Self-amplified spontaneous emission free-electron laser with an energy-chirped electron beam and undulator tapering

Ultrashort large-bandwidth X-ray free-electron laser generation with a dielectric-lined waveguide

Large-bandwidth pulses produced by cutting-edge X-ray free-electron lasers (FELs) are of great importance in research fields like material science and biology. In this paper, a new method to generate high-power ultrashort FEL pulses with tunable spectral bandwidth with spectral coherence using a dielectric-lined waveguide without interfering operation of linacs is proposed. By exploiting the passive and dephasingless wakefield at terahertz frequency excited by the beam, stable energy modulation can be achieved in the electron beam and large-bandwidth high-intensity soft X-ray radiation can be generated. Three-dimensional start-to-end simulations have been carried out and the results show that coherent radiation pulses with duration of a few femtoseconds and bandwidths ranging from 1.01% to 2.16% can be achieved by changing the undulator taper profile.

Controllable X-Ray Pulse Trains from Enhanced Self-Amplified Spontaneous Emission

We report the demonstration of optical compression of an electron beam and the production of controllable trains of femtosecond, soft x-ray pulses with the Linac Coherent Light Source (LCLS) free-electron laser (FEL). This is achieved by enhanced self-amplified spontaneous emission with a 2  μm laser and a dechirper device. Optical compression was achieved by modulating the energy of an electron beam with the laser and then compressing with a chicane, resulting in high current spikes on the beam which we observe to lase. A dechirper was then used to selectively control the lasing region of the electron beam. Field autocorrelation measurements indicate a train of pulses, and we find that the number of pulses within the train can be controlled (from 1 to 5 pulses) by varying the dechirper position and undulator taper. These results are a step toward attosecond spectroscopy with x-ray FELs as well as future FEL schemes relying on optical compression of an electron beam.

Longitudinal Phase-Space Manipulation with Beam-Driven Plasma Wakefields

The development of compact accelerator facilities providing high-brightness beams is one of the most challenging tasks in the field of next-generation compact and cost affordable particle accelerators, to be used in many fields for industrial, medical, and research applications. The ability to shape the beam longitudinal phase space, in particular, plays a key role in achieving high-peak brightness. Here we present a new approach that allows us to tune the longitudinal phase space of a high-brightness beam by means of plasma wakefields. The electron beam passing through the plasma drives large wakefields that are used to manipulate the time-energy correlation of particles along the beam itself. We experimentally demonstrate that such a solution is highly tunable by simply adjusting the density of the plasma and can be used to imprint or remove any correlation onto the beam. This is a fundamental requirement when dealing with largely time-energy correlated beams coming from future plasma accelerators.

Attosecond single-cycle undulator light: a review

Research at modern light sources continues to improve our knowledge of the natural world, from the subtle workings of life to matter under extreme conditions. Free-electron lasers, for instance, have enabled the characterization of biomolecular structures with sub-ångström spatial resolution, and paved the way to controlling the molecular functions. On the other hand, attosecond temporal resolution is necessary to broaden our scope of the ultrafast world. Here we discuss attosecond pulse generation beyond present capabilities. Furthermore, we review three recently proposed methods of generating attosecond x-ray pulses. These novel methods exploit the coherent radiation of microbunched electrons in undulators and the tailoring of the emitted wavefronts. The computed pulse energy outperforms pre-existing technologies by three orders of magnitude. Specifically, our simulations of the proposed Soft X-ray Laser at MAX IV (Lund, Sweden) show that a pulse duration of 50-100 as and a pulse energy up to 5 [Formula: see text]J is feasible with the novel methods. In addition, the methods feature pulse shape control, enable the incorporation of orbital angular momentum, and can be used in combination with modern compact free-electron laser setups.

Short-wavelength free-electron laser sources and science: a review

This review is focused on free-electron lasers (FELs) in the hard to soft x-ray regime. The aim is to provide newcomers to the area with insights into: the basic physics of FELs, the qualities of the radiation they produce, the challenges of transmitting that radiation to end users and the diversity of current scientific applications. Initial consideration is given to FEL theory in order to provide the foundation for discussion of FEL output properties and the technical challenges of short-wavelength FELs. This is followed by an overview of existing x-ray FEL facilities, future facilities and FEL frontiers. To provide a context for information in the above sections, a detailed comparison of the photon pulse characteristics of FEL sources with those of other sources of high brightness x-rays is made. A brief summary of FEL beamline design and photon diagnostics then precedes an overview of FEL scientific applications. Recent highlights are covered in sections on structural biology, atomic and molecular physics, photochemistry, non-linear spectroscopy, shock physics, solid density plasmas. A short industrial perspective is also included to emphasise potential in this area.

Generating Single-Spike Hard X-Ray Pulses with Nonlinear Bunch Compression in Free-Electron Lasers

A simple method for generating single-spike hard x-ray pulses in free-electron lasers (FELs) has been developed at the Linac Coherent Light Source (LCLS). This is realized by nonlinear bunch compression using 20-pC bunch charge, demonstrated in the hard x-ray regime at 5.6 and 9 keV, respectively. Measurements show about half of the FEL shots containing a single-spike spectrum. At 5.6-keV photon energy, the single-spike shots have a mean pulse energy of about 10  μJ with 70% intensity fluctuation and the pulse full width at half maximum is evaluated to be at 200-as level.

