This PDF file contains the front matter associated with SPIE Proceedings Volume 8778, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Contrary to nonlinear harmonic generation, harmonic lasing in a high-gain FEL can provide much more intense, stable,and narrow-band FEL beam which is easier to handle if the fundamental is suppressed. We performed thorough study of the problem withing framework of 3D model taking into account all essential effects. We found that harmonic lasing is much more robust than usually thought, and can be widely used in the existing or planned X-ray FEL facilities. LCLS after a minor modification can lase at the 3rd harmonic up to the photon energy of 25-30 keV providing multi-gigawatt power level. At the European XFEL the harmonic lasing would allow to extend operating range ultimately up to 100 keV.

The development of Free Electron Laser sources is opening up the possibility to probe dynamics at the femtosecondnanometer time-length scales. A remarkable step forward towards this goal would be achieved by extending the Four Wave Mixing (FWM) approach at VUV/soft x-ray wavelengths. FWM-based techniques allow a coherent control in both the stimulating and probing processes of photon-induced excitations. We propose to exploit the FERMI@Elettra seeded Free Electron Laser (FEL) source to put on practice the VUV/soft x-ray FWM approach, yet theoretically conceived one decade ago. Moreover, the exploitation of VUV/soft x-ray wavelengths allows adding site-sensitivity to FWM methods by exploiting core resonances of selected elements in the sample.

At the soft X-ray free electron laser FLASH, multiphoton ionization of free atoms has been studied by ion time-of-flight spectroscopy. Depending on the multiphoton mechanism, the ionization processes are influenced in different ways by the FEL pulse duration. This feature has been used, e.g., to measure the pulse duration of FLASH in the femtosecond regime by non-linear autocorrelation. In the present contribution, the impact of pulse duration on multiphoton ionization is discussed with an emphasis on the distinction between sequential and non-sequential processes, and collective electron excitation as well.

The recent success of the X-ray Free Electron Lasers has generated great interests from the user communities of a wide range of scientific disciplines including physics, chemistry, structural biology and material science, creating tremendous demand on FEL beamtime access. Due to the serial nature of FEL operation, various beam-sharing techniques have been investigated in order to potentially increase the FEL beamtime capacity. Here we report the recent development in using thin diamond single crystals for spectrally splitting the FEL beam at the Linac Coherent Light Source, thus potentially allowing the simultaneous operation of multiple instruments. Experimental findings in crystal mounting and its thermal performance, position and pointing stabilities of the reflected beam, and impact of the crystal on the FEL transmitted beam profile are presented.

We present the design, realization and characterization of active deformable gratings for extreme-ultraviolet monochromators for ultrashort pulses. The core device consists of a bimorph deformable mirror on the top of which a diffraction grating with laminar profile is realized by UV lithography. The curvature radius of the grating substrate can be varied changing the voltage applied to an underlying piezo-actuator. The advantage of this technology is to provide gratings with high optical quality, robust, compatible with any coating deposition and realized with only vacuumcompatible materials. We present the characterization of a time-delay compensated monochromator realized with these devices, showing that the active grating can optimize the beam focusing through its rotation and deformation. Two equal active gratings have been mounted in a compensated configuration to realize a grazing-incidence double-grating monochromator for the spectral selection of ultrashort pulses and the simultaneous compensation of the pulse front-tilt given by the diffraction. The wavelength scanning is performed by the first grating through rotation. The radiation is focused on the intermediate plane, where a slit carries out the spectral selection. Finally, the second grating compensates for the pulse front-tilt given by the first one. The spectral focusing of both gratings is maintained at the different wavelengths through the variation of the radii of curvature. The instrument has been tested with a Ti:Sa laser operated at 800 nm. We have been able to demonstrate that the double-grating configuration with active gratings compensates for the pulse front-tilt, that is reduced from 1 ps at the intermediate plane to 100 fs at the output. The final value is limited by the group delay dispersion of the monochromator within the 10-nm bandwidth of the laser. A configuration for the selection on XUV ultrashort pulses has been theoretically studied and the expected performances presented. Active gratings may be considered as a cheaper and more flexible alternative to standard gratings for the realization of extremeultraviolet monochromators for ultrafast pulses, such as free-electron lasers and high-order laser harmonics.

