Noise Spectrum of Electron Beam in Longitudinal Magnetic Field

Measurements of induced noise currents along drifting cylindrical electron beams have shown that noise fluctuations propagate as space-charge waves in the same fashion as RF signals of the same frequency. On many such beams, however, the regular standing-wave noise pattern is interrupted, after some drift distance, by a smooth steep increase in noise current, followed by slow, shallow undulations. This “growing noise” phenomenon, discovered by Smullin and his co-workers at M.I.T. several years ago, is the subject of study in this paper. Its importance is considerable, in a negative way, because it has hampered the development of medium-power traveling-wave-tube devices with acceptably low noise figures. The experimental measurements show the growing noise pattern to be the result of a two-stage process. Its primary cause is rippled-beam amplification of noise fluctuations over a wide band of microwave frequencies, much higher than the usual observation frequency. This explains its elusiveness. In the second stage, noise energy is transferred to lower frequencies, due to intermodulation and other non-linear processes within the gain band. As the beat-frequency noise increments are excited by continuous arrays of frequency pairs, their standing-wave patterns overlap one another, resulting in a smooth growing-noise pattern