The Linac Coherent Light Source (LCLS) Project will be an x-ray free-electron laser. It is intended to produce pulses of 800-8,000 eV photons. Each pulse, produced with a repetition frequency of up to 120 Hz, will provide >10¹² photons within a duration of less than 200 femtoseconds. The project employs the last kilometer of the SLAC linac to provide a low-emittance electron beam in the energy range 4-14 GeV to a single undulator. Two experiment halls, located 100m and 350m from the undulator exit, will house six experiment stations for research in atomic/molecular physics, pump-probe dynamics of materials and chemical processes, x-ray imaging of clusters and complex molecules, and plasma physics. Engineering design activities began in 2003, and the project is to be completed in March 2009. The project design permits straightforward expansion of the LCLS to multiple undulators.

The Berliner Elektronenspeicherring-Gesellschaft fuer Synchrotronstrahlung (BESSY) proposes a 2^nd generation FEL Soft X-ray user facility (BESSY FEL) in addition to the currently operating 3^rd generation light source BESSY II. The BESSY FEL will deliver photon pulses in the wavelength range of 51nm ≤ λ ≤ 1.2nm with pulse durations of a few tens of femtoseconds. To produce these short pulses and to ensure a high shot-to-shot stability a cascaded HGHG FEL scheme is proposed, where the wavelength and pulse duration is determined by an external seeding laser. It is expected to achieve a peak-brilliance of 10³¹ photons/(s mm² mrad² 0.1%BW), with GW peak power over the full spectral range. The main components of the BESSY FEL are a photo-cathode injector, a superconducting 2.3 GeV CW-Linac and three independently operating HGHG FEL sections, serving a total of 9 beamlines. A status report is given, including a detailed description of the layout and of the numerical simulations, performed to optimize the machine.

The French project of a fourth generation light source, ARC-EN-CIEL (Accelerator-Radiation for Enhanced Coherent Intense Extended Light), is a unique facility providing the user community with coherent femtosecond light pulses covering the UV, VUV and soft X ray spectral range. It is based on a CW 1 GeV superconducting linear accelerator delivering high charge, subpicosecond, low emittance electron bunches with high repetition rate (1 kHz), and adjustable polarisation until 1 keV. In addition to the High Gain Harmonic Generation (HGHG) experiment seeded with High Harmonics in Gases (HHG), allowing radiation down to 0.8 nm to be produced, two beam loops are foreseen to increase the beam current in using the energy recovery technique. They will accommodate fs synchrotron radiation sources in the IR, VUV and X ray ranges together with a FEL oscillator in the 10 nm range. Moreover, an important synergy is expected between accelerator and laser communities. Indeed, electron plasma acceleration will be tested and hard X ray femtosecond radiations will be produced by Thomson Scattering. The first phase of the project, ARC-EN-CIEL phase 1, is now under study. A general overview will be given.

The unusually long insertion devices being prepared for Ångstrom-wavelength Free Electron Lasers (FELs) will generate spectral-angular distributions in the proposed experimental areas that are substantially different from those conventionally calculated for the far field. In this paper we provide a general overview of near-field effects and report on initial computational simulations of near vs. far field distributions for the SLAC linac Coherent Light Source (LCLS) undulator, an insertion device approximately 140 meters long. The properties of the coherent radiation as a limiting case of the near-field emission, for the special condition of a microbunched beam radiating along the undulator axis, are reviewed.

Over the past few decades, scientists have focused their attention on the development of concepts and designs, leading to demonstrations, of unique x-ray sources to perform femtosecond and attosecond science. The rewards of such an effort in the x-ray wavelength range will revolutionize the subfields of atomic physics, molecular physics, biology, condensed matter physics and material science. A brief review of this subject and its impact on emerging areas of science will be presented. The storage-ring-based synchrotron radiation sources are today's workhorses in providing both time-averaged and time-resolved structural and chemical information with subnanosecond to subsecond resolution using x-ray imaging, spectroscopy and scattering techniques. On the other hand, many phenomena are ultrafast with characteristic periods of a few femtoseconds to tens of picoseconds. These include electronic motions around a nucleus in an atom, atomic and molecular vibrational motions in matter, spin dynamics, chemical and biological reactions, and phase transitions in response to photoexcitation. Probing such phenomena using photon-excited pump-probe experiments will require both optical and x-ray sources with comparable resolution. In the future, sources based on atypical concepts in storage-rings, table-top plasma sources, laser-based high harmonic generation (HHG) sources, linac-based sources, such as energy-recovery linacs (ERLs) and x-ray free-electron lasers (FELs), will likely meet these demands.