A hard x-ray free-electron laser (XFEL) provides an x-ray source with an extraordinary high peak-brilliance, a time structure with extremely short pulses and with a large degree of coherence, opening the door to new scientific fields. Many XFEL experiments require the x-ray beam to be focused to nanometer dimensions or, at least, benefit from such a focused beam. A detailed knowledge about the illuminating beam helps to interpret the measurements or is even inevitable to make full use of the focused beam. In this paper we report on focusing an XFEL beam to a transverse size of 125nm and how we applied ptychographic imaging to measure the complex wavefield in the focal plane in terms of phase and amplitude. Propagating the wavefield back and forth we are able to reconstruct the full caustic of the beam, revealing aberrations of the nano-focusing optic. By this method we not only obtain the averaged illumination but also the wavefield of individual XFEL pulses.

Free-electron lasers deliver EUV and soft x-ray pulses with the highest brilliance available and high spatial coherence. Users of such facilities have high demands on the coherence properties of the beam, for instance when working with coherent di ractive imaging (CDI). Experimentally, we are recovering the phase distribition with an EUV Hartmann wavefront sensor. This allows for online adjustment of focusing optics such as ellipsoidal or Kirkpatrick-Baez mirrors minimizing the aberrations in the focused beam. To gain highly resolved spatial coherence information, we have performed a caustic scan at beamline BL2 of the free-electron laser FLASH using the ellipsoidal focusing mirror and a movable EUV sensitized CCD detector. This measurement allows for retrieving the Wigner distribution function, being the two-dimensional Fourier transform of the mutual intensity of the beam. Computing the reconstruction on a four-dimensional grid, this yields the entire Wigner distribution which describes the beam propagation completely. Hence, we are able to provide comprehensive information about spatial coherence properties of the FLASH beam including the global degree of coherence. Additionally, we derive the beam propagation parameters such as Rayleigh length, waist diameter and M².

The Linac Coherent Light Source (LCLS) as the world’s first hard x-ray free electron laser in the range of 250 eV to 10 keV at its fundamental wavelength has been operated as a user facility since 2009 with six experimental stations and an increasing range of x-ray beam parameters now available to users. Various aspects of operating the LCLS accelerator to deliver the necessary electron beam in terms of bunch charge, energy, and length for the wide parameter range and different FEL operating modes will be discussed. An emphasis will be on the electron beam diagnostics that are most critical for generating the desired x-ray beam properties. Measurements of electron beam energy, energy loss, and transverse orbit will be shown as well as bunch duration and shape measurements.

The required high peak current in free-electron lasers (FELs) is realized by longitudinal compression of the electron bunches to sub-picosecond length. A novel in-vacuum polychromator (CRISP4) has been developed for measuring coherent radiation in the THz and infrared range. The polychromator is equipped with five consecutive dispersion gratings and 120 parallel readout channels. It can be operated either in short (5-44 μm) or in long wavelength mode (45-430 μm). Fast parallel readout permits the monitoring of coherent radiation from single electron bunches. Due to the large wavelength range covered and the absolute calibration of the device, Kramers-Kronig based phase retrieval allows to online reconstruct a longitudinal bunch profile from the measured coherent radiation spectrum. The device is used as a bunch length monitoring and tuning tool during routine operation at the Free-electron Laser in Hamburg (FLASH). Comparative measurements with the transverse deflecting structure show excellent agreement of both methods.

