Noise Spectrum of Electron Beam in Longitudal Magnetic Field

Sharp peaks are found in the UHF spectrum (10 to 500 mc) of an electron beam, emanating from a shielded diode. In the presence of a longitudinal magnetic field, the strongly rippled beam displays an additional set of peaks whose frequencies are proportional to the field strength. The largest of these, just above the cyclotron frequency, is connected with the overlap of a dense cluster of particle orbits, passing close to the beam axis. It can attain amplitudes of 65 db above background noise. The transverse distribution of UHF noise power is found to agree with that for ideal Brillouin flow, even in rippled beams. With long ripple wavelengths, two noise maxima are found to flank each beam waist. A small-signal wave analysis explains this pattern, and affords some insight into the energy-exchange processes in rippled-beam amplification. The reduction in “growing noise” due to positive ions is attributed to increased cancellation of net radial beam motion, due to overlap in particle orbits near the axis