A ''monochromatic'' RF-cavity

The Electron-Positron-Accumulation ring (EPA)in the LEP injector chain acts as a particle buffer between the fast-cycling (100Hz) LEP Injector Linac (LIL) and the slow-cycling (0.8 Hz) CERN Proton Synchrotron (CPS). EPA has a circumference of 126m and has operated after start-up in June 1986 at 500 MeV. Only e- have been stored while e+ operation is scheduled for March 1987. The nominal EPA intensities are 2.5 X 1010 e- per bunch corresponding to a total current of 76 mA and 38 mA in 8 bunches. However, a current of 240 MA has already been stored. A nominal LIL pulse is 12 ns long but 25 ns is typical. The intensity is < 3.109/pulse. In order to have a large longitudinal acceptance the lowest possible harmonic number h=8 was chosen for EPA yielding frf=19 MHz. The maximum r.f. voltage Vrf=50 kV is determined by the momentum acceptance (+-1.2%) of the bucket required by the e- LIL pulse. The longitudinal matching to the CPS did not impose further constraints except that adiabatic reduction of Vrf to about 10 kV prior to ejection is required if the CPS accelerates with frf=7.6 MHz. Since the 8 bunches are closely spaced, parasitic higher modes in the cavity have no time to decay. Therefore, efficient damping of these modes is required. Care was taken that other ring elements would not produce high-Q parasitic resonances