Microbunching instability is a well-known phenomenon that may deteriorate the performance of accelerators. The instability may be triggered by a shot-noise mechanism or by some initial intensity modulations at the generation of the electron bunch (or both) and can be amplified all along the machine. At SwissFEL, the free-electron laser (FEL) facility operating at the Paul Scherrer Institute (PSI), the initial design stipulated a shaping of the photocathode laser output to obtain a flat-top longitudinal profile. This scheme is attractive in terms of the uniformity of the beam properties along the bunch. The drawback of this approach is that some unavoidable modulations are generated along the laser pulse. We investigate, both experimentally and by numerical simulations, the longitudinal dynamics of a beam obtained illuminating a copper cathode with a laser profile shaped by the stacking technique. We repeat the analysis for several compression factors and initial laser profile modulations. We find that the microbunching instability gain renders the use of the stacking technique not efficient to run a free-electron laser facility using as photocathode a material with a short response time. We experimentally demonstrate that the use of a material with a longer response time efficiently damps the structures originating from the laser profile obtained with stacking, and helps to improve the performance of the facility. In general, this is an approach to minimize the microbunching instability at any FEL (also not using stacking) or at least reduce the use of other countermeasures, which, such as the laser heater, may degrade the final FEL performance.

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 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.

We report on a commercial, industrial-grade, sub-200 fs Yb-doped oscillator at 1041.3 nm central wavelength with an output power of 152 mW at 71.4 MHz repetition rate. The compact laser oscillator combines ultra-low amplitude and time jitter noise and it is well suited for seeding high-energy Yb-based amplifier systems. This technology developed for optical clocks and extended to this wavelength range will fill the gap at X-ray Free Electron Laser facilities [1] for an optical low-noise frequency-quadrupled photocathode drive laser source with excellent long-term performance and sub-ps pulse durations.

The ESB instrument at the SwissFEL ARAMIS hard X-ray free electron laser is designed to perform pump-probe experiments in condensed matter and material science employing photon-in and photon-out techniques. It includes a femtosecond optical laser system to generate a variety of pump beams, a X-ray optical scheme to tailor the X-ray probe beam, shot-to-shot diagnostics to monitor the X-ray intensity and arrival time, and two endstations operated at a single focus position that include multi-purpose sample environments and 2D pixel detectors for data collection.

The Compact Linear Collider (CLIC) is a multi-TeV high-luminosity linear e+e- collider under development. For an optimal exploitation of its physics potential, CLIC is foreseen to be built and operated in a staged approach with three centre-of-mass energy stages ranging from a few hundred GeV up to 3 TeV. The first stage will focus on precision Standard Model physics, in particular Higgs and top-quark measurements. Subsequent stages will focus on measurements of rare Higgs processes, as well as searches for new physics processes and precision measurements of new states, e.g. states previously discovered at LHC or at CLIC itself. In the 2012 CLIC Conceptual Design Report, a fully optimised 3 TeV collider was presented, while the proposed lower energy stages were not studied to the same level of detail. This report presents an updated baseline staging scenario for CLIC. The scenario is the result of a comprehensive study addressing the performance, cost and power of the CLIC accelerator complex as a function of centre-of-mass energy and it targets optimal physics output based on the current physics landscape. The optimised staging scenario foresees three main centre-of-mass energy stages at 380 GeV, 1.5 TeV and 3 TeV for a full CLIC programme spanning 22 years. For the first stage, an alternative to the CLIC drive beam scheme is presented in which the main linac power is produced using X-band klystrons.

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.

Radio-frequency (rf) photoinjectors are used to generate high-brightness electron beams for a wide range of applications. Because of their outstanding beam quality, they are particularly well-suited as sources for X-ray free-electron lasers (FELs). The beam emittance, which is significantly influenced by the intrinsic emittance of the cathode, is fundamental for FELs, since it has a strong impact on the lasing performance and it defines the length and cost of the facility. In this paper we present measurements of the intrinsic emittance as a function of the rf field for a copper photocathode. Our measurements match with the theoretical expectations, showing that the intrinsic emittance can be reduced by decreasing the rf field at the cathode. We obtained normalized intrinsic emittances down to 350 nm/mm, the lowest values ever measured in a rf photoinjector.

Intense ultrashort pulse lasers are used for fs resolution pump-probe experiments more and more at large scale facilities, such as free electron lasers (FEL). Measurement of the arrival time of the laser pulses and stabilization to the machine or other sub-systems on the target, is crucial for high time-resolution measurements. In this work we report on a single shot, spectrally resolved, non-collinear cross-correlator with sub-fs resolution. With a feedback applied we keep the output of the TW class Ti:sapphire amplifier chain in time with the seed oscillator to ~3 fs RMS level for several hours. This is well below the typical pulse duration used at FELs and supports fs resolution pump-probe experiments. Short term jitter and long term timing drift measurements are presented. Applicability to other wavelengths and integration into the timing infrastructure of the FEL are also covered to show the full potential of the device.

