Antenna radiation pattern of cyclotron harmonic waves in a hot magnetoplasma


  • B. Lembège


The near and far field radiation pattern of a very short antenna is studied in the plasma regime. ωc < ω < 2ωc, ωp ≫ ωc (where ωp and ωc are the electron plasma and cyclotron frequencies, respectively). The experiment is performed in a large laboratory magnetoplasma. Cyclotron harmonic waves are detected in all directions, not only those corresponding to the purely perpendicular direction of detection as was observed previously. This new result is theoretically explained and studied in detail. Constant phase curves deduced from the dispersion relation for a hot Maxwellian electron magnetoplasma are considered. They are found to be described by the low damped part of the polar dispersion curves. The different shapes of the wave surfaces (convex and concave with respect to the excitation source) are in good agreement with experimental results. The effects of plasma and magnetic field inhomogeneities on the dispersion relation are discussed. Experimental isoamplitude curves are presented in the whole space around the transmitter. The theoretical dispersion curves show that the energy of the cyclotron harmonic waves tend to propagate principally outside a cone whose apex is the transmitter and whose axis is parallel to the direction of the magnetic field. The theoretical opening angle of this cone is given by the direction of the group velocity at the inflexion point of the polar dispersion curve where the Landau damping increases suddenly. This theoretical result is in reasonable agreement with the spatial distribution of energy observed experimentally. From this study it is shown that the simple dispersion relation for perpendicular propagation can be used to predict qualitatively the distribution of cyclotron harmonic waves in all directions for dense magnetoplasma conditions.