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IUCrJ 8(6), 905 - 920 (2021). doi:10.1107/S2052252521008046, Serial femtosecond crystallography (SFX) at X-ray free-electron lasers (XFELs) enables essentially radiation-damage-free macromolecular structure determination using microcrystals that are too small for synchrotron studies. However, SFX experiments often require large amounts of sample in order to collect highly redundant data where some of the many stochastic errors can be averaged out to determine accurate structure-factor amplitudes. In this work, the capability of the Swiss X-ray free-electron laser (SwissFEL) was used to generate large-bandwidth X-ray pulses [$����/��$ = 2.2% full width at half-maximum (FWHM)], which were applied in SFX with the aim of improving the partiality of Bragg spots and thus decreasing sample consumption while maintaining the data quality. Sensitive data-quality indicators such as anomalous signal from native thaumatin micro-crystals and de novo phasing results were used to quantify the benefits of using pink X-ray pulses to obtain accurate structure-factor amplitudes. Compared with data measured using the same setup but using X-ray pulses with typical quasi-monochromatic XFEL bandwidth (����/�� = 0.17% FWHM), up to fourfold reduction in the number of indexed diffraction patterns required to obtain similar data quality was achieved. This novel approach, pink-beam SFX, facilitates the yet underutilized de novo structure determination of challenging proteins at XFELs, thereby opening the door to more scientific breakthroughs., Published by Chester

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-A 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 first lasing results at SwissFEL, an X-ray free-electron laser, are presented, highlighting the facility’s unique capabilities. A general comparison to other major facilities is also provided.

The energy spread of the electron beam is a critical parameter in x-ray free-electron lasers (XFELs) and needs to be optimized for best performance. The uncorrelated energy spread of the electrons can be a few keV or less in XFEL injectors, thus very challenging to measure. The standard method to characterize the electron beam energy spread, consisting in streaking the beam with a transverse deflector and measuring the time-resolved beam size of the electrons in a dispersive location for a single electron beam energy, has a typical resolution of several keV. To overcome this limitation we introduce a novel method to measure the beam size at a dispersive location for different beam energies so that it is possible to disentangle the beam size contributions related to the energy spread, the intrinsic beam size and the monitor resolution. As a consequence, the energy spread can be characterized with a much higher precision and resolution than in the standard approach. We also suggest to perform measurements for different deflection amplitudes so that the energy spread induced by the transverse deflector can be subtracted properly. The scheme does not require any additional hardware and thus can be readily applied in any standard XFEL facility. Numerical simulations and experimental results at SwissFEL confirm the validity of our method. Our calculations show that the approach can be used to significantly overcome the resolution of the standard approach and measure energy spreads well below 1 keV. As an example we present energy spreads of few keV measured at the SwissFEL injector., Physical Review Accelerators and Beams, 23 (9), ISSN:2469-9888

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.

The advancement of the techniques of producing electromagnetic radiation in the terahertz range (1--20 THz) attracts a wide range of potential applications and the quest for powerful THz sources is ever-increasing. The means to generate THz is an active research field and the energy per pulse is limited due to breakdown of the medium used. In this paper, we propose a compact yet powerful narrowband THz source based on two pillars of active research in the field of novel accelerators: laser plasma wakefield acceleration and high gradient dielectric lined structures. The laser plasma wakefield (LPW) injector is used to provide the relativistic electron bunches to an interaction tube, a dielectric lined waveguide (DLW), in order to generate THz. Due to the high gradient nature of both devices, the combination ends up with a very compact solution for mJ scale THz pulse generation. Potentially these pulses can be used as drivers for nonlinear phenomena in condensed matter, which can be probed using ultrashort x-ray pulses at the same free electron laser facility.

We have produced hard x-ray free-electron laser (FEL) radiation with unprecedented large bandwidth tunable up to 2%. The experiments have been carried out at SwissFEL, the x-ray FEL facility at the Paul Scherrer Institute in Switzerland. The bandwidth is enhanced by maximizing the energy chirp of the electron beam, which is accomplished by optimizing the compression setup. We demonstrate continuous tunability of the bandwidth with a simple method only requiring a quadrupole magnet. The generation of such broadband FEL pulses will improve the efficiency of many techniques such as x-ray crystallography and spectroscopy, opening the door to significant progress in photon science. It has already been demonstrated that the broadband pulses of SwissFEL are beneficial to enhance the performance of crystallography, and further SwissFEL users plan to exploit this large bandwidth radiation to improve the efficiency of their measurement techniques.

