Slow cyclotron and Cherenkov instabilities in weakly relativistic oversized backward wave oscillator

We analyze instabilities of oversized backward wave oscillator (BWO) designed for K-band operations in a weakly relativistic region (less than 100 kV). For the oversized BWO, the electromagnetic field is a surface wave localized near the waveguide wall. Instabilities due to the beam interactions with the surface wave are examined by considering three-dimensional beam perturbations. There exist the slow cyclotron and Cherenkov instabilities as in the case of non-oversized BWO. Nonaxisymmetric as well as axisymmetric instabilities are excited, even in the completely axisymmetric system. The growth rates of both modes are in the same order. The growth rates decrease exponentially if the beam is apart from the SWS wall.