[en] The authors present a detailed study of field-particle interaction in a so-called orbitron configuration. In such a device, plasma produced electrons orbit around a positively charged wire and originate centimeter and millimeter wave emission. The results follow from the integration of a large number of electron trajectories performed in a digital computer. They either compute individual trajectories or follow an electron distribution. The natural initial state is loaded as a spatial uniform distribution with velocities given by a random maxwellian generator. Along with an electrostatic electric field imposed externally to the cavity the authors assumed an initial electromagnetic field in a whispering-gallery mode to be present. The study of individual trajectories shows that the most common, being the more eccentric ones, may contribute to significant energy exchange with the wave due to the radial wave electric field. In such trajectories, the time flight between two successive passages nearby the central conductor is very nearly half a period of a circular orbit, which indicates roughly the same estimates to the radiation frequency as in the case of the circular orbits. In practical configurations which have been reported, the authors think that the circular orbits cannot efficiently contribute to the radiation because the wave electric field is almost radial near the conductor, and only small radius trajectories can account for the radiation frequencies observed