The transverse emittance of the electron beam is a fundamental parameter in linac-based x-ray free-electron lasers (FELs). We present results of emittance measurements carried out at SwissFEL, a compact x-ray FEL facility at the Paul Scherrer Institute in Switzerland, including a description of the novel high-resolution measurement techniques and the optimization procedure. We obtained slice emittance values at the undulator entrance down to 200 nm for an electron beam with a charge of 200 pC and an rms duration of 30--40 fs. Furthermore, we achieved slice emittances as low as 100 nm for 10 pC beams with few fs duration. These values set new standards for electron linear accelerators. The quality, verification, and control of our electron beams allowed us to generate high-power FEL radiation for a wavelength as short as 0.1 nm using an electron beam with an energy of only 6 GeV. The emittance values demonstrated at SwissFEL would allow producing hard x-ray FEL pulses with even lower-energy beams, thus paving the way for even more compact and cost-effective FEL facilities.

A simple method to significantly enhance the coherence of X-ray free-electron laser (FEL) pulses by combining self-seeding and the harmonic generation mechanism is presented. This work is a fundamental step towards obtaining fully coherent FEL pulses, solving in this way a critical problem in the FEL field and its multidisciplinary scientific applications. In particular, the coherence improvement will be extremely beneficial for many photon science applications requiring a small bandwidth such as resonant inelastic X-ray scattering, therefore opening new areas of research and playing a crucial role in the future of all scientific users of the FEL facilities., X-ray free-electron lasers (FELs) are cutting-edge scientific tools able to generate transversely coherent radiation with very high power and ultra-short pulse durations. The self-seeding mechanism has been proven to increase the longitudinal coherence of the FEL radiation but its efficiency could be significantly improved, especially for soft X-rays. This paper proposes the enhancement of the performance of self-seeding by combining it with the harmonic generation mechanism. In particular, by starting the process with a subharmonic of the wavelength of interest, the coherence of the produced radiation is improved, the undulator beamline becomes more compact, and the monochromator realization is simplified. Numerical simulations for SwissFEL are presented showing that the method can be employed, within a given space, to increase the spectral brightness by one order of magnitude or more with respect to standard self-seeding. This coherence enhancement will be fundamental for many photon science applications and techniques such as resonant inelastic X-ray scattering.

The first experimental observation of transverse spatial echoes generated by forward Bragg diffraction of an X-ray beam propagating through a perfect thin crystal is reported., Time-delayed, narrow-band echoes generated by forward Bragg diffraction of an X-ray pulse by a perfect thin crystal are exploited for self-seeding at hard X-ray free-electron lasers. Theoretical predictions indicate that the retardation is strictly correlated to a transverse displacement of the echo pulses. This article reports the first experimental observation of the displaced echoes. The displacements are in good agreement with simulations relying on the dynamical diffraction theory. The echo signals are characteristic for a given Bragg reflection, the structure factor and the probed interplane distance. The reported results pave the way to exploiting the signals as an online diagnostic tool for hard X-ray free-electron laser seeding and for dynamical diffraction investigations of strain at the femtosecond timescale.

We demonstrate the operation of a compact x-ray free-electron laser (FEL) using the optical klystron mechanism. This effect speeds up the FEL process using the dispersion of magnetic chicanes placed between the undulator modules of the FEL beamline. The demonstration was performed at the soft x-ray beamline of SwissFEL, called Athos, which is, as far as we are aware, the only x-ray FEL to date with magnetic chicanes between every two undulator modules. In our measurements, we show that, compared to standard operation without chicanes, the required undulator length to achieve FEL saturation is reduced between 15% and 30% for radiation wavelengths between 1 and 2 nm. Fully exploiting the optical klystron effect represents an important step toward more compact FEL designs, rendering this key technology more affordable and hence accessible to a larger science community.

