A pseudospark (PS) plasma sourced electron beam was both computationally and experimentally studied for generation of millimetre wave and terahertz radiation. The beam-wave interaction region is a sinusoidal rippled-wall slow wave structure of a backward wave oscillator (BWO) in G-band. An electron beam of ˜1 mm diameter carrying a current of up to 10 A with a sweeping voltage of 42 to 25 kV and pulse duration of 25 ns propagated through the interaction region in a plasma environment without the need for a guiding magnetic field, which resulted in broadband millimetre radiation generation over a frequency range of 186-202 GHz with a maximum power of 20 W.