Seeded Free-Electron Lasers and Free-Electron Laser Applications

Abstract

Several decades after the groundbreaking discovery of X-rays in 1895 by Wilhelm Conrad Roentgen, it became quite evident that the possibility of building X-ray sources with controlled photon beam parameters was a revolutionary step to address some of the important challenges facing humanity. The awareness that these sources could dramatically improve mankind's scientific knowledge and technological ability was at the base of significant investments and scientific programs worldwide that brought X-ray sources into the present “golden age.”

The two milestones after Roentgen's discovery are represented by the observation of the first synchrotron radiation in 1947 at the General Electric research laboratory (see, for example, [1]) and, a few decades later, the first development of a free electron laser (FEL) by J. Madey [2]. The subsequent development of the Self-Amplified Spontaneous Emission (SASE) process paved the way for the X-ray FEL [3].

Nowadays, novel radiation sources are developed to a stage that thousands of scholars routinely carry on experiments using X-ray beams at storage-ring-based synchrotrons and LINAC-based FELs. Henceforth, important questions ranging from the microscopic structure and mechanisms of inanimate matter and even living matter have been unveiled. In the meantime, technological applications fundamental for improving human health, energy-related processes, and electronics and information processing have been made possible. Because of the very high peak brilliance and a photon-pulse time structure in the femtosecond-time regime, FELs have shown unmatched possibilities for performing coherent imaging experiments and studies of systems out of equilibrium by using stroboscopic-based time-resolved spectroscopies.

The operation of the first soft X-ray FEL (FLASH) at DESY, followed by the first hard X-ray FEL (the Linac Coherent Light Source, LCLS) at SLAC, has opened the route for a new era in science and a strong worldwide user community has been built. Currently, novel FEL facilities in Japan (SACLA) and in Italy (FERMI at the Elettra Sincrotrone) are operational and others are under construction in South Korea (Pohang FEL), Switzerland (Swiss FEL), the European XFEL in Hamburg, and a second FEL (LCLS II) at SLAC.

The FELs in Hamburg (FLASH and the European XFEL) and LCLS II, based on a modern superconducting linear accelerator, will operate with hard and soft ultrashort X-ray pulses at very high repetition rate, radically increasing the average brilliance and the signal statistics in several critical coherent optics scattering experiments and spectroscopy studies.

Despite the remarkable success of SASE FELs, their limited coherence properties have yet to allow the full potential of the X-ray laser. Only by inducing the FEL process with a “seed” pulse is it possible to achieve almost fully coherent EUV and soft X-ray pulses tailored to specific science needs. With the first external seeding performed at FERMI [4, 5] and self-seeding at LCLS [6], these coherent FEL sources are now available to users. FEL seeding, along with full control of polarization and fine photon energy tunability, enables the cinematic imaging of dynamics, revealing the structure of heterogeneous systems and allowing for development of novel nonlinear X-ray spectroscopy along with the study of magnetic dynamics in the sub-ps time scale.

The recent literature on these topics shows that seeded FELs have unlocked the route for studying more advanced and interesting systems such as “quantum materials,” including unconventional superconductors, multiferroics, topological insulators, colossal magneto-resistance compounds, and nanostructures. More advanced seeding schemes, such as the oscillator FEL [7], could push FEL bandwidths into nuclear physics regimes. Furthermore, the full coherence of seeded FELs pulses opens the possibility of studying the statistical properties of an X-ray beam after the interaction with matter, measuring the properties of the coherent state of matter both in and out of equilibrium.

Finally, the coherence properties of the FEL could even lead to improvements in the FEL itself; more efficient “tapering” to extract energy from the electron beam could pave the way to the TW power regime [8].

Altogether, this arsenal of tools and concepts holds the promise that we are soon at the edge of another stage in mankind's never-ending story of discovery and knowledge.