It is well-known that the loss of phase information at detection means that a diffraction pattern may be consistent with a multitude of physically different structures. This paper shows that it is possible to perform unique structural determination in the absence of a-priori information using x-ray fields with phase curvature. We argue that significant phase curvature is already available using modern x-ray optics and we demonstrate an algorithm that allows the phase to be recovered uniquely and reliably.

4GLS is a suite of accelerator-based light sources planned to provide state-of-the-art radiation in the low energy photon regime. Superconducting energy recovery linac (ERL) technology will be utilised in combination with a variety of free electron lasers (IR to XUV), undulators and bending magnets. The 4GLS undulators will generate spontaneous high flux, high brightness radiation, of variable polarisation from 3 - 800 eV, optimised in the lower harmonics up to about 200 eV. Viable radiation at energies up to several keV may be provided from multipole wiggler magnet radiation. The ERL technology of 4GLS will allow shorter bunches and higher peak photon fluxes than possible from storage ring sources. It will also give users the added bonuses of pulse structure flexibility and effectively an infinite beam lifetime. VUV and XUV FELs will be used to generate short pulses (in the fs regime) of extreme ultraviolet light that is broadly tuneable and more than a million times more intense than the equivalent spontaneous undulator radiation. A strong feature of the scientific programme planned for 4GLS is dynamics experiments in a wide range of fields. Pump probe experiments will allow the study of chemical reactions and short-lived intermediates on the timescale of bond breaking and bond making, even for very dilute species. The high intensity of the FEL radiation will allow very high resolution in imaging applications. Funding for the first three years of the 4GLS project was announced by the UK Government in April 2003. This includes the research and development work necessary to produce a design study report, with the construction of an ERL-prototype. Additional funds have recently been awarded that will enable a study of the production of ultra-short pulsed X-rays from the ERL-prototype via Thomson scattering. It is anticipated that the full 4GLS facility will be available to users in 2011.

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements demonstrate that the initial stage of crystal disordering results from inertial motion on a laser softened potential energy surface. These inertial dynamics dominate for the first half picosecond following laser excitation, indicating that inter-atomic forces minimally influence atomic excursions from the equilibrium lattice positions, even for motions in excess of an Å. This also indicates that the atoms disorder initially without losing memory of their lattice reference.

The multi-mJ, 21-nm soft-x-ray laser at the PALS facility was focused on the surface of amorphous carbon (a-C) coating, developed for heavily loaded XUV/x-ray optical elements. AFM (Atomic Force Microscopy) images show 3-micrometer expansion of the irradiated material. Raman spectra, measured with an Ar⁺ laser microbeam in both irradiated and unirradiated areas, confirm a high degree of graphitization in the irradiated layer. In addition to this highfluence (~ 1 J/cm²), single-shot experiment, it was necessary to carry out an experiment to investigate consequences of prolonged XUV irradiation at relatively low fluence. High-order harmonic (HH) beam generated at the LUCA facility in CEA/Saclay Research Center was used as a source of short-wavelength radiation delivering high-energy photons on the surface at a low single-shot fluence but with high-average power. a-C irradiated at a low fluence, i.e., < 0.1 mJ/cm² by many HH shots exhibits an expansion for several nanometers. Although it is less dramatic change of surface morphology than that due to single-hot x-ray-laser exposure even the observed nanometer-sized changes caused by the HH beam on a-C surface could influence reflectivity of a grazing incidence optical element. These results seem to be important for estimating damages to the surfaces of highly irradiated optical elements developed for guiding and focusing the ultraintense XUV/x-ray beams provided by new generation sources (i.e., VUV FEL and XFEL in Hamburg; LCLS in Stanford) because, up to now, only melting and vaporization, but not graphitization, have been taken into account.

