Bunch Cleaning Strategies and Experiments at the Advanced Photon Source

The Advanced Photon Source (APS) design incorporated a positron accumulator ring (PAR) as part of the injector chain. In order to increase reliability and accommodate other uses of the injector, APS will run with electrons, eliminating the need for the PAR, provided another method of eliminating rf bucket “pollution” in the APS is found. Satellite bunches captured from an up to 30-ns-long beam from the linac need to be removed in the injector synchrotron and storage ring. The bunch cleaning method considered here relies on driving a stripline kicker with an amplitude modulated (AM) carrier signal where the carrier is at a revolution harmonic sideband corresponding to the vertical tune. The envelope waveform is phased so that all bunches except a single target bunch (eventually to be injected into the storage ring) are resonated vertically into a scraper. The kicker is designed with a large enough shunt impedance to remove satellite bunches from the injection energy of 0.4 GeV up to 1 GeV. Satellite bunch removal in the storage ring relies on the single bunch current tune shift resulting from the machine impedance. Small bunches remaining after initial preparation in the synchrotron may be removed by driving the beam vertically into a scraper using a stripline kicker operating at a sideband corresponding to the vertical tune for small current bunches. In this paper both design specifications and bunch purity measurements are reported for both the injector synchrotron and storage ring. 1 INJECTOR SYNCHROTRON BUNCH CLEANING The APS storage ring is designed to be filled one bunch per cycle at a 2-Hz rate. The principal bunch cleaning requirement is to prepare a single bunch by removing any beam that gets captured in all but a single bucket of the 432 possible injector synchrotron buckets. The bunch cleaning method presented here is based on an rf stripline kicker that is used to deflect the beam vertically into a scraper. Each kicker stripline is driven by a 250-watt broad-band (10 kHz-220 MHz) amplifier. The injector synchrotron ramps linearly from injection (350 MeV) to extraction (7 GeV) in 223 ms. Bunch cleaning will be done at relatively low energies starting at the synchrotron injection energy to Work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Under Contract No. W-31-109-ENG-38. y Email: [email protected] about 1 GeV so that the kicker amplifier power requirement is minimized. Originally, injector synchrotron bunch cleaning was to be accomplished by driving the rf kicker with an amplitude modulated (AM) signal. The choice of AM is motivated by the fact that the kick must be zero at the bucket containing the bunch to be injected into the storage ring. The AM signal has the form, V (t) = A(n!o)cos((m )!ot) (1) A(n!o) = Vosin( n!ot