The Accelerator on a Chip International Program (ACHIP), funded by the Gordon and Betty Moore Foundation for a 5 year period, pursues basic research and development for a super-compact accelerator on a chip, where the accelerating structure is a dielectric microstructure excited by femtosecond laser pulses. The Paul Scherrer Institute (PSI) will contribute to this by providing the international collaboration access to the high-brightness electron beams in SwissFEL, where we plan to do a proof-of-principle demonstration of the acceleration of a highly relativistic beam. In this contribution we present the conceptual layout of the experiment, in which we will focus the beam down to sub-micrometer beam sizes. Start-to-end simulation results of the tracked electron beam, and first calculations of the accelerating field of the microstructure will be shown.

International journal of modern physics / A 34(36), 1942031 - (2019). doi:10.1142/S0217751X19420314, Dielectric Laser Acceleration (DLA) achieves the highest gradients among structure-based electron accelerators. The use of dielectrics increases the breakdown field limit, and thus the achievable gradient, by a factor of at least 10 in comparison to metals. Experimental demonstrations of DLA in 2013 led to the Accelerator on a Chip International Program (ACHIP), funded by the Gordon and Betty Moore Foundation. In ACHIP, our main goal is to build an accelerator on a silicon chip, which can accelerate electrons from below 100 keV to above 1 MeV with a gradient of at least 100 MeV/m. For stable acceleration on the chip, magnet-only focusing techniques are insufficient to compensate the strong acceleration defocusing. Thus, spatial harmonic and Alternating Phase Focusing (APF) laser-based focusing techniques have been developed. We have also developed the simplified symplectic tracking code DLAtrack6D, which makes use of the periodicity and applies only one kick per DLA cell, which is calculated by the Fourier coefficient of the synchronous spatial harmonic. Due to coupling, the Fourier coefficients of neighboring cells are not entirely independent and a field flatness optimization (similarly as in multi-cell cavities) needs to be performed. The simulation of the entire accelerator on a chip by a Particle In Cell (PIC) code is possible, but impractical for optimization purposes. Finally, we have also outlined the treatment of wake field effects in attosecond bunches in the grating within DLAtrack6D, where the wake function is computed by an external solver., Published by World Scientific Publ., Singapur

X-ray free-electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths. Echo-enabled harmonic generation in a free-electron laser enables 45th harmonic pulses from a 264 nm wavelength seed, yielding 5.9 nm wavelength coherent output.

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., Journal of Synchrotron Radiation, 26 (4), ISSN:0909-0495, ISSN:1600-5775

Simulation of THz streaking of photoelectrons created by X-ray pulses from a free-electron laser and reconstruction of the free-electron laser pulse lengths., Having accurate and comprehensive photon diagnostics for the X-ray pulses delivered by free-electron laser (FEL) facilities is of utmost importance. Along with various parameters of the photon beam (such as photon energy, beam intensity, etc.), the pulse length measurements are particularly useful both for the machine operators to measure the beam parameters and monitor the stability of the machine performance, and for the users carrying out pump–probe experiments at such facilities to better understand their measurement results. One of the most promising pulse length measurement techniques used for photon diagnostics is the THz streak camera which is capable of simultaneously measuring the lengths of the photon pulses and their arrival times with respect to the pump laser. This work presents simulations of a THz streak camera performance. The simulation procedure utilizes FEL pulses with two different photon energies in hard and soft X-ray regions, respectively. It recreates the energy spectra of the photoelectrons produced by the photon pulses and streaks them by a single-cycle THz pulse. Following the pulse-retrieval procedure of the THz streak camera, the lengths were calculated from the streaked spectra. To validate the pulse length calculation procedure, the precision and the accuracy of the method were estimated for streaking configuration corresponding to previously performed experiments. The obtained results show that for the discussed setup the method is capable of measuring FEL pulses with about a femtosecond accuracy and precision.

A simple and compact scheme that enhances the brightness of self-amplified spontaneous-emission (SASE) free-electron lasers is presented. The method combines the high-brightness SASE scheme and the optical klystron concept to increase the temporal coherence of the produced radiation and to reduce the required length of the undulator beamline at the same time. The scheme is very simple and only requires compact chicanes between the modules of the undulator beamline. Simulations show that, in comparison with SASE, the brightness can be improved by up to a factor of ten and the required length to achieve saturation can be reduced by 20% or more.

