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The microbunching instability has been a long-standing issue for high-brightness free-electron lasers (FELs) and is a significant showstopper to achieve full longitudinal coherence in the x-ray regime. This paper reports the first experimental demonstration of microbunching instability mitigation through transverse Landau damping, based on linear optics control in a dispersive region. Analytical predictions for the microbunching content are supported by numerical calculations of the instability gain. The effect is confirmed through the experimental characterization of the spectral brightness of the FERMI FEL under different transverse optics configurations of the transfer line between the linear accelerator and the FEL.

The basic theory of free-electron lasers (FELs) indicates that the output wavelength of a seeded FEL operated in the high-gain harmonic generation (HGHG) configuration is determined by the wavelength of the seed laser and light is emitted when the undulators are tuned to an exact harmonics of the seed laser. In a realistic case, when taking into account the electron beam imperfections and the finite bandwidths of the seed and of the amplification process, the output wavelength is influenced by these factors and can deviate from the exact harmonic resonance. These effects are responsible for the small wavelength fluctuations of the FEL pulses but can also be exploited for an accurate FEL wavelength tuning. In this work, we show how the dispersive section, the curvature of the electron beam longitudinal phase-space and frequency pulling can influence the FEL wavelength and can be, in principle, used to control it. Furthermore, we show how one can reconstruct the electron beam longitudinal phase-space from the analysis of the FEL wavelength sensitivity to the seed laser delay with respect to the beam arrival time.

The advent of x-ray free-electron lasers (FELs) drastically enhanced the capabilities of several analytical techniques, for which the degree of transverse (spatial) coherence of the source is essential. FELs can be operated in self-amplified spontaneous emission (SASE) or seeded configurations, which rely on a qualitatively different initialization of the amplification process leading to light emission. The degree of transverse coherence of SASE and seeded FELs has been characterized in the past, both experimentally and theoretically. However, a direct experimental comparison between the two regimes in similar operating conditions is missing, as well as an accurate study of the sensitivity of transverse coherence to key working parameters. In this paper, we carry out such a comparison, focusing in particular on the evolution of coherence during the light amplification process.

Echo-enabled harmonic generation free-electron lasers (EEHG FELs) are promising candidates to produce fully coherent soft x-ray pulses by virtue of efficient high-harmonic frequency up-conversion from ultraviolet lasers. The ultimate spectral limit of EEHG, however, remains unclear, because of the broadening and distortions induced in the output spectrum by residual broadband energy modulations in the electron beam. We present a mathematical description of the impact of incoherent (broadband) energy modulations on the bunching spectrum produced by the microbunching instability through both the accelerator and the EEHG line. The model is in agreement with a systematic experimental characterization of the FERMI EEHG FEL in the photon energy range 130–210 eV. We find that amplification of electron beam energy distortions primarily in the EEHG dispersive sections explains an observed reduction of the FEL spectral brightness proportional to the EEHG harmonic number. Local maxima of the FEL spectral brightness and of the spectral stability are found for a suitable balance of the dispersive sections’ strength and the first seed laser pulse energy. Such characterization provides a benchmark for user experiments and future EEHG implementations designed to reach shorter wavelengths.

FERMI is a seeded free electron laser (FEL) based on the high gain harmonic generation (HGHG) scheme and has been generating intense and fully coherent extreme ultra-violet and soft x-ray pulses for several years. The high-degree performance of the FEL leans on a high brightness electron beam, with small transverse emittance (∼1 μm) and a peak current of about 700-800 A. The main constraint on lasing at high harmonics of the seed is low electron time-slice energy spread. The optimization of the photoinjector and of some linac parameters has allowed a reduction of the relative slice energy spread to the level of few times of 10^{-5}. With these new conditions, the FEL can be operated without the need of a laser heater to suppress micro bunching instabilities and this “cold” beam has allowed generation of extreme UV pulses with pulse energy exceeding a mJ, and with peak power of about 10 GW. We describe the electron beam characterization and the FEL performance improvement, including the extension of the range of harmonics of the seed which can be amplified, up to the twenty-fifth harmonic, i.e., 10 nm.

High-brightness electron bunches, such as those generated and accelerated in free-electron lasers (FELs), can develop small-scale structure in the longitudinal phase space. This causes variations in the slice energy spread and current profile of the bunch which then undergo amplification, in an effect known as the microbunching instability. By imposing energy spread modulations on the bunch in the low-energy section of an accelerator, using an undulator and a modulated laser pulse in the center of a dispersive chicane, it is possible to manipulate the bunch longitudinal phase space. This allows for the control and study of the instability in unprecedented detail. We report measurements and analysis of such modulated electron bunches in the 2D spectrotemporal domain at the Fermi FEL, for three different bunch compression schemes. We also perform corresponding simulations of these experiments and show that the codes are indeed able to reproduce the measurements across a wide spectral range. This detailed experimental verification of the ability of codes to capture the essential beam dynamics of the microbunching instability will benefit the design and performance of future FELs.