Modern VUV and X-ray Free Electron Laser (FEL) facilities contain a number of ultrafast lasers (like photoinjector, seed and pump-probe lasers) whose performance is crucial for the generated FEL light quality as well as for the accuracy of the time resolved measurements performed using the FEL pulses. One of the very important laser related aspects, especially at seeded FELs, is the ability to precisely lock the ultrafast laser systems to the master clock signal, keeping the timing jitter and drifts of the generated pulses with respect to the machine timing as low as possible. The aim of this work is to review the main sources of timing jitter and drifts and present the schemes and solutions developed at FERMI for their characterization and compensation. The paper will first introduce a general scheme showing the architecture of the laser locking system developed for FERMI. Both the radio-frequency (RF) locking and the advanced balanced optical cross correlator electronics and optical setup design are described, together with data on the laser oscillator locking performance obtained in different modalities. Cross correlation measurements indicating the contribution of the ultrafast regenerative amplifier and optical beam transport part to the overall temporal jitter of the amplified ultrashort pulses arriving at destination are presented. The paper also includes examples of the influence of improved laser timing jitter and drifts on the seeded FEL performance and discusses foreseen future developments.

In this paper, we present the long-term stable synchronization of the FLASH pump-probe Ti:sapphire oscillator to an optical reference with sub-10 fs (rms) timing jitter employing a balanced optical cross-correlator. The reference pulse train, transmitted over an actively transit time-stabilized 500m long fiber link, is generated by the FLASH master laser oscillator. This laser also provides the reference for several electron bunch arrival time monitors with sub-10 fs resolution, which in turn enables a longitudinal feedback reducing the electron bunch arrival time jitter to below 25 fs (rms). Combining the precise synchronization of the laser and the longitudinal accelerator feedback enabled a proof-of-principle pump-probe experiment at FLASH, ultimately showing a significant reduction of the timing jitter between the optical laser and the XUV pulses generated by the FEL, compared to the present standard operation.

The development of Free Electron Lasers has opened the possibility to investigate ultrafast processes using femtosecond hard x-ray pulses. In optical/x-ray light pump/probe experiments, however, the time resolution is mainly limited by the ability to synchronize both light sources over a long distance (<100 fs FWHM) rather than their pulse length (<10 fs FWHM). We have implemented a spectrally encoding x-ray to optical laser timing diagnostic into the XPP beamline at LCLS with a timing uncertainty down to 10 fs. An x-ray induced change of refractive index in a solid target is temporally probed for single pulses by a chirped white light pulse [4]. By resorting single shot data to the timestamps obtained by the diagnostics, the temporal data quality can be improved to basically pulse length limited time resolution. By interchangable targets and adjustable x-ray and laser foci, the method was successfully applied for very different x-ray parameters. These are different photon energies in the range of 6-20 keV, which at LCLS also includes application of 3rd Harmonic radiation, pulse energy, and bandwidth, when using a Si(111) monochromator.

Free-electron lasers offer a variety of unique properties for spectroscopy and imaging. The combination of high peak brilliance and a high repetition rate opens a window to experiments that have not been feasible so far but also introduces challenges in sample preparation and refreshment. First experiments at the Linac Coherent Light Source (LCLS) in Stanford showed the potential of free electron lasers for serial X-ray crystallography as well as for imaging non-reproducible objects. Owing to the superconducting accelerator technology, the European X-ray Free-Electron Laser Facility (European XFEL) will allow an average repetition rate of up to 27 kHz with bunch separation in the order of 200 ns. This extremely high repetition rate gives great chances for the scientific impact of the European XFEL, but it also comes with challenges for providing fresh samples for each bunch. This contribution will give an overview of the sample environment techniques that are in consideration for the European XFEL Facility. These techniques include gas phase, liquid, and aerosol sources for life science and physics experiments.

The European X-ray Free Electron Laser (XFEL.EU) will provide as-yet-unrivaled peak brilliance and ultrashort pulses of spatially coherent X-rays with a pulse length of less than 100 fs in the energy range between 0.25 and 25 keV. The high radiation intensity and ultra-short pulse duration will open a window for novel scientific techniques and will allow to explore new phenomena in biology, chemistry, material science, as well as matter at high energy density, atomic, ion and molecular physics. The variety of scientific applications and especially the unique XFEL.EU time structure require adequate instrumentation to be developed in order to exploit the full potential of the light source. To make optimal use of the unprecedented capabilities of the European XFEL and master these vast technological challenges, the European XFEL GmbH has started a detector R and D program. The technology concepts of the detector system presently under development are complementary in their performance and will cover the requirements of a large fraction of the scientific applications envisaged for the XFEL.EU facility. The actual status of the detector development projects which includes ultra-fast 2D imaging detectors, low repetition rate 2D detectors as well as strip detectors for e.g. spectroscopy applications and the infrastructure for the detectors’ calibration and tests will be presented. Furthermore, an overview of the forthcoming implementation phase of the European XFEL in terms of detector R and D will be given.

