Study of power enhancement and frequency shifting of microwave emission in a plasma filled non-uniform BWO

Abstract

Summary form only given. Recent experiments reported an improved efficiency in high power microwave generation by applying a non-uniform slow wave structure (SWS) or using a plasma filling in a backward wave oscillator (BWO). A combined scheme of plasma filling in a non-uniform SWS is investigated to further increase microwave output power. An annular cathode of radius 1 cm produces a 500 keV electron beam of density 10/sup 12/ cm/sup -3/. A plasma of density 10/sub 12/ cm/sup -3/ with temperatures T/sub c//spl ap/T/sub i//spl ap/10 eV is launched into the SWS from a cylindrical plasma gun placed inside the annular cathode. In a 2 1/2 D simulation, the annular electron beam is injected, with the use of a guiding magnetic field, into a cutoff neck with a radius of 11 mm. This electron beam moves into an 18 cm long non-uniform SWS section with an average radius of 1.5 cm, and then is guided into the waveguide wall. The voltage pulse in the simulation rises to 500 kV in 1 ns and remains flat for the remainder of the 20 ns. These simulations have shown that the microwave output power of a few hundred megawatts is nearly doubled when compared with the case without plasma, while the microwave frequency is upshifted on the order of 100 MHz. Most of the plasma electrons are quickly (<13 ns) driven out of the core plasma, leaving only the beam electrons flowing in an ionic background. Such a now might lead to an instability that could help to further bunch the electron beam and increase microwave power production.