Results on fabrication and experimental characterization of wire scanners (WS) with submicrometer spatial resolution are presented. Independently fabricated at PSI and FERMI by means of nanolithography, the proposed WS solutions consist of 900 and 800 nm wide free-standing stripes ensuring a geometric resolution of about 250 nm. The nanofabricated WS were tested successfully at SwissFEL where low-charge electron beams with a vertical size of 400–500 nm were characterized. Further experimental tests at 200 pC confirmed the resilience to the heat-loading of the structures. With respect to conventional WS consisting of a metallic wire stretched onto a frame, the nanofabricated WS allow for an improvement of the spatial resolution from the micrometer to the submicrometer level as well as of beam invasiveness thanks to an equivalent reduction of the impact surface of the scanning stripe. The present work represents an important milestone toward the goal—that PSI and FERMI are pursuing—to realize a standard WS solution with minimal invasiveness and submicrometer resolution.

SwissFEL is a x-rays free electron laser (FEL) driven by a 5.8 GeV linac under construction at Paul Scherrer Institut. In SwissFEL, wire scanners (WSCs) will be complementary to view-screens for emittance measurements and routinely used to monitor the transverse profile of the electron beam during FEL operations. The SwissFEL WSC is composed of an in-vacuum beam-probe—motorized by a stepper motor—and an out-vacuum pick-up of the wire signal. The mechanical stability of the WSC in-vacuum hardware has been characterized on a test bench. In particular, the motor induced vibrations of the wire have been measured and mapped for different motor speeds. Electron-beam tests of the entire WSC setup together with different wire materials have been carried out at the 250 MeV SwissFEL Injector Test Facility (SITF, Paul Scherrer Institut, CH) and at FERMI (Elettra-Sincrotrone Trieste, Italy). In particular, a comparative study of the relative measurement accuracy and the radiation-dose release of Al(99)∶Si(1) and tungsten (W) wires has been carried out. On the basis of the outcome of the bench and electron-beam tests, the SwissFEL WSC can be qualified as a high resolution and machine-saving diagnostic tool in consideration of the mechanical stability of the scanning wire at the micrometer level and the choice of the wire material ensuring a drastic reduction of the radiation-dose release with respect to conventional metallic wires. The main aspects of the design, laboratory characterization and electron beam tests of the SwissFEL WSCs are presented.

FERMI is the first user facility based upon an externally seeded free-electron laser (FEL) and was designed to deliver high quality, transversely and longitudinally coherent radiation pulses in the extreme ultraviolet and soft x-ray spectral regimes. The FERMI linear accelerator includes a laser heater to control the longitudinal microbunching instability, which otherwise is expected to degrade the quality of the high brightness electron beam sufficiently to reduce the FEL output intensity and spectral brightness. In this paper, we present the results of the FERMI laser heater commissioning. For the first time, we show that optimizing the electron beam heating at an upstream location (beam energy, 100 MeV) leads to a reduction of the incoherent energy spread at the linac exit (beam energy, 1.2 GeV). We also discuss some of the positive effects of such heating upon the emission of coherent optical transition radiation and the FEL output intensity.

We present a new diagnostic method allowing one to reconstruct certain properties of a relativistic electron beam and of a laser pulse by looking at the result of their interaction. The latter takes place within an undulator and results in energy modulation of electrons. By taking advantage of the correlation between time and electron-beam energy, measurements of the electron-beam length, current and energy spread are possible simultaneously. Moreover, the scheme allows an accurate measurement of the timing jitter between the electron beam and the external laser and also of the laser’s actual pulse length at the interaction point. Experimental demonstration of the last two measurements is reported in this paper. Finally, this diagnostic method provides an independent measure of the energy transferred from the electron beam to free-electron laser light produced when seeded electrons are injected into a long undulator. Experimental results obtained at the FERMI free-electron laser are presented after a description of the proposed technique.

The accurate and stable synchronization between electron bunch and external laser is a key requirement for the successful operation of an externally seeded free electron laser. This requirement is particularly stringent when the electron bunch is longitudinally compressed to sub-ps durations. We present an analytical description of the electron bunch arrival-time jitter that, supported by experimental evidence, allows the identification of specific, dominant jitter sources. The arrival-time jitter measurements were carried out as a function of the bunch length compression factor in FERMI@Elettra linac. The experimental behavior of the pulse-to-pulse time jitter agrees well both with the analytical predictions and particle tracking simulations. Our modeling takes into account the photoinjector laser arrival time on the cathode, the jitter of phases and voltages of the radio-frequency accelerator, and fluctuations of the compressor’s dipole field.