Physical review accelerators and beams 19(9), 090702(2016). doi:10.1103/PhysRevAccelBeams.19.090702, X-ray free-electron lasers (XFELs) are modern research tools in disciplines such as biology, material science, chemistry, and physics. Besides the standard operation that aims at minimizing the bandwidth of the produced XFEL radiation, there is a strong scientific demand to produce large-bandwidth XFEL pulses for several applications such as nanocrystallography, stimulated Raman spectroscopy, and multiwavelength anomalous diffraction. We present a self-consistent method that maximizes the XFEL pulse bandwidth by systematically maximizing the energy chirp of the electron beam at the undulator entrance. This is achieved by optimizing the compression scheme and the electron distribution at the source in an iterative back-and-forward tracking. Start-to-end numerical simulations show that a relative bandwidth of 3.25% full-width can be achieved for the hard x-ray pulses in the SwissFEL case., Published by American Physical Society, College Park, MD

The advent of x-ray free electron lasers has made possible the study of matter at the characteristic space and time scales of atomic and molecular phenomena using intense coherent x-ray pulses. This article describes the physical principles and the theoretical models governing the interaction of charged particles, electromagnetic waves, and external magnetic fields that comprise the x-ray free-electron lasers. It also includes a discussion of existing facilities and avenues for increasing the peak power and improving the control of spectral and coherence properties to allow the exploration of an even larger range of phenomena.\hskip-0.22em

With the successful operation of free-electron lasers (FELs) as user facilities there has been a growing demand for experiments with two photon pulses with variable photon energy and time separation. A configuration of an undulator with variable-gap control and a delaying chicane in the middle of the beamline is proposed. An injected electron beam with a transverse tilt will only yield FEL radiation for the parts which are close to the undulator axis. This allows, after re-aligning and delaying the electron beam, a different part of the bunch to be used to produce a second FEL pulse. This method offers independent control in photon energy and delay. For the parameters of the soft X-ray beamline Athos at the SwissFEL facility the photon energy tuning range is a factor of five with an adjustable delay between the two pulses from −50 to 950 fs.

Physical review accelerators and beams 19(10), 100702(2016). doi:10.1103/PhysRevAccelBeams.19.100702, The SwissFEL Injector Test Facility operated at the Paul Scherrer Institute between 2010 and 2014, serving as a pilot plant and test bed 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 ultralow-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., Published by American Physical Society, College Park, MD

Several parameters determine the performance of free electron lasers: the slice and the projected emittance, the slice energy spread, and the peak current are the most crucial ones. The peak current is essentially obtained by magnetic compression stages along the machine or occasionally assisted by velocity bunching at low energy. The minimum emittance and the alignment of the slices along the bunch are mainly determined in the low energy part of the accelerator (injector). Variations at the per-mille level of several parameters in this section of the machine strongly influence these quantities with highly nonlinear dynamic. We developed a numerical tool to perform the optimization of the injector. We applied this code to optimize the SwissFEL injector, assuming different gun designs, initial bunch lengths and intrinsic emittances. We obtained an emittance along the bunch of 0.14 mm mrad and around 0.08 mm mrad for the maximum and the minimum SwissFEL charges (200 and 10 pC, respectively). We applied the same tool to a running injector, where we automatized the optimization of the machine.

Next-generation X-ray sources, based on the X-ray Free Electron Laser (XFEL) concept, will provide highly coherent, ultrashort pulses of soft and hard X-rays with peak intensity many orders of magnitude above that of a synchrotron. These pulses will allow studies of femtosecond dynamics at nanometer resolution and with chemical selectivity. They will produce coherent-diffraction images of organic and inorganic nanostructures without the deleterious effects of radiation damage.

Free-Electron Lasers as high-brilliance radiation sources, rely on a high quality of the electron beam driving the FEL process. The amount of energy, transferred from the electrons to the radiation field, and thus the efficiency of the FEL depends on the provided beam parameters. The presentation discusses the impact of various beam parameter and how current designs of FEL injector try to accomplish the demands on the beam quality for reaching saturation.

