A Laboratory Approximation of Whistler Mode Antenna Arrays

Abstract:

Magnetic loop antennas whose dipole moment, n, is oriented both along and across B₀,are used to excite cw whistler modes in a large laboratory plasma for parameters ω ≈ 0.3 ω_ce « ω_pe. These whistler "vortices" resemble m = 0 helicons in bounded plasmas when n parallel to B₀ and m = 1 helicon modes when n is perpendicular to B₀. Both dipole orientations produce conical phase fronts that cannot be directly compared to plane wave theories and are inclined at the Gendrin angle. The magnetic field topology exhibits evidence of linkage and helicity, whose sign is defined by propagation direction. The wave fields are force free. Using linear superposition, as demonstrated in Physics of Plasmas 7, 519-526 (2000), the measured fields are added in a variety of configurations to produce, for example, directional radiation patterns, whistler standing waves, and nearly plane whistler waves. The configurations are produced by adding the spatially and temporally shifted observed single-antenna magnetic field topology. The directional antenna configuration is shown to be more efficient than rotating field antennas. Whistler standing waves produce no perfect nodes and have wave polarizations varying spatially between linear and circular. Nearly plane whistler waves are created when the angle θ of wave propagation has been varied by a phase shift along an array of spatially separated antennas. These results are of interest to space and laboratory plasmas. (Work supported by DOE/NSF.)