical field and the accompanying forces are unnecessarily strong. For this reason, it makes sense to open the slit to the gap dimension. Combining this with the APPLE III concept results in a force-reduced symmetric situation. Breaking up the concept of having two I-beams with two magnet arrays each enables this symmetric situation over the entire wavelength range by a radial change of each of the four magnet arrays individually. We gave this configuration the name APPLE X. Of course, to vary the polarization, the axial shift of each magnet array remains. The entire undulator now has one gap and one shift motor axis, for a total of eight axes, but it allows more flexibility (see Figure 1).

The SwissFEL Injector Test Facility operated at the Paul Scherrer Institute between 2010 and 2014, serving as a pilot plant and testbed for the development and realization of SwissFEL, the X-ray Free-Electron Laser facility under construction at the same institute. The test facility consisted of a laser-driven rf electron gun followed by an S-band booster linac, a magnetic bunch compression chicane and a diagnostic section including a transverse deflecting rf cavity. It delivered electron bunches of up to 200 pC charge and up to 250 MeV beam energy at a repetition rate of 10 Hz. The measurements performed at the test facility not only demonstrated the beam parameters required to drive the first stage of an FEL facility, but also led to significant advances in instrumentation technologies, beam characterization methods and the generation, transport and compression of ultra-low-emittance beams. We give a comprehensive overview of the commissioning experience of the principal subsystems and the beam physics measurements performed during the operation of the test facility, including the results of the test of an in-vacuum undulator prototype generating radiation in the vacuum ultraviolet and optical range.

For the SwissFEL at PSI beside the hard x-ray beamline to start with a soft x-ray line is planned to cover the wavelength between 0.7 and 7.0nm. For full control of the polarization of the FEL light, APPLE undulators are forseen. In this paper the design of these devices is introduced and the preliminary magnetic configuration together with the optimization strategy is presented in details.

SwissFEL [1] has in its base line design two undulator lines for the hardand soft x-ray, U15 in-vacuum and U40 / UE40 APPLE II type undulators with 12 respectively 15 modules of 4m length each. All undulators are equipped with the same frame and gap drive system to profit best from the series production. The frame is built up from two identical bases and side frames made of cast mineral. In this design, the frame transfers its stiffness to the I-beam through a backslash-free wedge based gap drive system. The interfaces to the inner I-beam for the in-vacuum undulator have been rearranged allowing a large reduction in the number of columns. Magnets and poles are carried by an extruded Aluminium block-keeper, which will allow an automatized shimming of the magnet structure. The prototype of the support structure has been built up in 2012 and first mechanical results are presented. The entire prototype shall be ready by the end of 2012. SWISSFEL UNDUALTOR LINES SwissFEL will have two undulator lines. The one for hard X-rays from, 7 Å (2 keV) to 1 Å (12.4 keV), with an electron energy of 5.8 GeV, is named Aramis. The second one, covering the entire soft X-ray range, from about 200 eV to 2 keV with full polarization control, is named Athos. In the baseline design, the Aramis line has 12 invacuum undulators U15’s with 15mm period. However, there are free slots, which can be used i.e. for (self)seeding. An overview of the Aramis undulator line is given in [2], the magnet array is discussed in detail in [3]. The Athos line will follow in 2020 and has a selfseeding design with 6 planar U40’s and 9 APPLE II type UE40’s, both with 40mm period. This sums up to 27 undulators. All undulators are 4m long each and have an identical intersection length of 75cm. The design large number of identical undulators for linac driven FELs is well suited for a consequent industrial based small series production. SPECS AND DESIGN GUIDELINES The specifications for the SwissFEL undulators can be summarized as follows:  Short period undulators for Aramis beamline  Variable polarization for Athos beamline

All soft x-ray beamlines at the Swiss Light Source (SLS) are served with variable polarization from APPLE II [1] type and electromagnetic undulators. Three APPLE II type undulators are used: a twin and a single standard APPLE II (UE56 and UE54) and a fixed gap APPLE II (UE44) which follows the adjustable-phase undulator approach by R. Carr [2], [3]. The demand to rotate the linear polarization vector from 0-180 degrees required all four magnet arrays to be shiftable. This opened the possibility to also vary the energy by a suitable shift of the magnet arrays with a simplified support structure lacking in any gap drive system [4], [5]. The current photon beam quality in linear and circular mode and the pros and cons of the operation of the UE44 will be discussed, namely the underestimated influence of gradients in the complex field distribution. As a consequence the spectra are degraded, but can be recovered by use of distributed coils or by a simple change in the operation mode.

