Focusing of the CERN muon storage ring

The polarized muons circulate in a uniform magnetic field. Vertical focusing of the muons is achieved with an electrostatic quadrupole field. The combined action of magnetic and electric field enhances considerably the probability of breakdown. This is due to trapped electrons which circulate around the electrodes and ionize the molecules of the rest gas. However, with reasonable vacuum (<10/sup -6 / Torr) the build-up of charge is slow. Therefore a pulsed electrostatic quadrupole field is applied, the field being on during the storage time of the muons, i.e. 1 ms. In this way the system is operational. (4 refs).