Phase-merging enhanced harmonic generation free-electron laser with a normal modulator

A phase-merging enhanced harmonic generation free-electron laser (FEL) was proposed to increase the harmonic conversion efficiency of seeded FELs and promote the radiation wavelength towards the X-ray spectral region. However, this requires a specially designed transverse gradient undulator (TGU) as the modulator to couple the transverse and longitudinal phase space of the electron beam. In this paper, the generation of the phase-merging effect is explored using the natural field gradient of a normal planar undulator. In this method, a vertical dispersion on the electron beam is introduced and then the dispersed beam travels through a normal modulator in a vertical off-axis orbit where the vertical field gradient is selected properly in terms of the vertical dispersion strength and modulation amplitude. The phase-merging effect will be generated after passing through the dispersive chicane. Theoretical analysis and numerical simulations for a seeded soft X-ray FEL based on parameters of the Shanghai Soft X-ray FEL project are presented. Compared with a TGU modulator, using the natural gradient of a normal planar modulator has the distinct advantage that the gradient can be conveniently tuned in quite a large range by adjusting the beam orbit offset.

Chirped pulse amplification in an extreme-ultraviolet free-electron laser

Chirped pulse amplification in optical lasers is a revolutionary technique, which allows the generation of extremely powerful femtosecond pulses in the infrared and visible spectral ranges. Such pulses are nowadays an indispensable tool for a myriad of applications, both in fundamental and applied research. In recent years, a strong need emerged for light sources producing ultra-short and intense laser-like X-ray pulses, to be used for experiments in a variety of disciplines, ranging from physics and chemistry to biology and material sciences. This demand was satisfied by the advent of short-wavelength free-electron lasers. However, for any given free-electron laser setup, a limit presently exists in the generation of ultra-short pulses carrying substantial energy. Here we present the experimental implementation of chirped pulse amplification on a seeded free-electron laser in the extreme-ultraviolet, paving the way to the generation of fully coherent sub-femtosecond gigawatt pulses in the water window (2.3-4.4 nm).

Two-Color Radiation Generated in a Seeded Free-Electron Laser with Two Electron Beams

We present the experimental evidence of the generation of coherent and statistically stable two-color free-electron laser radiation obtained by seeding an electron beam double peaked in energy with a laser pulse single spiked in frequency. The radiation presents two neat spectral lines, with time delay, frequency separation, and relative intensity that can be accurately controlled. The analysis of the emitted radiation shows a temporal coherence and a shot-to-shot regularity in frequency significantly enhanced with respect to the self-amplified spontaneous emission.

Free-electron laser design for four-wave mixing experiments with soft-x-ray pulses

We present the design of a single-pass free-electron laser amplifier suitable for enabling four-wave mixing x-ray spectroscopic investigations. The production of longitudinally coherent, single-spike pulses of light from a single electron beam in this scenario relies on a process of selective amplification where a strong undulator taper compensates for a large energy chirp only for a short region of the electron beam. This proposed scheme offers improved flexibility of operation and allows for independent control of the color, timing, and angle of incidence of the individual pulses of light at an end user station. Detailed numerical simulations are used to illustrate the more impressive characteristics of this scheme.

Two color free-electron laser and frequency beating

We review the theory of two color high gain free-electron laser emission, derive the integral equation characterizing the evolution of the optical intensities, and provide a description of the relevant dynamics. The characteristic feature of this regime is the existence of a mutual bunching, whose origin and role are discussed.

Observation of time-domain modulation of free-electron-laser pulses by multipeaked electron-energy spectrum

We present the experimental demonstration of a new scheme for the generation of ultrashort pulse trains based on free-electron-laser (FEL) emission from a multipeaked electron energy distribution. Two electron beamlets with energy difference larger than the FEL parameter ρ have been generated by illuminating the cathode with two ps-spaced laser pulses, followed by a rotation of the longitudinal phase space by velocity bunching in the linac. The resulting self-amplified spontaneous emission FEL radiation, measured through frequency-resolved optical gating diagnostics, reveals a double-peaked spectrum and a temporally modulated pulse structure.

Characterization of the THz radiation source at the Frascati linear accelerator

The linac driven coherent THz radiation source at the SPARC-LAB test facility is able to deliver broadband THz pulses with femtosecond shaping. In addition, high peak power, narrow spectral bandwidth THz radiation can be also generated, taking advantage of advanced electron beam manipulation techniques, able to generate an adjustable train of electron bunches with a sub-picosecond length and with sub-picosecond spacing. The paper reports on the manipulation, characterization, and transport of the electron beam in the bending line transporting the beam down to the THz station, where different coherent transition radiation spectra have been measured and studied with the aim to optimize the THz radiation performances.

Superradiant cascade in a seeded free-electron laser

We report measurements demonstrating the concept of the free-electron laser (FEL) superradiant cascade. Radiation (λ(rad) = 200 nm) at the second harmonic of a short, intense seed laser pulse (λ(seed) = 400 nm) was generated by the cascaded FEL scheme at the transition between the modulator and radiator undulator sections. The superradiance of the ultrashort pulse is confirmed by detailed measurements of the resulting spectral structure, the intensity level of the produced harmonics, and the trend of the energy growth along the undulator. These results are compared to numerical particle simulations using the FEL code GENESIS 1.3 and show a satisfactory agreement.