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Electrons in an x-ray free electron laser (XFEL) develop periodic density fluctuations, known as microbunches, which enable the exponential gain of x-ray power in an XFEL. When an electron beam microbunched at a hard x-ray wavelength is kicked, microbunches are often washed out due to the dispersion and R_{56} of the bend. An achromatic (dispersion-free) bend with a small R_{56}, however, can preserve microbunches, which rotate to follow the new trajectory of the electron bunch. Rotated microbunches can subsequently interact in a repointed undulator to produce a new beam of off-axis x rays. In this work, we demonstrate hard x-ray multiplexing in the Linac Coherent Light Source hard x-ray undulator line using microbunch rotation through a 10 μrad first-order-achromatic bend created by transversely offsetting quadrupole magnets in the FODO lattice. Quadrupole offsets are determined analytically from beam-matrix theory. We also discuss the application of microbunch rotation to out-coupling a cavity-based XFEL.

Accelerator-based x-ray free-electron lasers (XFELs) are the latest addition to the revolutionary tools of discovery for the 21st century. The two major components of an XFEL are an accelerator-produced electron beam and a magnetic undulator, which tend to be kilometer-scale long and expensive. A proof-of-principle demonstration of free-electron lasing at 27 nm using beams from compact laser wakefield accelerators was shown recently by using a magnetic undulator. However, scaling these concepts to x-ray wavelengths is far from straightforward as the requirements on the beam quality and jitters become much more stringent. Here, we present an ultracompact scheme to produce tens of attosecond x-ray pulses with several GW peak power utilizing a novel aspect of the FEL instability using a highly chirped, prebunched, and ultrabright tens of MeV electron beam from a plasma-based accelerator interacting with an optical undulator. The FEL resonant relation between the prebunched period and the energy selects resonant electrons automatically from the highly chirped beam which leads to a stable generation of attosecond x-ray pulses. Furthermore, two-color attosecond pulses with subfemtosecond separation can be produced by adjusting the energy distribution of the electron beam so that multiple FEL resonances occur at different locations within the beam. Such a tunable coherent attosecond x-ray sources may open up a new area of attosecond science enabled by x-ray attosecond pump/probe techniques.

Vector field analysis is presented for a highly overmoded iris-line structure that can act as a medium of transportation for THz radiation. The axisymmetric structure is capable of supporting hybrid modes with desirable features such as low propagation loss, uniformly linear polarization and approximately Gaussian intensity profile across the iris. A specific application that can benefit from these desirable features is the transportation of THz undulator radiation over hundreds of meters to reach the experimental halls at the Linac Coherent Light Source (LCLS) facility at SLAC, Stanford. Such a structure has been modeled before as a boundary-value problem using Vainstein’s complex-impedance boundary condition and assuming infinitely thin screens. Given that physical realizations of such screens must have finite thickness and that the THz wavelength in the 3–15 THz range is expected to be smaller than convenient screen thicknesses in practice, the question of the impact of finite screen thickness on propagation performance becomes rather pressing. To address this question, we present a mode-matching analysis of the structure as an open resonator with finite screen thickness and perturbatively clustered (localized) field expansions, for computational feasibility. The effect of screen thickness is seen to lower the attenuation constant on the iris line, which is dominated by diffraction loss. Ohmic loss due to the finite conductivity of metallic surfaces at the screen edges is found to be small compared to diffraction loss. The propagation loss predictions based on the Vainstein model are compared with the numerical results from mode matching for infinitely thin screens, where the former method is observed to agree with the numerical results better at higher Fresnel numbers (highly overmoded structures). The properties of the dominant mode fields are formally derived from first principles and a recommended approach is discussed for the inclusion of screen-thickness effects into propagation loss estimations.

Generally, turn-to-turn fluctuations of synchrotron radiation power in a storage ring depend on the 6D phase-space distribution of the electron bunch. This effect is related to the interference of fields radiated by different electrons. Changes in the relative electron positions and velocities inside the bunch result in fluctuations in the total emitted energy per pass in a synchrotron radiation source. This effect has been previously described assuming constant and equal electron velocities before entering the synchrotron radiation source. In this paper, we present a generalized formula for the fluctuations with a non-negligible beam divergence. Further, we corroborate this formula in a dedicated experiment with undulator radiation in the Integrable Optics Test Accelerator (IOTA) storage ring at Fermilab. Lastly, possible applications in beam instrumentation are discussed.

