This article presents the design, construction, and high-power test of two $X$ -band radio frequency (RF) accelerating structures built as part of a collaboration between CERN and the Paul Scherrer Institute (PSI) for the compact linear collider (CLIC) study. The structures are a modified "tuning-free" variant of an existing CERN design and were assembled using Swiss free electron laser (SwissFEL) production methods. The purpose of the study is two-fold. The first objective is to validate the RF properties and high-power performance of the tuning-free, vacuum brazed PSI technology. The second objective is to study the structures' high-gradient behavior to provide insight into the breakdown and conditioning phenomena as they apply to high-field devices in general. Low-power RF measurements showed that the structure field profiles were close to the design values, and both structures were conditioned to accelerating gradients in excess of 100 MV/m in CERN's high-gradient test facility. Measurements performed during the second structure test suggest that the breakdown rate (BDR) scales strongly with the accelerating gradient, with the best fit being a power law relation with an exponent of 31.14. In both cases, the test results indicate that stable, high-gradient operation is possible with tuning-free, vacuum brazed structures of this kind.

We present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-Å wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation.

The SwissFEL soft X-ray free-electron laser (FEL) beamline Athos will be ready for user operation in 2021. Its design includes a novel layout of alternating magnetic chicanes and short undulator segments. Together with the APPLE X architecture of undulators, the Athos branch can be operated in different modes producing FEL beams with unique characteristics ranging from attosecond pulse length to high-power modes. Further space has been reserved for upgrades including modulators and an external seeding laser for better timing control. All of these schemes rely on state-of-the-art technologies described in this overview. The optical transport line distributing the FEL beam to the experimental stations was designed with the whole range of beam parameters in mind. Currently two experimental stations, one for condensed matter and quantum materials research and a second one for atomic, molecular and optical physics, chemical sciences and ultrafast single-particle imaging, are being laid out such that they can profit from the unique soft X-ray pulses produced in the Athos branch in an optimal way.

Rafael Abela, Arturo Alarcon, Jürgen Alex, Christopher Arrell, Vladimir Arsov, Simona Bettoni, Markus Bopp, Christoph Bostedt, Hans-Heinrich Braun, Marco Calvi, Tine Celcer, Paolo Craievich, Andreas Dax, Philipp Dijkstal, Sladana Dordevic, Eugenio Ferrari, Uwe Flechsig, Rolf Follath, Franziska Frei, Nazareno Gaiffi, Zheqiao Geng, Christopher Gough, Nicole Hiller, Stephan Hunziker, Martin Huppert, Rasmus Ischebeck, Haimo Jöhri, Pavle Juranic, Roger Kalt, Maik Kaiser, Boris Keil, Christoph Kittel, René Künzi, Thomas Lippuner, Florian Löhl, Fabio Marcellini, Goran Marinkovic, Cigdem Ozkan Loch, Gian Luca Orlandi, Bruce Patterson, Claude Pradervand, Martin Paraliev, Marco Pedrozzi, Eduard Prat, Predrag Ranitovic, Sven Reiche, Colette Rosenberg, Stephane Sanfilippo, Thomas Schietinger, Thomas Schmidt, Kirsten Schnorr, Cristian Svetina, Alexandre Trisorio, Carlo Vicario, Didier Voulot, Ulrich Wagner, Hans Jakob Wörner, Adriano Zandonella, Luc Patthey* and Romain Ganter*

The SwissFEL X-ray Free Electron Laser (XFEL) facility started construction at the Paul Scherrer Institute (Villigen, Switzerland) in 2013 and will be ready to accept its first users in 2018 on the Aramis hard X-ray branch. In the following sections we will summarize the various aspects of the project, including the design of the soft and hard X-ray branches of the accelerator, the results of SwissFEL performance simulations, details of the photon beamlines and experimental stations, and our first commissioning results.

The goal of SwissFEL is to provide a source of extremely bright and short X-ray pulses enabling scientific discoveries in a wide range of disciplines, from fundamental research to applied science. To accelerate the electrons to an energy of up to 5.8 GeV a linear accelerator (LINAC) consisting of 104 C-band (5.712 GHz) accelerating structures each of a length of 2m is foreseen. We present the mechanical design of the accelerating structures. High precision manufacturing is applied in order to avoid a tuning step during fabrication. Following this production process several 0.3 m test structures have been produced and tested. First results including RF power tests are presented.