BEAM-BEAM LIMIT WITH HOURGLASS EFFECT IN HERA

After the design luminosity had been exceeded in the electron-proton collider HERA, the interaction regions have been rebuilt to boost the luminosity by roughly a fac- tor 4 and are now being commissioned. While the specific luminosity is increased by a reduction of the beta functions at the interaction point and by a reduced horizontal electron emittance, the beam-beam tune shift of the e-beam will be increased and the vertical proton beta function at the inter- action point will become comparable to the proton bunch length. To analyze these new beam dynamical conditions, the beam-beam force's effect on the tune spread and on high order resonances under the presence of the hourglass effect has been investigated, and accelerator studies have been performed in conditions for which the tune spread and the resonance strength are comparable to the luminos- ity upgrade parameters. In these experiments the nominal beam-beam parameter per experiment has been pushed to a record high of 0.5. Based on these tests, the future perfor- mance limits of the HERA e/p collider are discussed.