Phase-locked MHz pulse selector for x-ray sources

Picosecond x-ray pulses are extracted with a phase-locked x-ray pulse selector at 1.25 MHz repetition rate from thepulse trains of the accelerator-driven multiuser x-ray source BESSY II preserving the peak brilliance at high pulsepurity. The system consists of a specially designed in-vacuum chopper wheel rotating with ≈1 kHz angular frequency.The wheel is driven in an ultrahigh vacuum and is levitated on magnetic bearings being capable of withstandinghigh centrifugal forces. Pulses are picked by 1252 high-precision slits of 70 μm width on the outer rim of thewheel corresponding to a temporal opening window of the chopper of 70 ns. We demonstrate how the electronicphase stabilization of +-2 ns together with an arrival time jitter of the individual slits of the same order of magnitudeallows us to pick short single bunch x-ray pulses out of a 200 ns ion clearing gap in a multibunch pulse train asemitted from a synchrotron facility at 1.25 MHz repetition rate with a pulse purity below the shot noise detectionlimit. The approach is applicable to any high-repetition pulsed radiation source, in particular in the x-ray spectralrange up to 10 keV. The opening window in a real x-ray beamline, its stability, as well as the limits of mechanicalpulse picking techniques in the MHz range are discussed

Tuning the instrument resolution using chopper and time of flight at the small-angle neutron scattering diffractometer KWS-2

Following demand from the user community regarding the possibility of improving the experimental resolution, the dedicated high-intensity/extended Q-range SANS diffractometer KWS-2 of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center in Garching was equipped with a double-disc chopper with a variable opening slit window and time-of-flight (TOF) data acquisition option. The chopper used in concert with a dedicated high-intensity velocity selector enables the tuning at will of the wavelength resolution [Delta][lambda]/[lambda] within a broad range, from 20% (standard) down to 2%, in a convenient and safe manner following pre-planned or spontaneous decisions during the experiment. The new working mode is described in detail, and its efficiency is demonstrated on several standard samples with known properties and on a completely new crystallizable copolymer system, which were investigated using both the conventional (static) and TOF modes