Wheel with triple sound velocity for pulse selection at BESSY II

In order to pick out only one pulse per turn out of the 400 possible x-ray flashes at BESSY II, a joint team of physicists and engineers from Forschungszentrum Jülich, MPI of Microstructure Physics and HZB have developed an extremely fast rotating “MHz-pulse selector”, which is now at the core of the Uppsala Berlin joint Lab to extract the hybrid bunch within the 200 nanosecond ion clearing gap of BESSY II. The device consists of a wheel made of a special Aluminum alloy which has tiny slits of 70 micrometer width at its outer rim. They move frictionless in a vacuum at triple sound velocity perpendicular to the beam. Users can now decide to operate their experiment in a single bunch mode even during normal multibunch operation of BESSY II.

Ultrashort x-ray flashes as used at one of the more than 50 beamlines at BESSY II are usually generated in electron storage rings by circulating short electron bunches. However, many experiments don’t actually need all of the up to 400 pulses per turn but only one of them. One solution could consist of a wheel equipped with a hole, synchronized with the electron motion, to allow only one pulse to pass through the hole while the others are blocked. But this is not as easy as it sounds. The wheel has to be pretty fast because the pulse arrives every 800 nanoseconds (ns) which means that we are talking about triple sound velocity of roughly 1 km/s, meaning enormous stress on the material!

Indeed, this kind of device has been developed by a joint team of physicists and engineers from Forschungszentrum Jülich, Max-Planck-Institute of Microstructure Physics Halle/S. and HZB and is now available for regular use at a BESSY II beamline. The device, a “MHz-pulse selector” consists of a wheel made of a special Aluminum alloy which has tiny slits of 70 µm width at its outer rim. They move frictionless in a vacuum at triple sound velocity perpendicular to the beam. A high precision “cruise control” keeps the arrival time of the holes with respect to the beam within a margin of 2 ns and makes sure that only one x-ray pulse out of BESSY II’s pulse train arrives at the experiment.

Experimenters at this beamline may now select what they want: a single pulse mode or the quasi-continuous x-ray beam. “This kind of pulse selection will be particularly important for our upgrade project BESSY-VSR that will provide a number of selectable x-ray pulses at different pulse length” Karsten Holldack from the HZB Institute Methods and Instrumentation for Synchrotron Radiation Research explains.

The work is now published in Optics Letters: Phase-locked MHz pulse selector for x-ray sources, Daniel F. Förster, Bernd Lindenau, Marko Leyendecker, Franz Janssen, Carsten Winkler, Frank O. Schumann, Jürgen Kirschner, Karsten Holldack, and Alexander Föhlisch