Observation of the cross-excitation instability in a relativistic backward wave oscillator

Experimental observation of the cross-excitation instability in a relativistic backward wave oscillator is reported. The transition between single frequency operation and the cross-excitation regime is measured. The initial mode excited in the cross-excitation regime has a relatively low efficiency, while the second mode has considerably higher efficiency and a slightly higher frequency. The two modes are observed to be beating for a considerable fraction of the radiated microwave pulse. These results are consistent with earlier theoretical predictions for the onset and voltage scaling for this instability.