A fast X-ray chopper for single-bunch extraction at synchrotron sources

A new X-ray chopper system based on air-bearing technology was developed in a joint project between the Paul Scherrer Institut and DESY. The X-ray chopper wheel of 150 mm diameter has a slit width of 50 µm, which corresponds to an opening time of 230 ns at 30 000 r min^-1. The wheel is equipped with either one or two slits at slightly different radii, resulting in repetition frequencies of 500 and 1000 Hz. To reduce air friction the wheel is operated in a reduced-pressure environment. The chopper is controlled by a specially developed phase lock loop controller, which is synchronized to the master bunch clock of the synchrotron. The very small and constant friction of the bearing and the high mechanical accuracy of all mechanical components in combination with the high-precision controller resulted in a very small long-term jitter of less than 3 ns. The chopper is capable of isolating X-rays generated from a single bunch of the future PETRA III storage ring at DESY operated in 40-bunch mode.