A characterization of the transverse emittance of a 200 pC, 6 ps long electron bunch has been performed in the Fermi@Elettra Free Electron Laser (FEL) first bunch compressor area. This region includes a magnetic bunch length compressor, diagnostics, and quadrupole magnets. The beam is time compressed in one stage, without linearization of the longitudinal phase space. Some growth of the normalized emittance has been measured in the compressor area. To understand this effect, we have investigated mechanisms of emittance growth such as coherent synchrotron radiation emission, chromatic aberration, and spurious dispersion. We show that careful optics control inside the compressor is essential for emittance growth reduction. The final configuration of one-stage magnetic compression limits the emittance to values below the design goal of 2 mm mrad, up to a compression factor of about 5. This machine configuration has been adopted to optimize FEL radiation output at wavelengths in the range 30–60 nm.

Specific requirements for the electron beam in harmonic cascade-free electron lasers, together with means to produce such beams, are presented. All results are illustrated with simulations and particle tracking studies.

This paper is an addendum to reference (Brynes et al 2018 New J. Phys. 20 073035), in which we studied the transverse emittance growth due to coherent synchrotron radiation (CSR) in the bunch compressor of a free-electron laser. Analytic results based on the theory of CSR were compared with results from experiment and simulation. Recent results have been published (Stupakov 2019 arXiv: 1901.10745 ) which correct the theoretical emittance growth, predicting a smaller increase than that which was expected based on previous results. When comparing these new calculations with the emittance growth that was measured experimentally, and with 3D simulations, we find an improved agreement when the bunch compression approaches its maximum value.

The effect of multiple small-angle Coulomb scattering, or intrabeam scattering (IBS) is routinely observed in electron storage rings over the typical damping time scale of milliseconds. So far, IBS has not been observed in single pass electron accelerators because charge density orders of magnitude higher than in storage rings would be needed. We show that such density is now available at high brightness electron linacs for free-electron lasers (FELs). We report measurements of the beam energy spread in the FERMI linac in the presence of the microbunching instability, which are consistent with a revisited IBS model for single pass systems. We also show that neglecting the hereby demonstrated effect of IBS in the parameter range typical of seeded VUV and soft x-ray FELs, results in too conservative a facility design, or failure to realise the accessible potential performance. As an example, an optimization of the FERMI parameters driven by an experimentally benchmarked model, opens the door to the extension of stable single spectral line emission to the water window (2.3–4.4 nm), with far-reaching implications for experiments in a variety of disciplines, ranging from physics and chemistry to biology and material sciences, and including nonlinear x-ray optics based on the four-wave-mixing approach.

An understanding of collective effects is of fundamental importance for the design and optimisation of the performance of modern accelerators. In particular, the design of an accelerator with strict requirements on the beam quality, such as a free electron laser (FEL), is highly dependent on a correspondence between simulation, theory and experiments in order to correctly account for the effect of coherent synchrotron radiation (CSR), and other collective effects. A traditional approach in accelerator simulation codes is to utilise an analytic one-dimensional approximation to the CSR force. We present an extension of the 1D CSR theory in order to correctly account for the CSR force at the entrance and exit of a bending magnet. A limited range of applicability to this solution—in particular, in bunches with a large transverse spot size or offset from the nominal axis—is recognised. More recently developed codes calculate the CSR effect in dispersive regions directly from the Liénard–Wiechert potentials, albeit with approximations to improve the computational time. A new module of the General Particle Tracer code was developed for simulating the effects of CSR, and benchmarked against other codes. We experimentally demonstrate departure from the commonly used 1D CSR theory for more extreme bunch length compression scenarios at the FERMI FEL facility. Better agreement is found between experimental data and the codes which account for the transverse extent of the bunch, particularly in more extreme compression scenarios.

FERMI@Elettra is a free electron-laser (FEL)-based user facility that, after two years of commissioning, started preliminary users' dedicated runs in 2011. At variance with other FEL user facilities, FERMI@Elettra has been designed to deliver improved spectral stability and longitudinal coherence. The adopted scheme, which uses an external laser to initiate the FEL process, has been demonstrated to be capable of generating FEL pulses close to the Fourier transform limit. We report on the first instance of FEL wavelength tuning, both in a narrow and in a large spectral range (fine- and coarse-tuning). We also report on two different experiments that have been performed exploiting such FEL tuning. We used fine-tuning to scan across the 1s–4p resonance in He atoms, at ≈23.74 eV (52.2 nm), detecting both UV–visible fluorescence (4p–2s, 400 nm) and EUV fluorescence (4p–1s, 52.2 nm). We used coarse-tuning to scan the M _4,5 absorption edge of Ge (∼29.5 eV) in the wavelength region 30–60 nm, measured in transmission geometry with a thermopile positioned on the rear side of a Ge thin foil.