Nowadays the sources of the X-rays based on a storage ring with low beam energy and Compton scattering of intense laser beam are under development in several laboratories. In the paper the state-of-art in development and construction of cooperative project of a Kharkov advanced X-ray source NESTOR based on electron storage ring with beam energy 40 - 225 MeV and Nd:YAG laser is described. The layout of the facility is presented and main results and constructing timetable are described. The designed lattice includes 4 dipole magnets with combined focusing functions, 20 quadrupole magnets and 19 sextupoles with octupole component of magnetic field. At the present time a set of quadrupole magnet is under manufacturing and bending magnet reconstruction is going on. The main parameters of developed vacuum system providing residual gas pressure in the storage ring vacuum chamber up to 10^-9 torr are presented along with testing measurement at NSC KIPT vacuum bench. The basic parameters of the X-rays source system such as laser system, diagnostic system, injection system are presented. The facility is going to be in operation in the middle of 2006 and generated X-rays flux is expected to be of about 10¹³ phot/s.

The results of theoretical and numerical considerations of linear Compton scattering are used to evaluate characteristics of X-rays produced by collision between a low emittance electron beam and intensive laser light in an X-rays generator NESTOR of NSC KIPT. Two main generation modes have been under consideration at preliminary NESTOR design. There are the operation mode for medicine 33.4 keV X-rays production using 43 MeV electron beam and Nd:YAG laser beam and higher energy X-rays production mode providing X-rays with energy up to 900 keV with 225 MeV electron beam and Nd:YAG laser beam. It is supposed to use an optical cavity for laser beam accumulation of about 2.6 m long and an interaction angle of about 3° in both operation modes. A few more operation modes provide ossibility to expand operation range of NESTOR. Using interaction angle 10° and 150° along with optical resonator 42 or 21 cm long and the second mode of laser light it is possible to produce X-rays in energy range from a few keV till 1.5 MeV. The intensity and spectral brightness of the X-rays is expected to be ~ 1013 phot/s and ~ 10¹³ phot/s/mm²/mrad²/0.01%BW respectively.

We present measurements of x-ray emission from relativistic electrons passing through crystals and multilayer nanostructures mounted inside betatrons. Both spectra and yields have been measured. The measured spatial distributions and orientation dependencies are presented and are found to be in good agreement with theory. Betatrons developed over the past 30 years in Russia are compact and relatively inexpensive compared to LINACs and Storage Rings, and thus can be used in small laboratory settings. Various thin novel radiators mounted inside the betatron toroid can be used to generate (1) monochromatic tunable x-rays from crystalline and multilayer targets (2) tailored gamma-ray emission spectra from single thin foils and (3) soft x-ray spectra from multiple thin foils (transition radiation). Although betatrons have relatively low current, the thin radiators permit the multipassing of the electron beam for increased efficiency.

A calculation was carried out to evaluate the capabilities of Secondary X-ray Imaging (SXI), applied to human angiography. A primary photon pencil beam is rastered through the human heart, in two directions perpendicular to the primary photon beam. The signal is generated by fluorescent photons from a contrast agent, registered by a wide angle detector. One result is clearer images and a reduction of shadowing by obstructions inside the body. Sharp imaging is compatible with locally quantitative measurements, and also with pixel by pixel elemental analysis. The detector need not be position sensitive. Most of the primary beam will be scattered before they reach the target, but unscattered primary beam remains well focused. To discriminate against scattered background, the photons have to pass through a position/momentum selector, a W - Hf absorber shield, and a time window. The calcualation gives the approximate energy spectrum for the scattered photons, for the photons passing through the position/momentum selector, and for those at thefar side of the absorber shield. The last two are evaluated for time windows between 1000 and 167 ps. The surviving background causes relative image intensity fluctuations of the order of a percent. The primary beam intensity required for SXI is comparable or less than the intensity needed for Iodine K-edge subtraction (KES) imaging, but for SXI the primary photon energy spread may be one or two orders higher than what is needed for KES. Therefore, the requirements on the primary photon source an be relaxed. With an undulator as source, monochromatization may not be needed. That would further reduce the cost of the photon source, which may be a small low energy electron ring.

