Observations and cure of head-tail effect in the Argonne rapid-cycling synchrotron

Radial head-tail effects have been observed in the 500-Mev Rapid Cycling Synchrotron (RCS). The threshold intensity of accelerated beam to induce the instability was 4*10/sup 11/ protons/pulse, and the mode numbers observed were m.1 and m.2. Detailed study showed that the radial chromaticity of the lattice at this particular time in the acceleration cycle was positive. This is consistent with theoretical predictions for a machine operating below the transition /gamma/. In order to prevent the onset of this instability, a set of programmable sextupole magnets has been introduced that can be programmed to vary the chromaticity at any point during the acceleration period. With these sextupoles, the machine intensity has passed 2*10/sup 12/ protons/pulse. 3 refs