Millimeter Wave Overmoded Relativistic Backward Wave Oscillator

A millimeter wave relativistic backward wave oscillator (RBWO) is proposed to provide megawatts of power with Gaussian output. An overmoded rectangular slow wave structure (SWS) with a relativistic hollow electron beam was used to increase the interaction impedance and to avoid RF breakdown. The rectangular SWS can be fabricated more easily than a sinusoidal one. The RBWO consisting of an axisymmetric SWS generates more than 90 MW of power at 78 GHz. A mode selection technique was utilized to a select higher order mode as the operating mode. A corrugated waveguide mode converter can be used to convert the operating TM03 output mode into a Gaussian output mode.