Instability caused by electron cloud in combined function magnets: the FERMILAB experience

Electron cloud can lead to a fast instability in intense proton and positron beams in circular accelerators. In the Fermilab Recycler the electron cloud is confined within its combined function magnets. We show that the field of combined function magnets trapsthe electron cloud, pre-sent the results of analytical estimates of trapping, and compare them to numerical simulations of electron cloud formation. The electron cloud is located at the beam center and up to 1% of theparticles can be trapped by themag-netic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electrons signifi-cantly increases the density of the cloud on the next revo-lution. In a Recycler combined function dipole this multi-turn accumulationallows the electron cloud reaching final intensities orders of magnitude greater than in a pure di-pole. The multi-turn build-up can be stopped by injection of a clearing bunch of 1010p at any position in the ring.