The operation of modern free-electron lasers (FELs) necessitates precise knowledge of electron beam properties at the undulator to ensure the level of control required by increasingly demanding experiments. In seeded FELs, where only electrons interacting with the seed laser contribute to the process, it is crucial to determine the local values of these properties. We present a novel method, based on accurate modeling of the FEL process in high-gain harmonic generation, to accurately retrieve the electron beam slice energy spread, current and laser-induced energy modulation. Understanding these values is essential for enabling advanced FEL schemes and optimally setting advanced seeding schemes such as echo-enabled harmonic generation. We describe the method and provide an experimental application to the FERMI FEL-1, where a slice energy spread in the range 40-100 keV with a few keV accuracy is measured., (open access.)

Light manipulation at the nanoscale is essential both for fundamental science and modern technology. The quest to shorter lengthscales, however, requires the use of light wavelengths beyond the visible. In particular, in the extreme ultraviolet regime these manipulation capabilities are hampered by the lack of efficient optics, especially for polarization control. Here, we present a method to create periodic, polarization modulations at the nanoscale using a tailored configuration of the FERMI free electron laser and demonstrate its capabilities by comparing the dynamics induced by this polarization transient grating with those driven by a conventional intensity grating on a thin ferrimagnetic alloy. While the intensity grating signal is dominated by the thermoelastic response, the polarization grating excitation minimizes it, uncovering helicity-dependent responses previously undetected. We anticipate nanoscale polarization transient gratings to become useful for the study of physical, chemical and biological systems possessing chiral symmetry., Competing Interests: Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)

Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency 1,2 , with applications ranging from quantum information to photochemistry 3-7 . Although strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation 8 , comparable phenomena have been only recently observed with short wavelengths, accessing highly excited multi-electron and inner-shell electron states 9,10 . However, the quantum control of strong-field processes at short wavelengths has not been possible, so far, because of the lack of pulse-shaping technologies in the extreme ultraviolet (XUV) and X-ray domain. Here, exploiting pulse shaping of the seeded free-electron laser (FEL) FERMI, we demonstrate the strong-field quantum control of ultrafast Rabi dynamics in helium atoms with high fidelity. Our approach reveals a strong dressing of the ionization continuum, otherwise elusive to experimental observables. The latter is exploited to achieve control of the total ionization rate, with prospective applications in many XUV and soft X-ray experiments. Leveraging recent advances in intense few-femtosecond to attosecond XUV to soft X-ray light sources, our results open an avenue to the efficient manipulation and selective control of core electron processes and electron correlation phenomena in real time., Competing Interests: Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)

We demonstrate the generation of extreme-ultraviolet (XUV) free-electron laser (FEL) pulses with time-dependent polarization. To achieve polarization modulation on a femtosecond timescale, we combine two mutually delayed counterrotating circularly polarized subpulses from two cross-polarized undulators. The polarization profile of the pulses is probed by angle-resolved photoemission and above-threshold ionization of helium; the results agree with solutions of the time-dependent Schrödinger equation. The stability limit of the scheme is mainly set by electron-beam energy fluctuations, however, at a level that will not compromise experiments in the XUV. Our results demonstrate the potential to improve the resolution and element selectivity of methods based on polarization shaping and may lead to the development of new coherent control schemes for probing and manipulating core electrons in matter.

Femtosecond extreme ultraviolet wave packet interferometry (XUV-WPI) was applied to study resonant interatomic Coulombic decay (ICD) in the HeNe dimer. The high demands on phase stability and sensitivity for vibronic XUV-WPI of molecular-beam targets are met using an XUV phase-cycling scheme. The detected quantum interferences exhibit vibronic dephasing and rephasing signatures along with an ultrafast decoherence assigned to the ICD process. A Fourier analysis reveals the molecular absorption spectrum with high resolution. The demonstrated experiment shows a promising route for the real-time analysis of ultrafast ICD processes with both high temporal and high spectral resolution.

The optimal performance of high-brightness free-electron lasers (FELs) is limited by the microbunching instability, which can cause variations in both the slice energy spread and longitudinal profile of electron beams. In this paper, we perform 2D Fourier analysis of the full bunch longitudinal phase space, such that modulations in both planes can be studied simultaneously. Unlike the standard 1D analysis, this method is able to reveal modulations in a folded phase space, which would otherwise remain uncovered. Additionally, the plasma oscillation between energy and density modulations is also revealed by this method. The damping of the microbunching instability, through the use of a laser heater, is also analysed with this technique. We confirm a mitigation of the amplitude of modulation and a red-shift of the microbunching frequency as the energy spread added increases. As an outcome of this work, a systematic experimental comparison of the development of the instability in the presence of different compression schemes is here presented for the first time.