Laboratory generation of multiple periodic arrays of Alfvénic vortices

A novel antenna has been constructed to generate shear Alfvén waves with large k⊥ such that k⊥≃1−6.5 cm−1. The antennas comprise multiple current loops, aligned with their normals transverse to the background magnetic field. Each loop is a generator of Alfvén cones. The observed spatial pattern is the interference of thousands of these cones. This is completely a linear effect; in the low-power regime of this experiment, the waves do not modify the plasma density or temperature. The cones overlap and the electric and magnetic fields associated with them can lead to stochastic motion of electrons and ions passing through them. This type of antenna can serve as a platform for the generation of magnetic Alfvénic turbulence. In this work, we present measurements in three dimensions of the wave magnetic fields and currents for several antennas and a pair of antennas. Close to the antennas, the wave number spectra match what is expected from the geometry of the antenna. In the region where the waves collide, additional modes are observed. At large wave amplitudes and large k⊥, the ions can E→×B→ drift from an “O” to an “X” point in the wave pattern and thereafter undergo stochastic motion.