The research activities of our group comprise the investigation of advanced seeding techniques at free electron lasers and the improvement of superconducting resonators for modern particle accelerators.
Free Electron Lasers - Advanced Seeding Techniques for XUV and Angstrom Radiation
Free Electron Lasers (FEL) have proven to be a powerful source of XUV and Angstrom radiation. Most FELs worldwide use the so-called SASE principle, where the radiation intensity increases exponentially from shot-noise. This principle has turned out to be very robust. However, it suffers from fluctuations in intensity and spectral properties. In addition, the generated radiation has a rather limited longitudinal coherence.
In contrast to SASE, seeding schemes are based on lasing on an integer harmonic of an externally supplied (external seeding) or on a direct amplification of an internally produced and filtered (self-seeding, FEL oscillator) radiation field. It enables the generation of narrow-band and longitudinally coherent radiation with stable intensity and paves the way to a significant improvement of the emission properties of FELs. In addition, external seeding principally allows a full phase control of the generated radiation and the transfer of special properties like frequency chirp to the XUV radiation.
In order to exploit and further improve the properties of seeded radiation while accessing radiation wavelengths over a wide range extending from 1 to 100 Angstrom, our research focuses on systematic investigations of novel seeding schemes, applying different approaches individually dedicated to specific wavelengths. Our work comprises in-detail studies of “high gain harmonic generation” (HGHG) and “echo-enabled harmonic generation” (EEHG) addressing wavelengths from 60 nm down to a few nm. Since high repetition seed lasers are currently not available, we have just started investigating a novel approach where a single seed laser pulse is introduced into an optical cavity around the modulator and then repeatedly further amplified by the modulated electron beam thus compensating the resonator losses. Furthermore, we are preparing a demonstration experiment at the European XFEL where a diamond-mirror based FEL oscillator (XFELO) will generate fully coherent Angstrom radiation.
Superconducting Radio-Frequency Cavities
For more than two decades, superconducting radio frequency systems (srf cavities) continue to gain importance as one of the key technologies for modern accelerators. Research and development efforts carried out over many years have led to dedicated recipes for optimum construction and treatment of srf cavities made from pure niobium, yielding a reliable performance and high accelerating fields as successfully demonstrated in large scale facilities like the European XFEL.
Single resonators have already shown field gradients close to the performance limit of niobium. In order to achieve groundbreaking improvements in srf technology, new concepts for further enhancing accelerating fields and cavity performance have to be conceived, addressing the challenging demands posed by the planned upgrades of existing or the construction of future accelerators.
Our approach is based on two pillars: A further reduction of RF losses of niobium, for a continuous-wave operation of accelerators, where we tailor the near-surface properties of niobium by recently discovered surface annealing’s in different atmospheres, and study the surface dynamics on samples and cavities to understand the impact of the surface development on the srf performance. Aiming at developing a game-changer in srf, detailed investigations and characterization of the potential of next generation materials and structures have been started. These materials have the potential to pave the way to more compact and efficient next generation accelerators.