A Hybrid Magnetic Focusing System for Microwave Tubes

By using a three-dimensional particle-in-cell code, this paper reports on the electron-beam propagation behavior in a drift tube under two situations. First, it investigated the influence of the acceleration potential on the beam trajectory when using a periodic permanent magnet (PPM) focusing structure upon keeping constant either the electric current or the beam perveance. Next, taking for granted the optimum focalization parameters found in the previous evaluation, it examined the feasibility of using a hybrid magnetic focusing system (HS)-a PPM array clamped at the ends by a pair of solenoids-so that the value of the magnetic flux density at the entrance to the PPM system satisfies the Brillouin field condition. An experimental setup of the HS using PPM NdFeB permanent magnets without pole pieces is also reported