AC space charge effects in plasma-assisted slow-wave devices

Summary form only given. In conventional linear-beam devices, electron beam transport is provided by strong guiding magnetic fields. Therefore, ballistic electron bunching results in formation of electron bunches, in which the space charge forces deteriorate the bunch and, hence, lower the device efficiency. A distinctive feature of plasma-assisted slow-wave devices (pasotrons) is the absence of guiding magnetic fields. The beam transport is provided there by the ion focusing; thus, beam electrons propagate in a so-called Bennett pinch regime. In this regime, the repulsive space charge force caused by the beam electric self-field is compensated by the beam magnetic self-field and the electric field produced by ions. This equilibrium, however, can be disturbed by the AC space charge forces, which occur when electrons start forming the bunches in the process of interaction with electromagnetic waves. In the present paper, the role of AC space charge forces is analyzed. It is shown that, under certain conditions, these forces may play a dominant role in the beam propagation along the device axis. It is also shown that, in contrast to conventional linear-beam devices, in the pasotrons, the AC space charge effects do not deteriorate the quality of electron bunches, but cause their radial motion outwards. Then, the saturation occurs when these bunches reach the wall of a slow wave structure (SWS). Thus, there is an optimal relation between the initial radius of an electron beam, the wall radius of a SWS and the radiated microwave power. 2D-simulations predict reasonably high interaction efficiency of both traveling-wave and backward-wave configurations of pasotrons