Multipacting (MP) is a resonant electron discharge, often plaguing radiofrequency structures, produced by the synchronization of emitted electrons with the RF fields and by the electron multiplication at the impact point with the surface of the structure. The current of re-emitted electrons grows via true secondary re-emission when the secondary yield for the primary electron impact energy is greater than one. A simple example (MP in short-gap accelerating axial-symmetric cavities) allows an analytical solution of the equation of motion, giving both the synchronization (kinematics) and multiplication (impact energy) conditions as a function of the gap voltage (or accelerating field). Starting from this example a thorough discussion of MP discharges in axial-symmetric accelerating structures will be given and some poor mans rules are given to estimate the critical cavity field levels to meet the kinematic condition for resonance. The results of these poor mans rules are compared with computer simulations of MP discharges obtained by a statistical analysis of the re-emission yield for impinging electrons versus RF field level in the accelerating structure.
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