The European XFEL will generate extremely brilliant pulses of X-rays organized in pulse trains consisting of 2700 pulses <100 fs long, with <10¹² photons, and with a 220 ns spacing. The pulse trains are running at a 10Hz repetition rate. The detector to be used under these conditions will have to face several challenges: the dynamic range has to cover the detection of single photons and extend up to <104 photons/pixel/pulse in the same image, framing rates of 4.5 MHz (220 ns) are required in order to record one image per pulse, and as many images as possible have to be recorded during the pulse trains. Due to the high flux, the detector will have to withstand a dose up to 1GGy integrated over 3 years. To meet these challenges a consortium, consisting of Deutsches Elektronensynchrotron (DESY), Paul-Scherrer-Institut (PSI), University of Hamburg and University of Bonn, is developing the Adaptive Gain Integrating Pixel Detector (AGIPD). It is a hybrid-pixel detector, featuring a charge integrating amplifier with dynamic gain switching to cope with the extended dynamic range, and an analogue on-pixel memory for image storage at the required 4.5 MHz frame rate. The readout chip consists of 64×64 pixels of (200μm)², 8×2 of these readout chips are bump-bonded to a monolithic silicon sensor to form the basic module with 512 × 128 pixels. 4 of these modules are stacked to form a quadrant of the 1k ×1k detector system. Each quadrant is independently moveable in order to adjust a central hole, needed for the direct beam to pass through. Special designs are employed for both the sensor and the readout chip to withstand the integrated dose for 3 years.

We present the design and the characteristics of a portable and compact photon spectrometer to be installed in freeelectron- laser (FEL) beamlines for photon in – photon out experiments, in particular single-shot X-ray emission spectroscopy. The instrument is operated in the 30 – 800 eV energy range with two channels and is designed to be initially used in the LDM (Low-Density Matter) and TIMEX (TIme-resolved studies of Matter under EXtreme conditions) beamlines of the FERMI@ELETTRA FEL, covering the whole spectral range of FERMI-1 and FERMI-2 emissions. The design of the instrument is tailored to achieve high spectral resolution in the whole interval of operation, high acceptance angle and high dynamic range. These characteristics are achieved in a compact environment to give a portable instrument that may be easily installed in different experimental chambers. The design consists of an entrance slit, a grazing-incidence diffraction grating and a detector. The number of elements within the optical path is kept to a single component, to minimize the losses due to reflectivity. The grating is spherical with variable line spacing along its surface, to provide an almost flat spectral focal plane that is perpendicular to the direction of the diffracted light. The detector is a back-illuminated CCD. The spectral resolution is better than 0.2% in the 30 – 800 eV region and the acceptance angles are 10 × 17 mrad in the 30-250 eV and 5 × 17 mrad in the at 250-800 eV.

Beam parameters of the free-electron laser FLASH @13.5 nm in two different operation modes were determined from beam profile measurements and subsequent reconstruction of the Wigner distribution function behind the ellipsoidal focusing mirror at beamline BL2. 40 two-dimensional single pulse intensity distributions were recorded at each of 65 axial positions around the waist of the FEL beam with a magnifying EUV sensitized CCD camera. From these beam profile data the Wigner distribution function based on different levels of averaging could be reconstructed by an inverse Radon transform. For separable beams this yields the complete Wigner distribution, and for beams with zero twist the information is still sufficient for wavefront determination and beam propagation through stigmatic systems. The obtained results are compared to wavefront reconstructions based on the transport of intensity equation. A future setup for Wigner distribution measurements of general beams is discussed.

