Multipacting can cause breakdown in high power rf components like couplers, windows, etc. This phenomenon starts if certain resonant conditions for electron trajectories are fulfilled and if the impacted surface has a secondary yield larger than one. A general cure against multipacting is to avoid the resonant conditions. Therefore we investigated the dynamics of the electron trajectories in order to find rules for these resonances and thus suppress multipacting by appropriate design. We

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... ed a new code which combines standard trajectory calculations with advanced searching and analyzing methods for multipacting resonances. As a first step, coaxial power lines are investigated. We characterize multipacting behavior in straight and tapered lines and give scaling laws with respect to dimension, frequency and impedance. The calculations are compared with experimental observations.