SwissFEL is aiming to produce X-ray pulses from 30 fs down to the attosecond time scale.This requires the compression of the several picosecond long electron bunches produced by a photo-injector to sub-fs level.To achieve this, 40fs accurate injection of the electron bunches into the main linear accelerator is necessary.Therefore high timing accuracy is required from the drive laser of the electron gun.Furthermore fs scan capability is foreseen for the experimental stations of the FEL.The ultra-short pulse pump-probe lasers therefore need to exhibit outstanding, below 10fs short term jitter relative to the X-rays.Timing tools for both the electron gun laser and for the experiments are developed.The former is based on electro-optical modulation of the optical reference at 1560nm by a signal produced from the gun laser at 260nm, a concept similar to beam arrival monitors in the linear accelerator, with an expected resolution below 20fs.The latter will use spectrally resolved cross-correlation technique to determine relative jitter between the optical reference and the laser used at the experiments at 800nm, with fs resolution.These systems will be complemented by electron and X-ray timing tools.In this paper we present the general concept for the laser arrival time measurement and correction, with first results obtained on a Ti:sapphire chirped pulse amplifier system.Shot to shot, short term jitter and long term timing drift measurements are presented, with discussion on the sources of the noise.Plans for the feedback stabilization and the resolution and limitation of the systems are also covered.

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.

To produce short X-ray pulses for SwissFEL(1), it is necessary to compress the electron bunches by a factor of 300, down to sub-fs for the attosecond operational modes. To achieve stable EEL output accurate timing of the initial electron injection at the main linear accelerator is necessary. Tolerance studies show, that to reach the final performance goals, less than 40 fs relative rms jitter is required from the electron gun, relative to the reference. Here we present independent residual RF phase noise measurements of the laser oscillators, showing an exceptional similar to 30 fs integrated rms jitter. Moreover timing studies at the SwissFEL Injector Test Facility, based on charge detection at the sharp rising edge of the Schottky-scan were performed allowing for systematic correlation studies and showing a residual jitter of similar to 150 fs at 10 Hz from the pulsed laser system and beam transport respect to the reference. For future development, available relative jitter measurement techniques for pulsed laser systems will be reviewed and their applicability for laser arrival time monitoring and feedback for EEL applications will be discussed. (C) 2013 Published by Elsevier B.V.

Octave-spanning, 12.5 fs, (1.9 cycle) pulses with 115 μJ energy in the short-wavelength mid-infrared spectral range (1-2.5 μm) have been generated via phase-mismatched cascaded nonlinear frequency conversion using organic DAST (4-N, N-dimethylamino-4'-N'-methylstilbazolium tosylate) crystal. Such ultrafast cascading effect is ensured by the interaction of a pump pulse with the exceptionally large effective nonlinearity of the DAST crystal and experiencing nonresonant, strongly phase-mismatched, Kerr-like negative nonlinearity.

This paper focuses on the investigation of detrimental UV induced effects during fourth harmonic generation (FHG) in BBO crystals. An accumulation of partly recoverable UV two-photon induced optical defects has been observed in the 140 s train generated by frequency quadrupled 1.5 GHz Nd:YLF laser with about 300 W UV power per train, even for a relatively low peak pulse intensity of about 100 MW/cm 2 . BBO crystals with length of 4.2, 8.5 and 12 mm have been tested. In a shorter crystal less distortion have been observed for the same UV output power. Although the growth of optical defects is attributed to the UV two-photon absorption, the already accumulated defects lead to a linear absorption of green and UV co-propagating pulses which in turn results in train envelope degradation due to deterioration of a phase-matching and thermal beam distortion. The UV beam exhibited focusing behavior on a short distance (crystal length) which cannot be explained by temperature dependence of BBO refractive indexes. The UV beam focusing is addressed as a strong photo-elastic effect, caused by the stress induced by essentially non-uniform temperature distribution inside the beam propagation volume.

To reach nominal bunch compression and FEL performance of SwissFEL with stable beam conditions for the users, less than 40fs relative rms jitter is required from the injector. Phase noise measurement of the gun laser oscillator shows an exceptional 30 fs integrated rms jitter. We present these measurements and analyze the contribution to the timing jitter and drift from the rest of the laser chain. These studies were performed at the SwissFEL Injector Test Facility, using the rising edge of the Schottky-scan curve and on the laser system using fast digital signal analyzer and photodiode, revealing a residual jitter of 150 fs at the cathode from the pulsed laser amplifier and beam transport, measured at 10Hz. Spectrally resolved cross-correlation technique will also be reviewed here as a future solution of measuring timing jitter at 100 Hz directly against the pulsed optical timing link with an expected resolution in the order of 50 fs. This device will provide the signal for feedback systems compensating for long term timing drift of the laser for the gun as well as for the pulsed lasers at the experimental stations.

