Fusion Science and Technology / Volume 51 / Number 2T / February 2007 / Pages 331-333
Technical Paper / Open Magnetic Systems for Plasma Confinement
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.