Extension of the CERN ISR Energy by Acceleration of Stacked Beams

The proton beams in the CERN Intersecting Storage Rings (ISR) have been accelerated to achieve colliding beams at 31.4 GeV/c, equivalent to a momentum of above 2000 GeV/c if a conventional accelerator with a stationary target were used. High density stacks were made at 26.5 GeV/c from which a part was rebunched with the maximum available RF voltage. The rebunched beam was kept near central orbit while the magnetic field was gradually increased to its maximum value. The major problem was to keep the betatron oscillation frequencies unchanged while changing the magnetic field thus avoiding any blow-up of the beam due to resonances. The corrections of the highly non-linear saturation effects of the main magnet were achieved by a computer controlled tuning of the field by excitation of the poleface windings and sextupole lenses. Closed orbit corrections were applied throughout the acceleration. Colliding beams with currents above 0.5 A were obtained.