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Experiments are reported in which a relativistic electron beam, having an energy of 650 keV and carrying a beam current of about 0.8 kA, was used to excite slow (negative energy) space‐charge and cyclotron waves. The waves, which yielded megawatt powers in the Ka and V bands, were generated as a result of the unstable interaction between a beam wave, a static rippled magnetic field, and a waveguide mode. The instability growth was observed to increase with both increasing guide and ripple magetic field strengths, and saturated when the ripple field was about 10% of the guide field. The observations are mainly consistent with theoretical predictions, although selection rules for excited modes violate predictions.