Abstract
The successful realization of high gain free-electron lasers has opened new possibilities to X-ray scientists for investigating matter in different states. The availability of unprecedented photon properties stimulated the development of new experimental techniques capable of taking full advantage of these options and has started a virtuous collaboration between machine experts and photon users to improve further and optimize the generated X-ray pulses. Over the recent years, this has led to the development of several advanced free-electron laser (FEL) schemes to tailor the photon properties to specific experimental demands. Presently, tunable wavelength X-ray pulses with extremely high brilliance and short pulse characteristics are a few of the many options available at FELs. Few facilities can offer options such as narrowband or extremely short pulses below one fs duration and simultaneous pulses of multiple colors enabling resonant X-ray pump—X-ray probe experiments with sub fs resolution. Fully coherent X-ray radiation (both spatial and temporal) can also be provided. This new option has stimulated the application of coherent control techniques to the X-ray world, allowing for experiments with few attoseconds resolution. FELs often operate at a relatively low repetition rate, typically on the order of tens of Hz. At FLASH and the European XFEL, however, the superconducting accelerators allow generating thousands of pulses per second. With the implementation of a new seeded FEL line and with an upgrade at FLASH linac, all the new features will become available in the soft X-ray spectral range down to the oxygen K edge with unprecedented average photon flux due to the high repetition rate of pulses.
Highlights
The advent of high gain single-pass free-electron lasers (FEL) [1,2,3] and the subsequent development of FEL user facilities [4,5,6,7,8,9,10] have led the scientific progress in many fields making completely new options available to scientists
FLASH is the XUV and soft X-ray free-electron laser (FEL) user facility operated by DESY [4,34]
To verify that the double stage compression scheme designed for FLASH2020+ will be capable of supporting external seeding, dedicated numerical and analytical studies of microbunching amplification along the linac have been performed [69,70,71]
Summary
The advent of high gain single-pass free-electron lasers (FEL) [1,2,3] and the subsequent development of FEL user facilities [4,5,6,7,8,9,10] have led the scientific progress in many fields making completely new options available to scientists. With such an upgrade new scientific avenues are opened in various fields: in general, the higher repetition rate ideally caters to photon-hungry experiments that need to average over many (thousands) shots to accumulate statistically meaningful data Very often though, such experiments provide an unprecedented information depth making them very interesting for scientists. Such techniques often provide information that is unavailable to more traditional methods, such as the spatial motion of excitations in transient grating methods [54,55] Some of those experiments can be performed in a quasi-background-free manner, but full harvesting of results from such novel X-ray methods requires fully coherent beams at high repetition rate in order to bring weak signal levels above noise floors. To meet the stringent requirements of a seeded FEL at the full repetition rate of the FLASH superconducting accelerator
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