VERTICAL INSTABILITY AT IPNS RCS∗

The Rapid Cycling Synchrotron (RCS) of the Intense Pulsed Neutron Source (IPNS) at ANL accelerates> 3.0× 1012 protons from 50 MeV to 450 MeV with 30-Hz repe-tition frequency. During the acceleration cycle, the rf fre-quency varies from 2.21 MHz to 5.14 MHz. Presently, the beam current is limited by a vertical instability. By analyz-ing turn-by-turn beam position monitor (BPM) data, large-amplitude mode 0 and mode 1 vertical beam centroid os-cillations were observed in the later part of the acceleration cycle. The oscillations start in the tail of the bunch, build up, and remain localized in the tail half of the bunch. This vertical instability was compared with a head-tail instabil-ity that was intentionally induced in the RCS by adjusting the trim sextupoles. It appears that our vertical instability is not a classical head-tail instability [1]. More data anal-ysis and experiments were performed to characterize the instability