We present a concept of an accelerator based source of powerful, coherent IR/THz radiation for pump-probe experiments at the European XFEL. The electron accelerator is similar to that operating at the PITZ facility. It consists of an rf gun and a warm accelerating section (energy up to 30 MeV). The radiation is generated in an APPLE-II type undulator, thus providing polarization control. Radiation with wavelength below 200 micrometers is generated using the mechanism of SASE FEL. Powerful coherent radiation with wavelength above 200 micrometers is generated in the undulator by a tailored (compressed) electron beam. Properties of the radiation are: wavelength range is 10 to 1000 micrometers (30 THz - 0.3 THz), radiation pulse energy is up to a few hundred microjoule, peak power is 10 to 100 MW, spectrum bandwidth is 2 - 3 %. Pump-probe experiments involving ultrashort electron pulses can be realized as well. The time structure of the THz source and x-ray FEL are perfectly matched since the THz source is based on the same technology as the injector of the European XFEL. A similar scheme can also be realized at LCLS, SACLA, or SWISS FEL with S-band rf accelerator technology.

The design of optical systems for micro-focusing of extreme-ultraviolet (XUV) and soft X-ray pulses through grazingincidence toroidal mirrors is presented. Aim of the configuration here presented is to provide a micro-focused image through a high demagnification of the source with almost negligible aberrations and a long exit arm to accommodate at the output a large experimental chamber. We present the analytical and numerical study of two configurations to fulfill these requirements with two toroidal mirrors. The first mirror provides a demagnified image of the source in the intermediate plane that is free from defocusing but has a large coma aberration, the second mirror is mounted in Z-shaped geometry with respect to the previous one, in order to give a stigmatic image with a coma that is opposite to that provided by the first one. Some examples are provided to demonstrate the capability to achieve spot sizes in the 5-15 μm range both applied to high-order laser harmonics and free-electron-laser radiation.

We report on online measurements of photon beam parameters during mirror alignment in the soft x-ray spectral region of FLASH, the free-electron laser in Hamburg. A compact Hartmann sensor operating in the wavelength range from 6 to 35nm was used to determine the wavefront quality of individual free-electron laser (FEL) pulses during the alignment procedure as well as aberrations. Beam characterization and alignment of beamline BL3 was performed with λ_13.5𝑛𝑚/ 116 accuracy for wavefront rms (W_𝑟𝑚𝑠). Second moment beam parameters are computed using a spherical reference wavefront generated by a 5μm pinhole. The Hartmann sensor was used for alignment of the ellipsoidal focusing mirror of beamline BL3, resulting in a reduction of (W_𝑟𝑚𝑠) by 33%.

Effect of the parametric beam instability (PBI) of the relativistic electron beam in a crystal was analyzed theoretically in¹ (see also²). This effect is analogous to self-amplification of spontaneous emission (SASE) mechanism for X-ray Free Electron Laser (XFEL) with an undulator. However, in the case of PBI the transversal modulation of the beam is defined by channeling of the electron in a crystal and the longitudinal modulation arises because of the parametric X-ray radiation mechanism. It is shown in the present paper that the current density J in the electron bunch typical for FEL facility is enough for the beam self-modulation within the X-ray range if the crystal thickness is larger than the crystal absorption length L ≥ L_abs. This process could be used for generation of the coherent X-ray pulses if the time τ_d of the crystal destruction affected by the electron bunch is less than its passing time τ_d < L=c. The value τ_d (J) is estimated for the case of the FLASH electron bunch propagating through a Si crystal.

FLASH2 is a major extention to the soft X-ray free-electron laser FLASH at DESY. An additional variable-gap undulator line in a new separate tunnel and a new experimental hall will turn FLASH into a multi-beamline FEL user facility. Years of experience as single user facility have high impact on the planned photon diagnostics. Online measurements of intensity, position, wavelength, wavefront, and pulse length are optimized as well as photon beam manipulation tools such as a gas absorber and filters. The beamline system will be set up to cover a wide wavelength range with beamlines capable to deliver down to 0.8 nm in the 5th harmonic and 1st harmonics in the water window to cover the user community's high intrest in this wavelength range. In addition, other beamlines will cover the longer wavelengths from 6 nm - 40 nm and beyond. Proven concepts like the optical laser pump-and-probe instrument are taken over from the current operation scheme in an established way. Permanent endstations with specialized beamline layouts are foreseen. Civil construction and installations in the new FLASH2 tunnel are on-going, first beam is expected for end of 2013, and a first user experiment is anticipated for summer 2014.

