Deutsche Tagung f ¨ur Forschung mit ... - SNI-Portal

Methoden und Instrumentierung Vortrag: Do., 11:30–11:50 D-V43
Wavefront Studies at the Free-Electron Laser FLASH
Elke Plönjes 1 , Marion Kuhlmann 1 , Sven Toleikis 1 , Philippe Zeitoun 2 ,
Julien Gautier 2 , Thierry Lefrou 2 , Denis Douillet 2 , Pascal Mercere 3 , Guillaume
Dovillaire 4 , Marta Fajardo 5
1 HASYLAB at DESY, Notkestr. 85, D-22603 Hamburg, Germany – 2 Laboratoire
d’Optique Appliquee, ENSTA, Chemin de la Huniere, F-91761 Palaiseau cedex, France
– 3 Synchrotron SOLEIL, L’Orme des Merisiers - Saint-Aunin - BP 48, 91192 Gif-sur-
Yvette Cedex, France – 4 Imagine Optic, 18 rue Charles de Gaulle, Orsay France 91400,
France – 5 Centro de Fisica dos Plasmas, Institutio Superior Tecnico, Av. Rovisco Pais,
1049-001 Lisboa, Portugal
The Free-Electron Laser in Hamburg (FLASH) is operated in the “self-amplified spontaneous
emission” (SASE) mode and delivers sub-picosecond radiation pulses, with
gigawatt peak powers. At present, lasing has been observed down to about 13 nm
in the fundamental, the shortest wavelength ever achieved with a free electron laser.
User experiments started in August 2005 and were carried out between 45 nm and 13
nm. For these wavelengths, FEL pulse intensities from typical 5 µJ up to more than
50 µJ have been obtained with pulse lengths between 20 and 50 fs. FEL wavefronts
observations have been recorded using a Hartmann sensor (by Imagine Optic). The
Hartmann principle is based on a pinhole array, which divides the incoming beam into
a large number of sub-rays monitored in intensity and position of individual spots.
The identification of the local slope of the incident wavefront makes the aberrations
from a perfect spherical wavefront visible. Ray tracing backwards accesses the beam
focal point in size and position. The intense and coherent vacuum-ultraviolet FEL
beam of various repetition rates leads to unique requirements for the wavefront sensor
setup. The wavefront studies were carried out in the context of commissioning of
the experimental stations. A further goal is to provide an online diagnostic tool for
experiments which need to determine beam parameters which can vary with the shot
to shot characteristic of the FEL, e.g. the focal spot size. We report measurements of
the metrology of flat and curved mirrors at FLASH beamlines. The effects of solid and
gaseous filters are selectively described in the wavelength regime of 10 nm to 32 nm.
The use of wavefront measurements to provide reliable machine parameter is discussed.
The wavefront sensor proved to be a valuable tool to determine the FEL beam quality
and the performance of optical elements, filters and diagnostic tools.