In a storage ring, turn-to-turn fluctuations in the intensity of spontaneous synchrotron radiation occur due to two mechanisms. The first mechanism is the quantum uncertainty in the number of emitted photons. The second mechanism is the turn-to-turn variations in the relative positions of classical pointlike electrons in the bunch. We present a unified description of both effects in the framework of quantum optics. We derive an equation for the fluctuations for an arbitrary degree of coherence, which generalizes previously reported results for temporally incoherent radiation. We compare the predictions of our calculation with a previous experiment at Brookhaven National Laboratory, where the latter mechanism was dominant and propose a new dedicated experiment in the Integrable Optics Test Accelerator (IOTA) at Fermilab, where the two mechanisms may have comparable contributions to the fluctuations. Finally, our calculation shows that the magnitude of the fluctuations is rather sensitive to the dimensions and the shape of the electron bunch, thereby indicating possible applications in beam instrumentation. In particular, the small vertical size of the flat beams in IOTA may be estimated via these fluctuations, whereas measurement by a conventional synchrotron radiation monitor is difficult due to the diffraction limit.

The generation of femtosecond soft x-ray free-electron laser (XFEL) pulses with an energy of hundreds of microjoules has been demonstrated at the Linac Coherent Light Source (LCLS) based on fresh-slice multistage amplifications. In this paper, we present a comprehensive simulation analysis and methods to improve fresh-slice multistage XFELs. A time-dependent transverse kick along the bunch is generated by two passive corrugated structures. The resulting oscillating orbit is controlled by downstream kickers, enabling different slices of the bunch to lase. The simulations reproduce the LCLS experimental results in terms of the pulse energy, bandwidth, and statistics of spectral spikes. The simulations reveal time-domain pulse profile properties, such as the pulse duration and structure, that were unavailable experimentally. We discuss possible issues connected with the accelerator setup and propose a simpler and more robust variation of the scheme to generate XFEL pulses shorter than 3 fs.

Self-seeding is one of the most promising methods to improve the longitudinal coherence and spectral purity of free-electron lasers (FELs). Measurements of the multishot-averaged, soft x-ray self-seeding spectrum at the Linac Coherent Light Source (LCLS) FEL often have a pedestal-like distribution around the seeded wavelength. In this paper, a theoretical model based on the mechanism of spectral sideband generation is developed to explain the statistical behavior of the seed and pedestal. The model is in good agreement with statistical analysis of numerical FEL simulations and experimental measurements that show that pedestal fluctuations reflect the level of long-wavelength microbunching structures in the beam, and are driven by fluctuations of the seed.

Measurements of the soft x-ray, self-seeding spectrum at the Linac Coherent Light Source (LCLS) free-electron laser generally display a pedestal-like distribution around the central seeded wavelength. This pedestal limits the spectral purity and can negatively affect some user applications not employing a post-undulator monochromator. In this paper, we investigate the detailed experimental characteristics of both the amplified seed and its accompanying pedestal using data from a number of separate LCLS shifts over the 2015-2018 time period. We find that the amplified seed shows excellent wavelength stability and an exponential growth rate whose dependence upon energy detuning is consistent with theory. The pedestal’s spectral distribution and integrated strength vary strongly shot by shot, independent of electron beam energy jitter. Its shot-averaged strength relative to that of the seed grows at least linearly with z and can approach values of 15% or more. The pedestal is comprised of two separate components: (1) normal self-amplified spontaneous emission (SASE) whose total strength is nominally insensitive to energy detuning and laser heater (LH) strength; (2) sideband-like emission whose strength positively correlates with that of the amplified seed and negatively with energy detuning and LH strength. We believe this latter, non-SASE component arises from comparatively long wavelength (i.e., λ∼0.3–3 μm) amplitude and phase modulations of the main seeded radiation line. Its shot to shot variability and LH sensitivity suggests an origin connected to growth of the longitudinal microbunching instability on the electron beam.

