Higher Order Mode Radiations of Weakly Relativistic Oversized Backward Wave Oscillator

Higher order mode radiations of oversized K-band backward wave oscillator (BWO) are examined experimentally. By injecting an annular beam in the weakly relativistic region less than 100 kV, a beam mode with harmonic number n = 1 excites BWO radiations based on the TM01 surface wave. The same beam mode excites higher order modes leading to radiations in the U- and E-bands. The power levels of the higher order modes are hundreds of times smaller than that of BWO due to the surface wave. For a beam mode with n = 2, higher order mode radiations are in the F- and D-bands, up to about 100 GHz, which is about four times higher than the frequency of the TM01 surface wave. The power level of higher order mode radiation by the n = 2 beam mode decreases by a factor of hundreds from that of the n = 1 beam mode.