Beam-Plasma Interaction with Transverse Modulation

This paper describes experimental observations of electron beam-plasma interaction in low-pressure mercury-vapor discharges. Comprehensive data have been obtained of rf growth, phase velocity, axial and radial electric field profiles in the interaction region, and Faraday rotation, for the case of transverse modulation, i.e., for growth in the first azimuthally-varying mode (m = 1). The work complements previous parallel studies of the axisymmetric mode (m = 0) made with axial modulation. The results are compared with cold- and warm-plasma theories of the interaction. It is concluded that the warm-plasma theory gives a satisfactory approximation to the experimental results, and that the remaining discrepancies are due to lack of sharpness of the experimental beam edge.