Pulse shapes for absolute and convective cyclotron-resonance-maser instabilities

A linear analysis is presented for pulse shapes produced by a spatial and temporal delta-function disturbance of cyclotron-resonance-maser modes for the case where the initial equilibrium state is free of radiation. A pinch-point analysis based on the theory of R.J. Briggs (1964) and A. Bers (1983) is employed. Numerical and analytical techniques are developed for the straightforward calculation of pinch-point coordinates in a reference frame moving with arbitrary velocity in the axial direction. Examples analyzed include the absolute instability in the waveguide operating mode, in higher harmonics of the operating mode, and in lower-frequency waveguide modes when the operating mode is a higher-order waveguide mode. Effects of waveguide wall resistance on pulse shapes and the effectiveness of such resistance in suppressing or reducing the growth rates of absolute instabilities are also analyzed