4th Generation Light Sources will have brilliance performances which will exceed those of the 3rd Generation Light Sources by 10 orders of magnitude in peak value. 3rd Generation Light Sources were based on storage ring. Those sources had improved the quality of the X-Ray produced with respect to the 2^nd generation of machines by reducing the emittance and by increasing the current of the stored beam. The great stability and small emittance were intrinsically obtained by the damping produced by the radiation emission itself. In 4th generation sources, the X-Ray source quality (brilliance, peak brilliance, coherence) will directly depend on the quality of the injected beam (emittance, peak current, average current). Also, its stability will primarily depend on that of their injector. 4^th generation sources include both X-Ray FELs and ERL based sources. The technological challenges of injectors for X-Ray FELs include small emittances, high peak current, high stability and reliability. ERL based sources aim at the same type of performances, but in addition average current as high as those of the 3^rd generation light sources is desired. The focus of this review will be on the technological challenges of X-Ray FELs sources but for solutions proposed by the ERL injector community which could benefit X-Ray FELs sources. Photoinjectors are the primary source choice for many of the X-Ray FELs under design and construction. Critical issues for this technology include optimum laser pulse shaping and high quality of emission from the photocathode. Beam performances obtained from photoInjectors up to date just fulfill the requirements of X-Ray FEL drivers, but adequate stability and reliability remain to be demonstrated. Alternate technologies to X-Ray FEL sources will also be briefly discussed.

Energy Recovering Linacs (ERLs) offer an attractive alternative as drivers for light sources as they combine the desirable characteristics of both storage rings (high efficiency) and linear accelerators (superior beam quality). Using superconducting RF technology allows ERLs to operate more efficiently because of the inherent characteristics of SRF linacs, namely that they are high gradient-low impedance structures and their ability to operate in the long pulse or CW regime. We present an overview of the physics challenges encountered in the design and operation of ERL based light sources with particular emphasis on those issues related to SRF technology. These challenges include maximizing a cavity's Q_o to increase cryogenic efficiency, maintaining control of the cavity field in the presence of the highest feasible loaded Q and providing adequate damping of the higher-order modes (HOMs). If not sufficiently damped, dipole HOMs can drive the multipass beam breakup (BBU) instability which ERLs are particularly susceptible to. Another challenge involves efficiently extracting the potentially large amounts of HOM power that are generated when a bunch traverses the SRF cavities and which may extend over a high range of frequencies. We present experimental data from the Jefferson Lab FEL Upgrade, a 10 mA ERL light source presently in operation, aimed at addressing some of these issues. We conclude with an outlook towards the future of ERL based light sources.

The temporal resolution of pump-flash interactions in the ultrashort (fs-as) regime is limited by the characteristic time constants of the excited states in the detector material. If the relaxation time constant is appreciably longer that the time interval between the pump and probe signals the response of the detector material to the probe represents a temporal convolution with the pump and probe responses, setting a lower limit on the resolution to which the interval between the two pulses can be measured. In most of the solid state ultrafast detection schemes that are being considered for the ultra-short pulse x-ray sources under current development at SLAC and elsewhere the characteristic time constants are related to the bound states of the atoms comprising the material or to the relaxation times of phase transitions or charge carrier populations of the lattice, setting a probable lower limit on the attainable resolution on the order of ~0.1 ps. In this paper we consider a novel detection principle predicated on the excitation of specially prepared unbound states in an ionized plasma with high pump and probe fields, and estimate its potential for extending the lower limit of resolution into the as regime.