A recent breakthrough in laser-plasma accelerators, based upon ultrashort high-intensity lasers, demonstrated the generation of quasi-monoenergetic GeV-electrons. With future Petawatt lasers ultra-high beam currents of ~100 kA in ~10 fs can be expected, allowing for drastic reduction in the undulator length of free-electron-lasers (FELs). We present a discussion of the key aspects of a table-top FEL design, including energy loss and chirps induced by space-charge and wakefields. These effects become important for an optimized table-top FEL operation. A first proof-of-principle VUV case is considered as well as a table-top X-ray-FEL which may open a brilliant light source also for new ways in clinical diagnostics., Comment: 6 pages, 4 figures; accepted for publication in Appl. Phys. B

X-ray free-electron lasers (XFELs) are cutting-edge research tools that produce almost fully coherent radiation with high power and short-pulse length with applications in multiple science fields. There is a strong demand to achieve even shorter pulses and higher radiation powers than the ones obtained at state-of-the-art XFEL facilities. In this context we propose a novel method to generate terawatt-attosecond XFEL pulses, where an XFEL pulse is pushed through several short good-beam regions of the electron bunch. In addition to the elements of conventional XFEL facilities, the method uses only a multiple-slotted foil and small electron delays between undulator sections. Our scheme is thus simple, compact, and easy to implement both in already operating as well as future XFEL projects. We present numerical simulations that confirm the feasibility and validity of our proposal.

Free electron lasers in the X-ray regime require a good slice alignment along the electron bunch to achieve their best performance. A transverse beam slice shift reduces this alignment and spoils projected emittance and optics. Coherent synchrotron radiation specifically for over-compression and transverse wakefields are major contributors to this. In the case of the large-bandwidth operation, based on a strictly monotonic energy chirp of the bunch, the here introduced correction additionally enhances the spectral bandwidth of the FEL pulse. Well-defined leaking of dispersion at places with a strictly monotonic longitudinal phase space can compensate a beam tilt. This work presents a way to characterize the beam tilt as well as a method to correct for it within a linear accelerator with at least one high dispersive section with corrector magnets.

We review and discuss the theoretical and experimental work that has led to the development of short wavelength free-electron lasers operating as single pass amplifiers, starting from the spontaneous undulator radiation, in the self amplified spontaneous emission mode. This work has led to several projects to build this type of free-electron lasers operating at a wavelength of about 0.1 nm, producing coherent X-ray pulses with an unprecedented brilliance and peak power, and pulse length in the femtosecond range. One such project, the LCLS, is presently under construction and is expected to be operational in 2008.

SPARC-X is a two branch project consisting in the SPARC test facility dedicated to the development and test of critical subsystems such as high brightness photoinjector and a modular expandable undulator for SASE-FEL experiments at 500 nm with seeding, and the SPARK facility aiming at generation of high brilliance coherent radiation in the 1.5-13 nm range, based on the achieved expertise. The projects are supported by MIUR (Research Department of Italian Government) and Regione Lazio. SPARC has completed the commissioning phase of the photoinjector in November 2006. The achieved experimental results are here summarized together with the status of the second phase commissioning plans. The SPARK project is based on the generation of ultra high peak brightness electron beams at the energy of 1 and 2 GeV generating radiation in the 1.5-13 nm range. The construction is at the moment planned in two steps starting with a 1 GeV Linac. The project layout including both RF-compression and magnetic chicane techniques has been studied.

The BESSY soft X-ray FEL is planned as a linac-based single pass FEL multi user facility covering the VUV to soft X-ray spectral range (0.02 keV≤hω≤ 1 keV) [ 1 , 2 ]. A Photoinjector and a superconducting CW linac [ 3 ] will feed three independent SASE-FEL-lines consisting of APPLE II type undulators providing variable polarized radiation [ 2 , 4 ]. The performance of the SASE-FEL can be enhanced using a coherent seed which dominates the spontaneous emission at the beginning of the process. We present simulation studies for three BESSY-SASE-FELs as well as for possible multistage high-gain harmonic generation (HGHG) FELs and discuss their performance in terms of pulse power, pulse duration, pulse shape and signal-to-noise ratio. © 2004 Elsevier B.V. All rights reserved. PACS: 41.60.Cr;07.85.Fv

Chirped beam manipulations are of the great interest to the free electron laser (FEL) community as potential means of obtaining ultra short X-ray pulses. The experiment is under way at the accelerator test facility (ATF) at Brookhaven National Laboratory (BNL) to study the FEL process limits with the under-compressed chirped electron beam. High gain near-saturation SASE operation was achieved with the strongly chirped beam (∼2.8% head-to-tail). The measured beam dynamics and SASE properties are presented, as well as the design parameters for the next round of experiment utilizing the newly installed UCLA/ATF chicane compressor.

