Physical Understanding Of Enhanced Radiation From A High-power Plasma-filled Backward Wave Oscillator

In order to understand the physical meaning of the enhanced radiation from plasma-filled BWO, some heuristic linear numerical analyses have been made. We here try to show that the presence of a plasma inside slow wave structure may strengthen the interaction between IREB and the radiation. (1) Assuming the practical parameters of the plasma-filled corrugated axisymmetric waveguide without the beam, RF fields patterns of TM
₀₁
mode and the distribution of the Poynting vector have been mapped. It is found that, even for uniform plasma case, E
_z
on the axis and E on the corrugated wall have tendencies to concentrates themselves, as the plasma density increases. Here, the total Poynting flux is kept constant. This fact suggests that an incident annular IREB near the wall may excite a strong RF E
_z
on the axis, if the plasma is present. (2) We extended the previous linear analysis [2] of a plasma BWO to the case of localized ( pinched) plasma and annular IREB. This model may be more realistic to compare with the experiments [1] than the previous one [2]. First, in the case of vacuum BWO, spatial growth rate (SGR) increases with beam radius for constant beam current. SGR is considered to be a measure of strength of radiation from BW0. [3] SGR gets the maximum, when the beam radius critically touches inner wall. Second, in plasma BWO case, SGR increases with decreasing plasma radius for constant line density of the plasma column. These facts suggest that BWO has large output, when plasma is located near the axis, while the beam propagetes near the wall. (3) We have extended our previous linear analysis [2],[3] for infinitely strong axial static magnetic field to the case of strong but finite field. It is found that SGR increases with decreasing magnetic field. This fact may account for some of the experimental results.