No abstract

X-ray Free-electron Lasers (X-ray FELs) give us for the first time the possibility to explore structures and dynamical processes of atomic and molecular systems at the Ångstrom-femtosecond space and time scales. They generate coherent photon pulses with time duration of a few to 100 femtosecond, peak power of 10 to 100 GW, over a wavelength range extending from about 100 nm to less than 1 Å. Using these novel and unique capabilities new scientific results are being obtained in atomic and molecular sciences, in areas of physics, chemistry and biology. In this paper we review the physical principles, the theoretical models and the numerical codes on which X-ray FELs are based, starting from a single electron spontaneous undulator radiation to the FEL collective instability of a high density electron beam, strongly enhancing the electromagnetic radiation field intensity and its coherence properties. We present also a short review of the main experimental properties of X-ray FELs, and discuss the results of the most recent research to improve their longitudinal coherence properties, increase the peak power and generate multicolor spectra.

We show that a free-electron laser oscillator generating x rays with wavelengths of about 1 A is feasible using ultralow emittance electron beams of a multi-GeV energy-recovery linac, combined with a low-loss crystal cavity. The device will produce x-ray pulses with 10{9} photons at a repetition rate of 1-100 MHz. The pulses are temporarily and transversely coherent, with a rms bandwidth of about 2 meV, and rms pulse length of about 1 ps.

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.

Laser-plasma accelerators are prominent candidates for driving next-generation compact light sources, promising high-brightness, few-femtosecond x-ray pulses intrinsically synchronized to an optical laser, and thus are ideally suited for pump-probe experiments with femtosecond resolution. So far, the large spectral width of laser-plasma-driven beams has been preventing a successful free-electron laser (FEL) demonstration using such sources. In this paper, we study the application of an optimized undulator design and bunch decompression to large-energy-spread beams in order to permit FEL amplification. Numerically, we show a proof-of-principle scenario to demonstrate FEL gain in the vacuum ultraviolet range with electron beams from laser-plasma accelerators as currently available in experiments.

The SwissFEL X-ray Free Electron Laser (XFEL) facility started construction at the Paul Scherrer Institute (Villigen, Switzerland) in 2013 and will be ready to accept its first users in 2018 on the Aramis hard X-ray branch. In the following sections we will summarize the various aspects of the project, including the design of the soft and hard X-ray branches of the accelerator, the results of SwissFEL performance simulations, details of the photon beamlines and experimental stations, and our first commissioning results.

No abstract

Energy extraction efficiency of a free electron laser (FEL) can be greatly increased using a tapered undulator and self-seeding. However, the extraction rate is limited by various effects that eventually lead to saturation of the peak intensity and power. To better understand these effects, we develop a model extending the Kroll-Morton-Rosenbluth, one-dimensional theory to include the physics of diffraction, optical guiding, and radially resolved particle trapping. The predictions of the model agree well with that of the GENESIS single-frequency numerical simulations. In particular, we discuss the evolution of the electron-radiation interaction along the tapered undulator and show that the decreasing of refractive guiding is the major cause of the efficiency reduction, particle detrapping, and then saturation of the radiation power. With this understanding, we develop a multidimensional optimization scheme based on GENESIS simulations to increase the energy extraction efficiency via an improved taper profile and variation in electron beam radius. We present optimization results for hard x-ray tapered FELs, and the dependence of the maximum extractable radiation power on various parameters of the initial electron beam, radiation field, and the undulator system. We also study the effect of the sideband growth in a tapered FEL. Such growth induces increased particle detrapping and thus decreased refractive guiding that together strongly limit the overall energy extraction efficiency.