The Free Electron Laser FLASH at the German Electron Synchrotron (DESY) in Hamburg is a linear accelerator, which uses superconducting technology to produce soft x-ray laser light ranging from 4,1 to 45 nm. To ensure the operation stability of FLASH, monitoring of the beam is mandatory. Among various detectors located at the beam pipe, two Ionization Profile Monitors (IPM) detect the lateral x and y position changes. The functional principle of the IPM is based on the detection of electrons, generated by interaction of the photon beam with the residual gas in the beam line. The newly designed IPM enables the combined determination of the FEL’s horizontal and vertical position as well as the beam’s profile. This is made possible by a compact monitor, consisting of a cage in a vacuum chamber, two microchannel plates (MCP) and two structural repeller plates with toggled electric fields at the opposite sides of the MCPs. The electrons created by the FEL beam, drift in a homogenous electrical field towards the respective micro-channel plate, which produces an image of the beam profile on an attached phosphor screen. A CCD camera for each MCP in combination with a computer is used for the evaluation. This indirect detection scheme operates over a wide dynamic range and allows the detection of the center of gravity and the shape of the photon beam without affecting the FEL beam. Exact knowledge of the path taken by the electrons permits a recursive determination of the beam position. Within a beam variance of less than 10 mm, an accuracy better than ±8 um seems to be possible.

After the generation of the laser light, a dipole deflects the highly energetic electron beam of FLASH (Free Electron Laser Hamburg) into a dump. A detector is developed to monitor the position, dimensions and profile of the electron beam. Scintillation light is emitted due to the electrons hitting a luminescent screen located in front of the dump aperture. This light is guided by an optical system external to the vacuum to a CCD camera for optical analysis of the generated image. In this paper the layouts of two different optical systems are presented, both of which will be redundantly installed at FLASH II. The conventional lens-mirror-arrangement, consisting of three single collecting lenses, two mirrors and a zoom lens, is supposed to have a theoretical resolution of 0.25 mm. The second optical system is based on radiation-hard optical fibres. For the latter it is planned to test the impact of radiation on the optical qualities of the bundle by installing it into a “radioactive hot spot” at the bunch compressor in the FLASH accelerator. This test setup will also be presented.

For the European XFEL [1] an x-ray split- and delay-unit (SDU) is built covering photon energies from 5 keV up to 20 keV [2]. This SDU will enable time-resolved x-ray pump / x-ray probe experiments as well as sequential diffractive imaging [3] on a femtosecond to picosecond time scale. Further, direct measurements of the temporal coherence properties will be possible by making use of a linear autocorrelation. The set-up is based on geometric wavefront beam splitting, which has successfully been implemented at an autocorrelator at FLASH [4]. The x-ray FEL pulses will be split by a sharp edge of a silicon mirror coated with Mo/B₄C multi layers. Both partial beams will then pass variable delay lines. For different wavelengths the angle of incidence onto the multilayer mirrors will be adjusted in order to match the Bragg condition. For a photon energy of hν = 20 keV a grazing angle of θ = 0.57° has to be set, which results in a footprint of the beam (6σ) on the mirror of l = 120 mm. At this photon energy the reflectance of a Mo/B₄C multi layer coating with a multi layer period of d = 3.2 nm and N = 200 layers amounts to R = 0.92. In order to enhance the maximum transmission for photon energies of hν = 8 keV and below, a Ni/B4C multilayer coating can be applied beside the Mo/B₄C coating for this spectral region. Because of the different incidence angles, the path lengths of the beams will differ as a function of wavelength. Hence, maximum delays between +/- 2.5 ps at hν 20 keV and up to +/- 23 ps at hν = 5 keV will be possible.