The method of enhanced self-amplified spontaneous emission (eSASE) is one of the strongest candidates for the generation of sub-femtosecond x-ray pulses in a free-electron laser (FEL). The optimization of an eSASE experiment involves many independent parameters, which makes the exploration of the parameter space with thee-dimensional (3D) simulations computationally intensive. Therefore, a robust theoretical analysis of this problem is extremely desirable. We provide a self-consistent, analytical treatment of such a configuration using a one-dimensional (1D), time-dependent FEL model that includes the key effects of electron beam chirp and undulator taper. Verified via comparison with numerical simulation, our formalism is utilized in parameter studies that seek to determine the optimum setup of the FEL.

The microbunching instability is known to be detrimental to x-ray free electron laser performance. At the Linear Coherent Light Source, the microbunching instability is suppressed with a laser heater, which increases the uncorrelated energy spread of e-beam in the injector. While the current system has been shown to improve x-ray brightness, other laser architectures could further enhance performance. In this study, we model the interaction between a laser and e-beam with arbitrary transverse profiles and examine the effect of various laser designs on the energy distribution of the electrons after the injector and laser heater, as well as their ability to suppress microbunching instability. This simulation incorporates random transverse jitter in order to reproduce physically representative operation of the Linac Coherent Light Source. We compare Gaussian and Laguerre-Gaussian modes, and explore composite beams in the form of an array of Gaussian beamlets. We conclude that the Gaussian laser profile is highly susceptible to e-beam ellipticity and random transverse jitter. The Laguerre-Gaussian profile, a mathematically ideal solution to suppressing microbunching, is less susceptible to these effects and can provide effective suppression even with a distorted e-beam, though performance can be improved by increasing stabilization. The array of beamlets presents a solution that produces consistent and smooth energy distributions with significantly less variance in heating than the Laguerre-Gaussian profile.

The RadiaBeam/SLAC dechirper, a structure consisting of pairs of flat, metallic, corrugated plates, has been installed just upstream of the undulators in the Linac Coherent Light Source (LCLS). As a dechirper, with the beam passing between the plates on axis, longitudinal wakefields are induced that can remove unwanted energy chirp in the beam. However, with the beam passing off axis, strong transverse wakes are also induced. This mode of operation has already been used for the production of intense, multicolor photon beams using the fresh-slice technique, and is being used to develop a diagnostic for attosecond bunch length measurements. Here we measure, as a function of offset, the strength of the transverse wakefields that are excited between the two plates, and also for the case of the beam passing near to a single plate. We compare with analytical formulas from the literature, and find good agreement. This report presents the first systematic measurements of the transverse wake strength in a dechirper, one that has been excited by a bunch with the short pulse duration and high energy found in an x-ray free electron laser.

The microbunching instability driven by beam collective effects in a linear accelerator of a free-electron laser (FEL) facility significantly degrades the electron beam quality and FEL performance. A conventional method to suppress this instability is to introduce an additional uncorrelated energy spread by laser-electron interaction, which has been successfully operated in the Linac Coherent Light Source and Fermi@Elettra, etc. Some other ideas are recently proposed to suppress the instability without increasing energy spread, which could benefit the seeded FEL schemes. In this paper, we propose a reversible electron beam heater using two transverse gradient undulators to suppress the microbunching instability. This scheme introduces both an energy spread increase and a transverse-to-longitudinal phase space coupling, which suppress the microbunching instabilities driven by both longitudinal space charge and coherent synchrotron radiation before and within the system. Finally the induced energy spread increase and emittance growth are reversed. Theoretical analysis and numerical simulations are presented to verify the feasibility of the scheme and indicate the capability to improve the seeded FEL radiation performance.

