A method is proposed to generate trains of few-cycle x-ray pulses from a free-electron laser (FEL) amplifier via a compact ``afterburner'' extension consisting of several few-period undulator sections separated by electron chicane delays. Simulations show that in the hard x ray (wavelength $$\sim${}0.1\text{ }\text{ }\mathrm{nm}$; photon energy $$\sim${}10\text{ }\text{ }\mathrm{keV}$) and with peak powers approaching normal FEL saturation (GW) levels, root mean square pulse durations of 700 zs may be obtained. This is approximately two orders of magnitude shorter than that possible for normal FEL amplifier operation. The spectrum is discretely multichromatic with a bandwidth envelope increased by approximately 2 orders of magnitude over unseeded FEL amplifier operation. Such a source would significantly enhance research opportunity in atomic dynamics and push capability toward nuclear dynamics.

A method to achieve high-brightness self-amplified spontaneous emission (HB-SASE) in the free-electron laser (FEL) is described. The method uses repeated nonequal electron beam delays to delocalize the collective FEL interaction and break the radiation coherence length dependence on the FEL cooperation length. The method requires no external seeding or photon optics and so is applicable at any wavelength or repetition rate. It is demonstrated, using linear theory and numerical simulations, that the radiation coherence length can be increased by approximately 2 orders of magnitude over SASE with a corresponding increase in spectral brightness. Examples are shown of HB-SASE generating transform-limited FEL pulses in the soft x-ray and near transform-limited pulses in the hard x-ray. Such pulses may greatly benefit existing applications and may also open up new areas of scientific research.

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 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.

The UK New Light Source (NLS) project identified a strong scientific case for a next-generation light source to deliver continuous coverage of the photon energy range 50–1000 eV with variable polarisation, 20 fs pulse widths, a high degree of temporal coherence and 1 kHz repetition rate. Three separate seeded free-electron lasers (FELs) were proposed which in combination satisfy these requirements. It was proposed to use a high harmonic generation (HHG) seed source tuneable from 50–100 eV to give direct seeding at the fundamental FEL photon energy up to 100 eV, with one or two stages of harmonic up-conversion within the FEL to reach the higher photon energies. This paper focuses on the optimisation of a two-stage harmonic cascade FEL design operating at the highest (fundamental) photon energy of 1000 eV. Topics investigated include the configuration of the modulator undulators, the required seed power and the effects of the temporal structure of the unfiltered HHG seed on the FEL output. FEL simulations using realistic electron beam distributions tracked from the gun to the FEL are presented, illustrating the predicted coherence properties of the FEL output.

The compact linear accelerator for research and applications (CLARA) is a 250 MeV ultrabright electron beam test facility at STFC Daresbury Laboratory. A user beamline has been designed to maximize the exploitation of CLARA in a variety of fields, including novel acceleration and new modalities of radiotherapy. In this paper, we present the specification and design of this beamline for full energy beam exploitation. We outline the key elements that will provide users access to ultrashort, low emittance electron bunches in two large experiment chambers. The results of start-to-end simulations are reported that verify the expected beam parameters delivered to these chambers. Key technical systems are detailed, including those which facilitate a combination of electron bunches with high-power laser pulses. Published by the American Physical Society 2024

X-ray free-electron lasers (FELs) are powerful photon sources offering a wide wavelength range, subfemtosecond pulse duration, and high brightness. Most x-ray FELs are based on self-amplified spontaneous emission (SASE). SASE-FEL radiation has excellent transverse but only limited longitudinal coherence, with power and spectral profiles consisting of multiple randomly distributed spikes. In this Letter, we present the first experimental demonstration of mode-locked SASE, which generates periodic trains of phase-locked subfemtosecond pulses, thus providing an x-ray analog of the optical frequency comb. Our approach combines the mode-coupled SASE scheme, where magnetic chicanes between the undulator modules of the FEL increase the coherence of the output radiation, and an external optical laser that restricts the FEL amplification to periodic and short regions of the electron bunch. The work relies on evidence in the frequency and time domains for photons and electrons, respectively, and will benefit investigations of ultrafast dynamics as well as coherent spectroscopy, and enable new types of experiments requiring phase-correlated x-ray pulses.