Abstract. Equivalent linear and truly nonlinear analyses, and total

A linear dispersion relation of beam loaded slow wave structure (SWS) with finite strength guiding axial magnetic field is derived and analyzed numerically.In most theoretical analyses of high power backward wave oscillators (BWO's), the axial magnetic field to guide electron beam in the SWS has been assumed to be infinitely large. In the present paper, we derive an exact linear dispersion relation for a finite magnetic field in which space charge modes, cyclotron modes and the structure modes can interact resonantly.We consider an axisymmetric mode in an infinitely long SWS with sinusoidally corrugated metal wall in the axial direction. A solid cylindrical beam is present on the axis. The dielectric tensor in the beam region has 9 non-zero components [l]. Coupled ordinary and extra-ordinary modes must exist to satisfy boundary conditions at the beam and metal surfaces. We assume practical size and beam parameters that have been studied in the University of Maryland and Niigata University.The calculated results for real wavenumber versus complex frequency indicate that conventional instability of BWO's caused by slow space charge modes is almost unchanged even in the case of finite magnetic fields. In addition, normal and anomalous Doppler shifted cyclotron modes give rise to instabilities near the intersections with TM,, structure mode. The negative or positive feedback mechanism can be expected, if two instabilities with identical frequency are present.The authors would like to acknowledge Thomas M. Antonsen, Jr., for his valuable guidance and discussion.[l] H. P. Freund et al.; IEEE Trans. P-S, 21,654 (1993). AbstractThe coaxial virtual cathode oscillator (vircator) is an attractive microwave device due to its simplicity, but suffers from low efficiency. The coaxial vircator geometry has many physical parameters that can be changed to alter performance and possibly increase the efficiency. The typical polarity for the coaxial vircator at Texas Tech has been the cathode grounded (machine ground) with a positive potential applied to the anode. Many geometry variations on this polarity, termed the positive polarity, have been tried. An alternate polarity, the negative polarity, has been tried with the anode grounded and a negative, pulsed potential applied to the cathode. Though these polarity arrangements initially appear to be identical, subtle differences exist. The two major differences are the proximity of the virtual cathode to the overmoded output waveguide and the change of the static electric field in the diode region into a non-uniform negative polarity field which gives low energy electrons in the interaction region a push away from, and out of the diode region. This, as well as decreasing the cathode width, increases the efficiency of the vircator substantially (-factor of 10). The coaxial vircator pulsed power source consists of a 31 stage Marx generator capable of storing 7.75 W. The energy from the Marx generator is switched into a 12.5 ns, 10 SZ pulse forming line with a single channel, selfbreaki...

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