The growth of peak velocity, noise, and signals in O-type electron beams

A new phenomenon, the growth of peak velocity of an electron beam originating from a shielded gun, has been investigated. Its behavior with distance, magnetic field, and phasing between beam voltage pulse and heater current zero is presented. This phenomenon bears a general but not a detailed resemblance to the noise growth observed by previous investigators. The noise growth phenomenon has been re-examined in detail on the beam used for peak velocity growth measurements. The single most striking observation is that whereas peak velocity and 3-kmc noise growth take place at magnetic field strength primarily above the Brillouin flow value, 3-kmc signal growth occurs only at magnetic fields below the Brillouin value. A combination of linear axially symmetric surface-wave mode theory and simple intermodulation theory is unable to describe quantitatively the noise growth rate. Consideration of other axially-symmetric modes on near-Brillouin beams, namely body modes and near-synchronous modes, also leads to poor correlation with experiment. It seems likely that the correct explanation involves consideration of one or more of the following: 1) theta-varying dc beam media, 2) theta-varying RF excitation, or 3) a more accurate nonlinear representation of noise.