VISA (Visible to Infrared SASE Amplifier) is a high-gain self-amplified spontaneous emission FEL, which achieved saturation at 840 nm within a single-pass 4-m undulator. A gain length shorter than 18 cm has been obtained, yielding the gain of 2 ×108 at saturation. The FEL performance, including spectral, angular, and statistical properties of SASE radiation, has been characterized for different electron beam conditions. The results are compared to 3-D SASE FEL theory and start-to-end numerical simulations of the entire injector, transport, and FEL system. Detailed agreement between simulations and experimental results is obtained over the wide range of the electron beam parameters.© 2003 Elsevier Science B.V. All rights reserved.

The experimental characterization of nonlinear harmonic generation (NHG) and electron beam microbunching at saturation from a visible SASE FEL are presented in this report. The gain lengths, spectra and energies of NHG were experimentally measured up to the third harmonic, and agree with theoretical predictions. Electron beam microbunching in both the fundamental and the second harmonic as the function of the SASE output were experimentally observed over the full range of SASE gain. The bunching factors for both the fundamental (b1) and second harmonic (b2) were experimentally characterized at saturation. The microbunching data provides another test of SASE saturation as well as correlating the NHG and electron beam microbunching modes to the fundamental SASE.© 2003 Published by Elsevier Science B.V.PACS: 41.60. Cr;41.60. Ap;4185. Ja

Visible to Infrared SASE Amplifier is a free electron laser (FEL) designed to saturate at a radiation wavelength of 800 nm within a 4 m long, strong focusing undulator. Large gain is achieved by driving the FEL with 72 MeV, high brightness beam of BNL's accelerator test facility. We present measurements that demonstrate saturation in addition to the frequency spectrum of the FEL radiation. Energy, gain length and spectral characteristics are compared and shown to agree with simulation and theoretical predictions.

The emittance of the electron beam is crucial for Free-Electron Laser facilities: it has a strong influence on the lasing performance and on the total length of the accelerator. We present our procedure to measure and minimize the projected and slice emittance at the SwissFEL Injector Test Facility. The normalized slice emittance resolution achieved is about 3 nm and the longitudinal resolution is about 13 fs, with measurement errors estimated to be below 5%. After performing a full optimization we have obtained, for uncompressed beams, a slice emittance of about 200 nm for a beam charge of 200 pC, and a slice emittance of about 100 nm for 10 pC. These values are consistent with our simulations and are well below the requirements of the SwissFEL under construction at the Paul Scherrer Institute. At these bunch charges our measured slice emittances are, to our knowledge, the lowest reported so far for an electron linear accelerator.

In this paper we report on start-to-end simulation of a next generation light source based on a high repetition rate free electron laser (FEL) driven by a CW superconducting linac. The simulation integrated the entire system in a seamless start-to-end model, including birth of photoelectrons, transport of electron beam through 600 m of the accelerator beam delivery system, and generation of coherent x-ray radiation in a two-stage self-seeding undulator beam line. The entire simulation used the real number of electrons ($\ensuremath{\sim}2$ billion electrons/bunch) to capture the details of the physical shot noise without resorting to artificial filtering to suppress numerical noise. The simulation results shed light on several issues including the importance of space-charge effects near the laser heater and the reliability of x-ray radiation power predictions when using a smaller number of simulation particles. The results show that the microbunching instability in the linac can be controlled with 15 keV uncorrelated energy spread induced by a laser heater and demonstrate that high brightness and flux 1 nm x-ray radiation ($\ensuremath{\sim}{10}^{12}\text{ }\text{ }\mathrm{photons}/\mathrm{pulse}$) with fully spatial and temporal coherence is achievable.

The Visible to Infrared SASE Amplifier (VISA) FEL is designed to obtain high gain at a radiation wavelength of 800 nm. The FEL uses the high brightness electron beam of the Accelerator Test Facility (ATF), with energy of 72 MeV. VISA uses a novel, 4 m long, strong focusing undulator with a gap of 6 mm and a period of 1.8 cm. To obtain large gain the beam and undulator axis have to be aligned to better than 5 μm. Results from initial measurements on the alignment, gain, and spectrum will be presented and compared to theoretical calculations and simulations.

