Beam confinement in periodic permanent magnet focusing klystrons

Abstract

The confinement of a tightly bunched electron beam is studied in a periodic permanent magnet (PPM) focusing klystron. By analyzing the Hamiltonian dynamics of a train of collinear periodic point charges interacting with a conducting drift tube, an rf field, and an applied PPM focusing field, a space-charge limit is derived for the radial confinement of slightly bunched electron beams, and is shown to be significantly below the well-known Brillouin density limit for an unbunched beam. Several state-of-the-art PPM klystrons developed at SLAC are found to operate close to this limit, shedding some light on the origin of observed beam losses.