A novel method using high-power laser as a circular deflector is proposed for the measurement of femtosecond (fs) and sub-fs electron beam. In the scheme, the electron beam interacts with a laser pulse operating in a radially polarized doughnut mode (TEM_{01^{*}}) in a helical undulator, generating angular kicks along the beam in two directions at the same time. The phase difference between the two angular kicks makes the beam form a ring after a propagation section with appropriate phase advance, which can reveal the current profile of the electron beam. Detailed theoretical analysis of the method and numerical results with reasonable parameters are both presented. It is shown that the temporal resolution can reach up to ∼100 attosecond, which is a significant improvement for the diagnostics of ultrashort electron beam.

We propose a method based on the slice energy spread modulation to generate strong subpicosecond density bunching in high-intensity relativistic electron beams. A laser pulse with periodic intensity envelope is used to modulate the slice energy spread of the electron beam, which can then be converted into density modulation after a dispersive section. It is found that the double-horn slice energy distribution of the electron beam induced by the laser modulation is very effective to increase the density bunching. Since the modulation is performed on a relativistic electron beam, the process does not suffer from strong space charge force or coupling between phase spaces, so that it is straightforward to preserve the beam quality for terahertz (THz) radiation and other applications. We show in both theory and simulations that the tunable radiation from the beam can cover the frequency range of 1–10 THz with high power and narrow-band spectra.

In a free-electron laser, a transverse momentum offset (or “kick”) results in an oscillation of the centroid of the electron beam about the undulator axis. Studying the influence of this effect on the free-electron laser (FEL) interaction is important both from a tolerance point of view and for its potential diagnostic applications. In this paper, we present a self-consistent theoretical analysis of a high-gain FEL driven by such a “kicked” beam. In particular, we derive a solution to the three-dimensional, linearized initial value problem of the FEL through an orthogonal expansion technique and also describe a variational method for calculating the average FEL growth rate. Our results are benchmarked with genesis simulations and provide a robust theoretical background for a comparison with previous analytical results.

Utilizing laser-driven plasma accelerators (LPAs) as a high-quality electron beam source is a promising approach to significantly downsize the x-ray free-electron laser (XFEL) facility. A multi-GeV LPA beam can be generated in several-centimeter acceleration distance, with a high peak current and a low transverse emittance, which will considerably benefit a compact FEL design. However, the large initial angular divergence and energy spread make it challenging to transport the beam and realize FEL radiation. In this paper, a novel design of beam transport system is proposed to maintain the superior features of the LPA beam and a transverse gradient undulator (TGU) is also adopted as an effective energy spread compensator to generate high-brilliance FEL radiation. Theoretical analysis and numerical simulations are presented based on a demonstration experiment with an electron energy of 380 MeV and a radiation wavelength of 30 nm.

We propose a novel scheme to generate attosecond soft x rays in a self-seeded free-electron laser (FEL) suitable for enabling attosecond spectroscopic investigations. A time-energy chirped electron bunch with additional sinusoidal energy modulation is adopted to produce a short seed pulse through a self-seeding monochromator. This short seed pulse, together with high electron current spikes and a cascaded delay setup, enables a high-efficiency FEL with a fresh bunch scheme. Simulations show that using the Linac Coherent Light Source (LCLS) parameters, soft x-ray pulses with a FWHM of 260 attoseconds and a peak power of 0.5 TW can be obtained. This scheme also has the feature of providing a stable central wavelength determined by the self-seeding monochromator.

Measurements of the multishot-averaged, soft x-ray, self-seeding spectrum at the LCLS free-electron laser often have a pedestal-like distribution around the seeded wavelength, which limits the spectral purity and can negatively affect some user applications not employing a post-undulator monochromator. In this paper, we study the origins of such pedestals, focusing on longitudinal phase space modulations produced by the microbunching instability upstream of the free-electron laser (FEL) undulator. We show from theory and numerical simulation that both energy and density modulations can induce sidebands in a high-gain, seeded FEL whose fractional strength typically grows as the square of the undulator length. The results place a tight constraint on the longitudinal phase space uniformity of the electron beam for a seeded FEL, possibly requiring the amplitude of long-wavelength modulations to be much smaller than the typical incoherent energy spread if the output sideband power is to remain only a couple percent or less of the amplified seed power.