title: FemtoSpeX: a versatile optical pump-soft X-ray probe facility with 100 fs X-ray pulses of variable polarization.
abstract: Here the major upgrades of the femtoslicing facility at BESSY II (Khan et al., 2006) are reviewed, giving a tutorial on how elliptical-polarized ultrashort soft X-ray pulses from electron storage rings are generated at high repetition rates. Employing a 6 kHz femtosecond-laser system consisting of two amplifiers that are seeded by one Ti:Sa oscillator, the total average flux of photons of 100 fs duration (FWHM) has been increased by a factor of 120 to up to 10(6) photons s(-1) (0.1% bandwidth)(-1) on the sample in the range from 250 to 1400 eV. Thanks to a new beamline design, a factor of 20 enhanced flux and improvements of the stability together with the top-up mode of the accelerator have been achieved. The previously unavoidable problem of increased picosecond-background at higher repetition rates, caused by `halo' photons, has also been solved by hopping between different `camshaft' bunches in a dedicated fill pattern (`3+1 camshaft fill') of the storage ring. In addition to an increased X-ray performance at variable (linear and elliptical) polarization, the sample excitation in pump-probe experiments has been considerably extended using an optical parametric amplifier that supports the range from the near-UV to the far-IR regime. Dedicated endstations covering ultrafast magnetism experiments based on time-resolved X-ray circular dichroism have been either upgraded or, in the case of time-resolved resonant soft X-ray diffraction and reflection, newly constructed and adapted to femtoslicing requirements. Experiments at low temperatures down to 6 K and magnetic fields up to 0.5 T are supported. The FemtoSpeX facility is now operated as a 24 h user facility enabling a new class of experiments in ultrafast magnetism and in the field of transient phenomena and phase transitions in solids.
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Karsten
authors.initials: K
authors.lastname: Holldack
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Bahrdt
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Balzer
authors.affiliation: Fakultät für Physik, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47048, Germany.
authors.forename: Uwe
authors.initials: U
authors.lastname: Bovensiepen
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Maria
authors.initials: M
authors.lastname: Brzhezinskaya
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Alexei
authors.initials: A
authors.lastname: Erko
authors.affiliation: Fakultät für Physik, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47048, Germany.
authors.forename: Andrea
authors.initials: A
authors.lastname: Eschenlohr
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Rolf
authors.initials: R
authors.lastname: Follath
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Firsov
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Winfried
authors.initials: W
authors.lastname: Frentrup
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Loïc
authors.initials: L
authors.lastname: Le Guyader
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Torsten
authors.initials: T
authors.lastname: Kachel
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Kuske
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Rolf
authors.initials: R
authors.lastname: Mitzner
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Roland
authors.initials: R
authors.lastname: Müller
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Niko
authors.initials: N
authors.lastname: Pontius
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Torsten
authors.initials: T
authors.lastname: Quast
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Ilie
authors.initials: I
authors.lastname: Radu
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Jan Simon
authors.initials: JS
authors.lastname: Schmidt
authors.affiliation: II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, Köln 50937, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Schüssler-Langeheine
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Mike
authors.initials: M
authors.lastname: Sperling
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Stamm
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Trabant
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin 12489, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Föhlisch
keywords: diffractive optics; elliptical undulators; femtosecond X-ray pulses; slicing; storage ring; time-resolved X-ray spectroscopy; ultrafast science