A self-amplified spontaneous emission (SASE) free-electron laser (FEL) is under construction at the Advanced Photon Source (APS). Five FEL simulation codes were used in the design phase: GENESIS, GINGER, MEDUSA, RON, and TDA3D. Initial comparisons between each of these independent formulations show good agreement for the parameters of the APS SASE FEL.

We report measurements of large gain for a single pass free-electron laser operating in self-amplified spontaneous emission (SASE) at 16 \ensuremath{\mu}m starting from noise. We also report the first observation and analysis of intensity fluctuations of the SASE radiation intensity in the high gain regime. The results are compared with theoretical predictions and simulations.

The measurement of the transverse phase-space map, or transport matrix, of a relativistic electron in a high-gradient, radio-frequency linear accelerator (rf linac) at the UCLA photoinjector is reported. This matrix, which indicates the effects of acceleration (adiabatic damping), first-order transient focusing, and ponderomotive second-order focusing, is measured as a function of both rf field amplitude and phase in the linac. The elements of the matrix, determined by observation of centroid motion at a set of downstream diagnostics due to deflections induced by a set of upstream steering magnets, compare well with previously developed analytical theory [J. Rosenzweig and L. Serafini, Phys. Rev. E {bold 49}, 1599 (1994)]. The determinant of the matrix is obtained, yielding a direct confirmation of trace space adiabatic damping. Implications of these results on beam optics at moderate energy in high-gradient linear accelerators such as rf photoinjectors are discussed. {copyright} {ital 1997} {ital The American Physical Society}

In this paper we investigate and compare the properties of two narrow-bandwidth free-electron laser (FEL) schemes, one using self-seeding and the other high gain harmonic generation (HGHG). The two systems have been thoroughly studied analytically and numerically in the past. The aim of this work is to compare their performances when the FEL is driven by an electron beam with nonideal properties, thus including effects such as shot-to-shot energy fluctuations and nonlinear energy chirp. In both cases nonlinearities produce a bandwidth larger than the Fourier transform limited value. However, our analysis indicates that, for approximately the same output power levels, the self-seeding scheme is less affected than the HGHG scheme by quadratic energy chirps in the electron beam longitudinal phase space. This is confirmed by a specific numerical example corresponding to SPARX parameters where the electron beam was optimized to minimize the FEL gain length. The work has been carried out with the aid of the time dependent FEL codes GENESIS 1.3 (3D) and PERSEO (1D).

Free-electron lasers (FELs) are radiation sources, based on the coherent emission of synchrotron radiation of relativistic electrons within an undulator or wiggler. The resonant radiation wavelength depends on the electron beam energy and can be tuned over the entire spectrum from micrometer to X-ray radiation. The emission level of FELs is several orders of magnitude larger than the emission level of spontaneous synchrotron radiation, because the interaction between the electron beam and the radiation field modulates the beam current with the periodicity of the resonant radiation wavelength. The high brightness and the spectral range of this kind of radiation source facilitate the study of physical and chemical process on a femtosecond scale with angstrom resolution. Keywords: free-electron laser; undulator; microbunching; SASE FEL; FEL oscillator; FEL amplifier; FEL parameter; gain length

Microbunching of a relativistic electron beam into a helix is examined analytically and in simulation. Helical microbunching is shown to occur naturally when an e beam interacts resonantly at the harmonics of the combined field of a helical magnetic undulator and an axisymmetric input laser beam. This type of interaction is proposed as a method to generate a strongly prebunched e beam for coherent emission of light with orbital angular momentum at virtually any wavelength. The results from the linear microbunching theory show excellent agreement with three-dimensional numerical simulations.

In the framework of the DS4 EUROFEL collaboration, a research work plan at the SPARC free-electron laser (FEL) test facility aiming at the investigation of seeded and cascaded FEL configurations is under implementation. The main goal of the collaboration is to study and test the amplification and the FEL harmonic generation process of an input seed signal obtained as higher-order harmonics generated both in crystals (400 and 266 nm) and in gases (266,160,114 nm). The SPARC FEL can be configured to test several cascaded FEL configurations. In this paper we introduce SPARC and its main parameters and we analyze the superradiant cascade and the harmonic cascade seeded with a signal with the typical time structure of the harmonics generated in gas. (C) 2008 Elsevier B.V. All rights reserved.

