Antenna radiation patterns in the whistler wave regime measured in a large laboratory plasma

Abstract

The radiation pattern of electric dipole and magnetic loop antennas is determined experimentally in the parameter regime 0.1 ≲ ω/ωc ≲ 1, 1 ≲ ωp/ωc ≲ 30 (ωp, ωc = electron plasma, cyclotron frequency, respectively). The measurements are performed in a uniform, dense magneto‐plasma (density 1010 < ne < 1012 cm−3, temperature 0.2 < kTe < 3 eV, magnetic field B0 ≲ 100 G) of very large dimensions (50 cm diameter, 300 cm length) where boundary effects are negligible even for many far‐field patterns. When the antenna dimensions are small compared to the parallel whistler wavelength (k¶ Lant≪ 1) the radiation pattern is characterized by resonance cones. For finite sized antennas (k¶ Lant≳ π) no cones are observed, but a dipole‐like pattern with a field‐aligned narrow lobe. Amplitude and phase distribution in the near zone and far zone are investigated. The antenna orientation with respect to the static magnetic field is varied. At large amplitudes (Brf/B0 ≃ 1%) the radiation pressure modifies the plasma density and the wave self‐focuses. In this regime nondiverging filaments of radiation are seen. Copyright 1976 by the American Geophysical Union.