The Materials Science beamline at the Swiss Light Source has been operational since 2001. In late 2010, the original wiggler source was replaced with a novel insertion device, which allows unprecedented access to high photon energies from an undulator installed in a medium-energy storage ring. In order to best exploit the increased brilliance of this new source, the entire front-end and optics had to be redesigned. In this work, the upgrade of the beamline is described in detail. The tone is didactic, from which it is hoped the reader can adapt the concepts and ideas to his or her needs.

Abstract SwissFEL [1] will start operation with the so-called Aramis FEL line which lases in the hard X-ray wavelength range from 1 to 7 Angstroem. First photons are foreseen for the end of 2016. In this first phase of the project only the transfer line (a dog-leg section) to the soft X-ray line will be assembled. The soft X-ray undulator line, Athos, will be completed at a later stage after 2018. The civil construction of SwissFEL has started in spring 2013 and will be completed by December 2014. Aramis line has 12 undulator segments but can host up to 20 segments. Tests of an undulator prototype have been recently completed and are described in a companion paper [2]. The layout and the design status of components are presented.

In the framework of the SwissFEL project, an R&D activity concerning in-vacuum undulator technology is ongoing at the Paul Scherrer Institut. The magnetic field configuration of the hard X-ray SwissFEL undulators has been designed on purpose for a single pass machine. Moreover the permanent magnet material (NdFeB) is manufactured following a novel procedure (Dy diffused in the grain boundaries) to improve the coercivity versus remanence. The assembly and tests of a 44 periods hybrid magnetic structure are presented. Procedures for the magnetic field, trajectory and phase optimization are reported versus experimental results.

The cryogenic permanent magnet undulator (CPMU) is an insertion device in which the permanent magnets are cooled down to cryogenic temperatures to improve the magnetic performance in terms of the remanent field and coercivity. As it was found recently, the peak field and coercivity of permanent magnet materials like NdFeB are increasing as the temperature is decreased and reached a maximum at around 130 K. This temperature is not directly attainable by boiling of standard cryogenic fluids like liquid nitrogen (77 K) or liquid helium (4.2 K). We present a practical and reliable cooling method based on the thermal shunt principle with either liquid nitrogen or a cryocooler as a cold source. Design criteria, cryogenic analysis and the layout of a CPMU based on this principle are presented. A new CPMU with a magnetic period of 14 mm and a magnetic length of 1.7 m, has been manufactured and is presently installed and in operation at the Swiss Light Source (SLS) as part of a collaboration between PSI (Paul Scherrer Institute), SPring-8 and Hitachi Metals, Ltd.

The cryogenic permanent magnet undulator (CPMU) is an insertion device in which permanent magnets are cooled down to cryogenic temperature (CT) to improve the magnetic performances. Toward realizataion of CPMUs, it is important to establish a technique to measure the magnetic field at CT and to correct it if necessary. A new method of the undulator magnetic correction has been developed at SPring-8 based on a mechanical adjustment of the invacuum beam. This method is available at CT without breaking the vacuum and thus enables the “in-situ” field correction. The feasibility of this method has been tested with the CPMU for the SLS storage ring, the results of which are reported in this paper.

