Backward Wave Modes of Partially Plasma Column Loaded Cylindrical Waveguide

When two waves traveling in the same or opposite direction are interfered, they can attenuate or increase each other. The backward wave oscillators work with the same principal that interference between two waves traveling in opposite directions increases the amplitude along propagation direction. There are backward wave oscillators which use interaction of plasma-electron beam, in the literature. The Method of Moment (MoM) is a widely used technique for numerical simulation of propagation and scattering problems. In this study, backward wave modes of the plasma column loaded cylindrical waveguide have been investigated by using Method of Moment (MoM). In previous studies, the authors presented the validation of the method for gyro-resonance region. For the structure, the backward waves appear in the plasma resonance region. Unlike gyro-resonance region modes (called plasma modes), to obtain the plasma resonance region modes (called cyclotron modes), it is necessary to use very large dimensions for the linear algebraic equations system used in the MoM. Indeed in the implementation of the MoM in the plasma resonance region for the cyclotron modes, we have found that the method exhibits discrepancies with the exact solution which do not disappear unless very large numbers of expansion functions are used. The objective of this work is to report these discrepancies and suggest a possible numerical technique to overcome them. Also presented are the dispersion curves of the backward wave modes. This technique consists in using additional computer time and memory space. In particular it is demonstrated that the mean relative error is confined to less than 0.001 over the whole frequency band of interest which would be impossible if insufficient computing power was used.