Influence of surface absorption on cyclotron oscillations in bounded systems

It is shown that, in a plasma with a sharp boundary, surface absorption leads to a build-up of cyclotron oscillations with negative energy. As a result, the instability region expands. A detailed analysis is made of the processes giving rise to surface absorption. By employing the concept of modulated beams (Van Kampen waves) it is possible to construct a clear picture of the phenomenon, which is compared with other processes in inhomogeneous systems leading to an exchange of energy between oscillations and plasma particles (cyclotron and Cherenkov resonance).