We describe the concepts and technical realization of the high-resolution soft-X-ray beamline ADRESS operating in the energy range from 300 to 1600 eV and intended for Resonant Inelastic X-ray Scattering (RIXS) and Angle-Resolved Photoelectron Spectroscopy (ARPES). The photon source is an undulator of novel fixed-gap design where longitudinal movement of permanent magnetic arrays controls not only the light polarization (including circular and 0-180 deg rotatable linear polarizations) but also the energy without changing the gap. The beamline optics is based on the well-established scheme of plane grating monochromator (PGM) operating in collimated light. The ultimate resolving power E/dE is above 33000 at 1 keV photon energy. The choice of blazed vs lamellar gratings and optimization of their profile parameters is described. Due to glancing angles on the mirrors as well as optimized groove densities and profiles of the gratings, high photon flux is achieved up to 1.0e13 photons/s/0.01%BW at 1 keV. Ellipsoidal refocusing optics used for the RIXS endstation demagnifies the vertical spot size down to 4 um, which allows slitless operation and thus maximal transmission of the high-resolution RIXS spectrometer delivering E/dE better than 11000 at 1 keV photon energy. Apart from the beamline optics, we give an overview of the control system, describe diagnostics and software tools, and discuss strategies used for the optical alignment. An introduction to the concepts and instrumental realization of the ARPES and RIXS endstations is given.

Insertion devices (IDs) of various types provide light of high brilliance to experimenters at the Swiss Light Source (SLS) beamlines. However, changes in the photon energy and polarization by movement of the ID gap and phase shift cause orbit distortions that result in a displacement of the photon beam in both angle and position at the beamline. A feed-forward correction scheme has been developed to quantify and precisely correct these effects using designated correctors local to the photon source. The settings for these correctors are determined using an orbit configuration consisting of 73 digital beam position monitors (DBPMs) and associated correctors; recently commissioned X-ray beam position monitors (XBPMs) located at the beamline front-end are also included in the correction algorithm to further constrain the photon beam to its specified position. The feed-forward correction procedure is finally implemented at the local processor level and applied at a rate of 10Hz. A photon pointing stability at the sub-microradian level is achieved. The entire gap scan, feed-forward generation and subsequent verification can be completed from within a few minutes to several hours depending on the complexity of the ID. The methodology of the procedure is described and the results from several ID analyses are presented. The effect of the ID on the betatron tune is also discussed. Both feed-forward and feedback procedures have been implemented to maintain the horizontal and vertical components of the tune at the desired working points.

FEMTO, a femtosecond (fs) X-ray source based on laser interaction with a relativistic electron beam, began operation in the fall of 2006. It is installed at the μXAS beamline of the Swiss Light Source (SLS) at the Paul Scherrer Institut, Villigen. "Laser slicing" of an electron beam has first been proposed and demonstrated at the ALS [] and has recently been implemented at BESSY [2 Khan, S. 2006. Phys. Rev. Lett, 97: 074801[Crossref], [Web of Science ®] , [Google Scholar]] to generate fs soft X-rays (1–2 keV) with variable polarization. FEMTO is the first undulator source providing tunable, fs hard X-rays in the range 4.5–12 keV for laser/X-ray pump-probe absorption and diffraction experiments.

The X-ray photo-emission monitors at the Swiss Light Source (SLS) are used for beam-position diagnostics and beam stabilization down to the sub-micron level. The main systematic change of the photon beam- position is induced by varying insertion device (ID) settings, such as photon energy, harmonic-selection or light polarization. An ID beam-position correction scheme is based on digital beam-position monitors (DBPM) located inside the storage ring, combined with analogue Bergoz BPMs, located inside the ID straight section, and analogue photon monitors (XBPMs), in beam line front-ends. The use of XBPMs in this correction scheme will be demonstrated. Moreover, in order to achieve sub-micron photon-beam stability while changing the ID parameters with XBPM- readouts requires precise XBPM alignment and characterization for each ID-setting (1-2). We present an account of the design and performance of the XBPMs as used for characterizing systematic effects of the U19, and the more challenging elliptical undulator UE44 (3), a newly designed fixed-gap APPLE II undulator recently installed at the advanced resonant spectroscopy beam line (4).

Corrections of insertion device (ID) induced orbit distortions at the SLS are performed by means of feed forward schemes down to the micron level at the corresponding photon beam position monitors (XBPMs). The remaining orbit fluctuations are suppressed by XBPM feedbacks which are an integral part of the fast orbit feedback system. As a result, sub-μm RMS stability at the XBPMs is achieved while the ID settings are varied.