In this paper, we study the production of low charge—in the 1 pC range—high brightness, ultra-short electron bunches, with a length shorter than 1 micromn, to produce sub-femtosecond pulses in an X-ray FEL. We show that the electron bunches have a brightness one or two orders of magnitude larger than the current photoinjectors run in their design regime. The ultra-short bunches can be used to drive a high gain SASE X-ray FEL with a small gain length, to produce femtosecond to attosecond X-ray pulses. This method to produce such short X-ray pulses has the advantage over other proposed methods in that it can be free from longer pulse duration background radiation. We also show, using nascent SPARX SASE FEL as an example, that the electron bunch thus produced can be shorter than the cooperation length of the X-ray FEL, leading to the production of a single spike, fully coherent, X-ray pulse. The proposed system for the production of ultra-short bunches uses the same injector hardware configuration of X-ray FELs like SPARX or the LCLS, operated with a different set of parameters, and does not require any new technical development in the injector and compressor systems.

It is proposed to modulate electrons longitudinally by the ponderomotive force of a CO2 laser beat-wave. For the 9.3 and 10.6 ?tm laser lines, the beam injected into the undulator radiates at ~ 77 ?m.

In this Letter we report the first experimental characterization of superradiance in a single-pass high-gain free-electron laser (FEL) seeded by a 150 femtosecond (FWHM) Ti:sapphire laser. The nonlinear energy gain after an exponential gain regime was observed. We also measured the evolution of the longitudinal phase space in both the exponential and superradiant regimes. The output FEL pulse duration was measured to be as short as 81 fs, a roughly 50% reduction compared to the input seed laser. The temporal distribution of the FEL radiation as predicted by a numerical simulation was experimentally verified for the first time.

Self-amplifying spontaneous radiation free-electron lasers, such as the LCLS or the European X-FEL, rely on the incoherent, spontaneous radiation as the seed for the amplifying process. Though this method overcomes the need for an external seed source one drawback is the incoherence of the effective seed signal. The FEL process allows for a natural growth of the coherence because the radiation phase information is spread out within the bunch due to slippage and diffraction of the radiation field. However, at short wavelengths this spreading is not sufficient to achieve complete coherence. In this presentation we report on the results of numerical simulations of the LCLS X-ray FEL. From the obtained radiation field distribution the coherence properties are extracted to help to characterize the FEL as a light source.

A comprehensive analysis is presented that describes amplification of a seed THz pulse in a single-pass free-electron laser (FEL) driven by a photoinjector. The dynamics of the radiation pulse and the modulated electron beam are modeled using the time-dependent FEL code, GENESIS 1.3. A 10-ps (FWHM) electron beam with a peak current of 50–100 A allows amplification of a ∼1 kW seed pulse in the frequency range 0.5–3 THz up to 10–100 MW power in a relatively compact 2-m long planar undulator. The electron beam driving the FEL is strongly modulated, with some inhomogeneity due to the slippage effect. It is shown that THz microbunching of the electron beam is homogeneous over the entire electron pulse when saturated FEL amplification is utilized at the very entrance of an undulator. This requires seeding of a 30-cm long undulator buncher with a 1–3 MW of pump power with radiation at the resonant frequency. A narrow-band seed pulse in the THz range needed for these experiments can be generated by frequency mixing of CO_{2} laser lines in a GaAs nonlinear crystal. Two schemes for producing MW power pulses in seeded FELs are considered in some detail for the beam parameters achievable at the Neptune Laboratory at UCLA: the first uses a waveguide to transport radiation in the 0.5–3 THz range through a 2-m long FEL amplifier and the second employs high-gain third harmonic generation using the FEL process at 3–9 THz.

The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser (FEL) project based on the SLAC linac. The nominal parameter set is founded on a 1-nC bunch charge and normalized emittance of about 1 micron. Many of the most challenging issues are associated with the relatively high charge. The study described here uses a 0.2-nC bunch charge and 0.85-μm emittance with only 30 A of peak current in the injector, producing the same FEL saturation length. The resulting performance is more stable, has negligible resistive wakefield, greatly reduced CSR effects, and no transverse wakefield emittance dilution in the linac, with no change to the baseline engineering design.