Confinement of Electron Orbits and the Interaction between the Electron Beam and Cavity Modes in the Orbitron Maser

The Orbitron maser, developed over recent years in the University of Tennessee Plasma Laboratory, is an alternative microwave source, capable of producing microwave radiation up to the submillimeter range. It operates on the basis of interaction between an electrostatically confined electron cloud and electromagnetic field in the Orbitron resonator. Good confinement of the cloud is very important for an efficient operation of the Orbitron, and is directly related to the shape of electron orbits in the device. It appears that the radial confinement can be determined by two simple criteria concerning the electron's angular momentum L and energy E. The longitudinal trapping is currently achieved through the etching of the central wire. A region in the (L²,E) parameter space, corresponding to the population of electrons trapped between etches can be established. Next, the coupling of cavity modes to the electron beam in the Orbitron's resonator is investigated. One possible mechanism of this interaction involves a "direct current coupling" between cavity modes and electron orbits in a tenuous electron beam. Although in principle, this type of interaction can result in a resonant exponential growth of cavity modes with time, it is concluded that under present operating conditions the observed maser action must be of a quite different origin. Then, the azimuthal harmonic perturbations of a circular beam, resulting from the "negative mass" instability in a logarithmic field, are considered. In particular, it is demonstrated how these perturbations can drive cavity modes through the accompanying perturbation currents and time variation of the electrostatic potential. On the other hand, the influence of an oscillating mode on the frequency of perturbations is found to be very small, even for the resonant parameters. Thus, a plausible theoretical argument is established which explains the origin of microwave production in the Orbitron maser.