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.

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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.

Abstract. Self-Referenced Spectral Interferometry is used for single shot pulse characterization over the 0.9-2.5 µm spectral range with a single spectrometer and an optimized optical setup. We characterize sub-55 fs pulses from 1.4 µm to 2 µm and broadband 2.5-cycle pulses at 1.65 µm (13 fs FWHM).The development of laser sources delivering tunable, ultra-broadband femtosecond pulses in the shortwavelength infrared (SW-IR) and mid-infrared (mid-IR) spectral ranges open-up the way for new laser pulse characterization devices. Recently developed laser systems make use of Optical Parametric amplification (OPA) [1] and Optical Parametric Chirped Pulse Amplification (OPCPA) [2] to produce sub-20 fs pulses covering bandwidth from 1.2 to 2.4 µm with a few milli-Joule energy per pulse. A broadband, single shot characterization technique is mandatory, since these sources are based on nonlinear parametric frequency conversion, and so intensity fluctuations in the pump lead to shot to shot phase fluctuations in the signal. Up to our knowledge, none of the usually used characterization techniques (like FROG or SPIDER) have been adapted yet to few-cycle pulse characterization in the SWIR [3].Self-Referenced Spectral Interferometry (SRSI) [4,5], discussed here, is a technique which offers intrinsically single-shot and frequency-conserving pulse reconstruction (i.e. the recorded signal is at the same wavelength as the input signal). It is thus possible to characterize both spectrum and phase with one single spectrometer. We demonstrate a SRSI based measurement device for ultra-short broadband SWIR pulse characterization with a single extended-InGaAS spectrometer. Single-shot measurement of sub-55 fs pulses in the 1.4 -2 µm spectral range is reported and attests to the broadband capability of the technique. In addition, we characterized few-cycle pulses (13 fs FWHM, λ 0 =1.65 µm) as an experimental confirmation of the device's capability to measure ultra-short pulses in the SWIR [3].

With the improvement of acceleration techniques, the intrinsic emittance of the cathode has a strong impact on the final brightness of a free electron laser. The systematic studies presented in this paper demonstrate for the first time in a radiofrequency photocathode gun a reduction of the intrinsic emittance when tuning the laser photon energies close to the effective work function of copper. The intrinsic emittance was determined by measuring the core slice emittance as a function of the laser beam size at laser wavelengths between 260 and 275 nm. The results are consistent with the measured effective work function of the cathode. Slice emittance values normalized to the laser beam size reached values down to 500 nm=mm, close to that expected from theory. A 20% reduction of the intrinsic emittance was observed over the spectral range of the laser.