The objective of the CLIC Test Facility CTF3 is to demonstrate the feasibility issues of the CLIC two-beam technology: the efficient generation of a very high current drive beam, used as the power source to accelerate the main beam to multi-TeV energies with gradients of over 100 MeV/m, and stable drive beam deceleration. Results of successful beam acceleration with over 100 MeV/m energy gain are shown. Measurements of drive beam deceleration over a chain of Power Extraction Structures (PETS) are presented. The achieved RF power levels, the stability of the power production and of the deceleration are discussed. Finally, we give an overview of the remaining issues to be addressed by the end of 2011. Presented at: 2nd International Particle Accelerator Conference, San Sebastian, Spain, 4 9 Sep 2011 Geneva, Switzerland Date 15/09/2011 CLIC – Note – 897

We report on a new concept for the gun laser system of the future hard and soft x-ray SASE FEL (SwissFEL) at the Paul Scherrer Institute and present first experimental verifications. The system consists of a hybrid Yb fiber and solid state Yb:CaF2 amplifier. The laser performance, such as energy stability, timing jitter, double pulse operation, temporal and spatial pulse shape of the ultra-violet laser pulses match the SwissFEL requirements. The mature and stable direct diode pumping technology and an optimized design allow for high reliability, long lifetime and lower maintenance cost compared to the widely used Ti:sapphire laser systems.

The SwissFEL compact accelerator design is based on extremely low emittance electron beam from an RF photoinjector. Proper temporal and spatial shaping of the photocathode drive laser is employed to reduce the space charge emittance contribution. However, the ultimate limit for the beam quality is the thermal emittance, which depends on the excess energy of the emitted photoelectrons. By varying the photocathode laser wavelength it is possible to reduce the thermal emittance. For this purpose, we applied a tunable Ti:sapphire laser and an optical parametric amplifier which allow to scan the wavelength between 250 and 305 nm. The system permits to study the thermal emittance and the quantum efficiency evolution as function of the laser wavelength for the copper photocathode in the RF gun of the SwissFEL injector test facility. The results are presented and discussed. INTRODUCTION AND MOTIVATIONS The Paul Scherrer Institute (PSI) is building an X-ray Free Electron Laser (FEL) user facility, which aims to deliver ultrashort coherent photon pulses with wavelengths ranging between 0.1 and 0.7 nm by the year 2017 [1]. For cost and space reasons the driving accelerator is foreseen with relatively modest final energy, thus calling for very low emittance. In preparation of SwissFEL, PSI is commissioning a 250 MeV photo-injector (SITF), which intends to demonstrate the generation of high-brightness electron beams and serves as a realistic test bed for crucial components for SwissFEL [2]. Modern linear accelerators demonstrated that it is feasible to preserve the electron beam emittance throughout acceleration. It becomes therefore important to generate the electron bunch at the source with the lowest possible emittance. Its growth due to the linear space charge forces is effectively counteracted by emittance compensation scheme. The photocathode drive lasers employ typically spatial and temporal pulse shaping in order to compensate the emittance dilution due to nonlinear space charge effect. Therefore the thermal or intrinsic emittance becomes a realistic limit for the beam quality. This parameter is a measure of the temperature of the electrons emitted from the cathode and it depends on the excess of energy of the photoelectrons in vacuum. Thermal emittance is function of the cathode material and surface quality, the accelerating electric field and laser wavelength. The value of the intrinsic emittance is linked directly to the quantum efficiency (QE) of the photocathode (number of emitted electrons per incident photons). In the presented work we characterize the intrinsic emittance and the QE in RF gun while varying the laser wavelength. Similar studies are reported for photocathodes in DC gun [3]. The intrinsic emittance, εin can be written as [4]: € ε in ω ( ) =σL ω −Φeff 3mc 2 (1) with ω the laser wavelength, ω the photon energy, σL the rms laser spot size and Φeff the effective work function of the copper cathode including the Schottky effect. The Schottky term accounts for the reduction of potential barrier due to the applied electric field on the cathode surface. The total emittance can be reduced by adapting the laser photon energy to the net work function of the cathode. A decrease in quantum efficiency (QE) is expected when the laser photon energy approaches the effective work function. The QE can be expressed as [4]: € QE ω ( )≈ K ⋅ ω −Φeff ( ) 2 (2) K takes into account the reflection of the laser at the cathode surface and the probability of the emission process. From equations 1 and 2 it is clear that lower εin can be obtained at the price of also lower QE. For the design of high brightness accelerator a trade-off between the maximum acceptable intrinsic emittance and the quantum efficiency need to be established. The lower QE calls for higher energy and more complex drive laser with consequent worsening of system stability and ability control the photon beam tridimensional shape.