From DESY: “Profiling FLASH electron bunches on a femtosecond scale”

DESY
DESY

2017/05/26
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Scientists use external seeding to monitor few-femtosecond slices of ultra-relativistic electron bunches.

The success of FELs, having a transformative impact on science with X-rays, relies on the capability of analysing and controlling ultra-relativistic electron beams on femtosecond timescales. One major challenge is to extract tomographic electron slice parameters for each bunch instead of projected electron beam properties. A team of scientists has developed an elegant method to derive the slice emittance from snapshots of electron bunches with femtosecond resolution. Mapping of electron slice parameters and seeded FEL pulse profiles is an important ingredient for both, today’s large scale facilities and future compact table-top FELs and creates new opportunities for tailored photon beam applications. The project team headed by Jörg Rossbach from the University of Hamburg, DESY photon scientist Tim Laarmann and DESY accelerator physicist Jörn Bödewadt, reports its work in the journal Scientific Reports.

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View into the seeding area of FLASH (photo: Dirk Nölle).

DESY/FLASH

Since 2005, DESY´s free-electron laser FLASH in Hamburg delivers ultra-short high-brilliance photon pulses to a wide range of scientific users. The light pulses are generated by electron bunches that are accelerated to a velocity close to the speed of light. These bunches have lengths of less than 100 μm, the diameter of a human hair. After acceleration, they traverse a series of magnets with alternating polarities, the undulator, and emit bright, soft X-ray light. While a synchrotron light source like PETRA III works very similar, a free-electron laser makes use of a further phenomenon: “During the emission process, different parts of the electron bunch organize themselves into thin microbunches with a distance of the wavelength of the emitted light,” explains principal author and PhD student Tim Plath. “Several parts of the bunch undergo this process with slightly different wavelengths and phases leading to a spiky structure of the spectrum. It is in the nature of this spontaneous amplification process that the properties are slightly different from shot to shot. This process is called self-amplified spontaneous emission (SASE) and is routinely used at many FEL facilities”.

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Experimental setup of the seeding experiment at FLASH. From left: The beam comes from the linear accelerator and is overlapped with an external seed laser. In the modulator the laser imprints an energy modulation on the electron bunch that gets transformed to a density modulation by the bunching chicane. The formed microbunches can then coherently emit radiation in the radiator. The experimental setup is followed by a diagnostic for the photons and the rf deflector that can diagnose the electron bunch distribution (picture: Tim Plath, UHH/DESY).

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DESY is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

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