FemtoSpeX: a versatile optical pump–soft X‐ray probe facility with 100 fs X‐ray pulses of variable polarization

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.