Investigation of the Coherent Beam-Beam Effects in the ISR

Although the beam-beam tune shift is relatively small in the ISR (0.001 per intersection) and below the value where stochastic effects might be expected, coherent beam-beam effects can be important. Each high-intensity ISR beam is close to the transverse single beam stability limit. The addition of the beam-beam force has led to the loss of one of the beams accompanied by coherent oscillations. The change of the betatron frequency distribution due to the other beam has been investigated. The resonance excitation by the non-linear beam-beam force has been measured and seems to influence the transverse stability. Quantitative information about the beam-beam force is obtained by exciting a coherent betatron oscillation in one beam and observing the response of the other beam. This beam-beam transfer function depends on the beam separation in the interaction point and can be used to centre the two beams. At the same time the effective beam height is obtained, which together with the beam currents determines the luminosity.