Beam - Beam Effect and Luminosity in J ’ ) Spear

Many measurements on the beam-beam limit in SPEAR have been performed over the past eight years since colliding beam operation began. The goal for these measurements was to find the proper parameterization of the beambeam effect. Earlier measurements I,21 in SPEAR, however, were limited in their validity by two circumstances. First, until 1978 we had no control over the so-called flip-flop phenomenon. 3) We did not even know about this effect because it seemed natural that due to the beam-beam interaction one of the beams-the "weaker" one-got vertically blown up when high current beams were brought into collision. In 1978 we found, however, that we could choose which beam gets blown up or, what is more important, we could manage to make the particle distribution in both beams the same. This can be done by adjusting the relative phase of the two rf systems located symmetrically on either side of the interaction points. As yet we do not understand this effect, but control of the flip-flop effect resulted in an increase in luminosity by a factor 1.5 to 2 (Fig. 1). All measurements in this report were done with both beams equally blown up. The second shortcoming of the earlier measurements was the limited energy variation possible in SPEAR. This led to erroneous energy scalings of the beam-beam incoherent tune shift parameter 2) 5. In 1979 the magnet power supplies were modified such that operation at energies as low as 400 MeV was possible. We have made colliding beam measurements at energies as low as 600 MeV and together with earlier measurements we can now present the scaling of some relevant storage ring parameters from 600 MeV up to almost 4 GeV. All measurements have been done with a natural beam emittanceof ex(rad m) =5.0x10s8 E2 (GeV2), the wiggler magnets off, and with the following beam dynamic parameters at the interaction point