Ebook: Physics of and Science with X-Ray Free-Electron Lasers

Since the first lasing of the Linac Coherent Light Source in the hard X-ray spectral region and FLASH and FERMI in the soft X-ray region, many other X-ray Free-Electron Lasers (X-FELs) have been designed, built and commissioned, in Asia, USA and Europe to explore atomic and molecular science at the Ångström-femtosecond space and time scales. Great progress has been made in improving their performance and extending the capabilities of X-FELs, improving the peak power and longitudinal coherence, generating pulses with femtosecond and shorter duration. At the same time, the X-FEL user community has made tremendous progress in developing experimental techniques that exploit the unique properties of X-FELs to explore atomic and molecular systems of interest to physics, chemistry, biology and material sciences, at the Ångström-femtosecond space and time scales. After the early proof-of-principle experiments, FEL-based X-ray science has been growing at a rapid rate, continuously adding new technical and experimental capabilities. As a result, our knowledge of atomic and molecular science has been largely extended. New results, possible only using X-ray FELs, have been obtained also in materials science and high energy density science, including phenomena relevant to the state of matter in the interior of planets and stars.

Even if great progress has been made, there is still much to be accomplished, and the potential for discovery with X-FELs is still largely unexplored. To take advantage of the unmatched flexibility of X-FELs, the next generation of scientists will have to be well versed in both particle beams/FEL physics and X-ray photon science.

The Italian Physical Society International School of Physics “Enrico Fermi”, on “Physics of and Science with X-ray Free Electron Lasers”, will be the seed of this approach to X-FEL based science. The multidisciplinary program includes basic accelerator and FEL physics, as well as an introduction to the main research topics in X-FEL based Biology, Atomic Molecular Optical Science, Material Sciences, High Energy Density Physics and Chemistry. The lectures presented at the school will introduce graduate students and young scientists to this fast growing and exciting scientific area and help bridge the gap between accelerator/FEL physicists and scientists working in all these disciplines, encouraging collaboration across different fields of X-FEL based science.

The lectures start with a discussion of the physics and technology of X-ray free-electron lasers, then cover the interaction of X-ray with matter and results obtained in different fields using X-ray FELs, including some very recent work, giving a wide view of the research being actively done.

We wish to express our gratitude to the Italian Physical Society, SLAC National Accelerator Laboratory, Sincrotrone Trieste, European X-FEL for supporting the course. Our gratitude and thanks go to the staff of the Varenna School for their help and assistance during the course and our memorable stay in Varenna.

J. Hastings, C. Pellegrini and A. Marinelli

The paper presents basic elements of the accelerator beam physics in application to free-electron lasers.

This paper is intended to introduce the physics of X-ray free-electron lasers in the one-dimensional framework.

We derive the equations for the growth of the power starting from a pre-bunched beam and provide a simple method to predict the power performances of a seeded free-electron laser device. The formalism introduced is used in the analysis of other features of a seeded FEL, as the pulse duration.

This contribution summarises the theoretical basics for the theory of X-ray matter interaction and is based on a quantised description of the electromagnetic field. In view of a formally transparent presentation, we adopt the approach of second quantisation for the electronic degrees of freedom. We start from the minimal-coupling Hamiltonian that in principle contains all levels of electron correlation and treat the X-ray matter interaction in lowest-order perturbation theory. We give the explicit derivation of the photoionisation (X-ray absorption) cross section and the doubly differential cross sections for elastic and inelastic Thomson scattering, as well as resonant inelastic X-ray scattering. A connection to relevant experimental techniques is attempted. At some instances, the electronic Hamiltonian is approximated by a mean-field Hamiltonian, in order to facilitate the interpretation of the results and connect experimental observables with the underlying fundamental quantities of the quantum system under study.

The development of x-ray free-electron lasers has provided a new x-ray source with unprecedented properties. Understanding the basics of x-ray scattering and its implications for x-ray optics to make optimal use of this new radiation source is critical. This paper introduces concepts in x-ray scattering, x-ray optics and coherent diffraction imaging. The basics are presented at a level for advanced undergraduates and first year graduate students. The intent is to give entry into the subject, not a comprehensive discussion, allowing the reader to go to one or more texts on the subject with the basics in hand.

