Experiments with bunch trains in LEP

During 1994 tests were made to evaluate the possibility of operating LEP with four equidistant bunch trains in each beam during 1995. A train will consist of up to four bunches with a bunch spacing of some 75 m. The bunch trains collide head-on. They are separated at the parasitic collision points on either side of the interaction points by vertical electrostatic separator bumps. The accumulation of bunch trains in a single beam was studied with magnetically simulated bumps. Following the installation of additional separators late in 1994, the parasitic encounters of the bunch trains near the interaction points were studied for one train of electron bunches colliding with one train of positron bunches in two diametrically opposite interaction points. The higher order mode losses in the superconducting RF cavities were measured for several arrangements of beams, trains and bunches. The synchrotron radiation background, enhanced by the off-centred beams in the quadrupoles inside the separator bumps, is reduced by super-imposing asymmetric magnetic bumps such that the offset of the incoming beams is reduced.