Dresden 2014 – scientific programme
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BE: Beschleunigerphysik
BE 12: Free-Electron Lasers
BE 12.5: Talk
Thursday, April 3, 2014, 10:45–11:00, MOL 213
Comparison of transverse beam profile measurement techniques — •Jan-Niclas Gruse1, Sven Ackermann1,2, Christoph Lechner1, Jörg Rossbach1, and Markus Drescher1 — 1University of Hamburg, Hamburg, Germany — 2Desy, Hamburg, Germany
Free-electron lasers in the UV and extreme ultraviolet are typically operated in self-amplified spontaneous emission (SASE) mode. Starting up from shot-noise, there are typically multiple longitudinal modes present, resulting in poor longitudinal coherence. Using so-called ’seeding’ techniques it is possible to produce photon pulses with greatly improved longitudinal coherence. For successful direct FEL-seeding the energy coupling of the electron bunch and the seed pulse inside the undulator is critical. This requires low divergence photon beams which can be characterized by the beam quality factor M2. One standard technique to determine the M2 is to measure the photon beam size at different longitudinal positions. Due to space constraints such a measurement would be challenging in the undulator. Therefore the seed beam characterization is performed with a dedicated setup. The drawback of using two separated beamlines is that error sources (i.e. deformed mirrors) in one of those branches will result in unpredictable seeding performance. In a recent publication a different approach using modal decomposition of a single image was proposed, holding the promise of a M2 estimation in the undulator beamline.
In this contribution we present studies comparing the results obtained with the longitudinal scanning technique and the modal decomposition method.