The recent advent of Free Electron Lasers (FEL) facilities permits to push experimental techniques typical of table top pulsed laser towards much shorter wavelength allowing to probe dynamical processes occurring in molecular and nanostructured materials with an unprecedented time-space resolution. Within the portfolio of FEL-based experimental methods we will discuss the new opportunities offered by the extension of non-linear spectroscopies in the vacuum ultraviolet to soft X-ray energy range. Pioneering wave mixing experiments have been successfully carried out at the FERMI FEL, demonstrating that second-harmonic generation and four-wave mixing experiments are now feasible at nanometer wavelength. These results pave the way to a new class of experiments like investigation of heat transfer at the nanoscale or energy transfer in light harvesting devices.

Extreme states of matter with high temperatures and pressures can be created by irradiating matter with either intense X-rays emitted by X-ray free-electron lasers (FELs), and by heating and/or compression with optical lasers and then using the FEL X-rays as a probe. We provide here a very basic introduction to this burgeoning field, highlighting a few specific experiments, and signposting some directions for future exploration.

Recent advances in laser technology have made possible the generation of precisely shaped strong-field pulses at terahertz frequencies. These pulses are especially useful to selectively drive collective modes of solids, for example exciting the lattice to very large amplitudes. One can consider different types of lattice excitations, including rectification and high-order harmonics. Here, I discuss the fundamentals of the coherent control of the lattice. I also show how lattice excitation can be used to switch the electronic and magnetic phases of solids. I discuss experiments in which lattice excitation drives changes in the conductivity or enhancement in superconductivity.

Modern photon science facilities, such as X-ray Free-Electron Lasers (XFEL) are becoming increasingly combined accelerator and laser facilities. Due to the pulse nature of an FEL, ultrafast lasers play an essential role starting from the photo injector laser creating the electron bunch in the gun, over a laser heater to a potential seed laser down to the experimental station, which usually houses pump and/or probe lasers to excite the sample under investigation or probe it from Terahertz to optical frequencies. Over the last decade also great progress has been made in synchronizing all lasers as well as critical microwave sources in such facilities to better than 10 fs r.m.s. and in special cases to the sub-femtosecond level using ultrafast optical techniques. This is one to two orders of magnitude better than possible with microwave techniques. In this tutorial, we first review the principles behind the optical synchronization techniques and show their further development to the sub-femtosecond level. These technological advances will make up the first part of this lecture. In the second part of this lecture we look at the possibility to create compact, fully coherent free-electron laser sources entirely laser driven for intrinsic synchronization of all components for attosecond X-ray imaging and spectroscopy. Here the relativistic electron beam is generated by strong THz pulses generated by optical rectification and difference frequency generation of high-energy picosecond to nanosecond laser pulses and wiggling of the electrons is proposed by a optical undulators.

A short presentation of the X-ray free-electron lasers currently operating, commissioning, or in an advanced state of construction worldwide is given. The main features of each facility are summarized, emphasizing the electron energy, the wavelength range and the time structure. Special attention is given to the Pohang X-ray Free-Electron Laser, to the European XFEL in Hamburg, and to the SwissFEL at the PSI in Villigen, that are currently in an advanced state of commissioning, with an imminent start of experimental activities on some of their planned instruments.

The development of X-ray free-electron lasers (XFELs) has launched a new era in X-ray science by providing ultrafast coherent X-ray pulses with a peak brightness that is approximately one billion times higher than previous X-ray sources. This dramatic advance in capabilities has already had significant impact across broad areas of science including atomic, molecular, and optical science; chemistry; condensed-matter physics; matter in extreme conditions; and biology. This paper presents some representative highlights from the Linac Coherent Light Source (LCLS) and outlines the science motivation for upgrades providing ultrafast coherent X-ray pulses at high repetition rates up to 1 MHz in the soft X-ray range (LCLS-II) and in the hard X-ray range (LCLS-II-HE).