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Profiling FLASH electron bunches on a femtosecond scale

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).
  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”.

In order to improve the spectral properties, i.e. the quality of the light pulse, the scientists arrange the electrons in their microbunches before the light is generated. This technique is called ‘seeding’ and allows for a large degree of control on the emission process. “It is based on the interaction of relativistic electron bunches with external seed laser pulses and gives the opportunity to tailor the time-frequency spectrum of the photon beams in modern soft X-ray FELs on demand”,  points out Jörn Bödewadt.

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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).
For the reconstruction of the photon power profiles, a measurement of the electron bunch after FEL lasing has taken place is sufficient and allows a non-invasive extraction of photon pulse properties. This measurement is conducted with a special diagnostic device called radio-frequency (rf) deflector. When traversing the rf deflector, the electrons get an arrival-time-dependent kick in the vertical plane by an electromagnetic rf field. This means that electrons in the head of the bunch get a kick of a different magnitude than electrons in the tail. On a screen installed at a proper phase behind the rf deflector this kick evolves into a transverse offset. This way, the arrival time of an electron is mapped into its vertical position on the observation screen, leading to a time profile of the bunch. Behind the rf deflector a dipole is installed that bends the electron trajectory based on the kinetic energy of the electron. This dipole spectrometer maps the energy of the electrons to their horizontal position on the observation screen. When generating a photon pulse, the electrons lose energy, since the total amount of energy in a system is conserved. On the observation screen after the rf deflector, the particle energy along the electron bunch can temporally be resolved and the energy drain within the electron bunch enables the non-invasive extraction of the longitudinal photon pulse profile.

“With this method one cannot only monitor the radiation power profile of the seeded FEL pulse, but also map the characteristics of electron slices within the bunch it was radiated from,” notes Jörg Rossbach. “It turns out that confined regions of the electron bunch can be seeded (stimulated to lase) very efficiently, while others only show a moderate amount or even no generated light at all.”

The screen shows the electron energy as a function of electron position (measured in fs units) in the electron bunch. The color codes the amount of electrons in the region. The spike towards lower energies is the trace left in the electron bunch by the seeded radiation process (picture: Tim Plath, UHH/DESY).
With a simple, yet elegant analysis of the measurements the reason for the non-uniform performance in different parts of the electron bunch can be traced back to its local slice properties. For this purpose the scientists superimpose the electron bunch with a seed pulse from the optical laser which is only covering a short part of the bunch, stimulating this part to emit coherent light. Monitoring the time-energy profile of bunches stimulated in that way reveals the performance of the radiation process of single bunch slices and confirms the reconstruction of the electron slice quality parameters and the predicted lasing behaviour calculated by the scientists. “Essential slice quality parameters such as electron energy, energy spread and emittance can be literally speaking caught on the flight with a temporal resolution of a few-femtoseconds only (1 fs = 10-15 s), while the electrons move at a velocity close to the speed of light,” emphasizes Tim Plath. “This extraction enables the prediction of the peak power of the radiated FEL pulse and with this the depth of the trace. The seed pulse then serves as a microscopic probe that locally stimulates the FEL process in a longitudinal section of the electron bunch and validates our predictions.”

Diagnostic tools of this type are of utmost importance for future realization of compact electron guns, electron accelerators, and novel free-electron laser architectures. “Experimental access to slice parameters of the electron bunch helps pushing the temporal and spectral qualities of high-brightness electron or photon beams to their performance limits and beyond for sophisticated user experiments,” concludes Tim Laarmann.


Mapping few-femtosecond slices of ultra-relativistic electron bunches; Tim Plath et al.; Scientific Reports, 2017; DOI: 10.1038/s41598-017-02184-3