Laboratory generation of multiple periodic arrays of Alfvenic vortices

A novel antenna (“Alfvén tennis racket”) has been constructed to generate shear Alfvén waves with large k⊥ such that k⊥ ≃ 1 − 10 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. The antenna has successfully generated two dimensional Alfvén vortices in planes transverse to B_{0z .} The experiments were performed in the Large Plasma Device at the University of California, Los Angeles, In these experiments the uniform background magnetic field was 400G, the plasma density n = 5X10¹² cm^-3, T_e = 4 eV and f_wave = 95 kHz. The wave patterns were validated in a theoretical calculation. Second antennas were constructed to generate different wavenumbers and were used simultaneously to launch counter-propagating waves. These antennas can serve as a platform for the generation magnetic Alfvenic turbulence which is relevant, for example, to solar wind studies. In this work we present measurements in three dimensions of the wave magnetic fields, currents and modes for several antennas and antennas pairs. 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 can⃗ E ×⃗ B drift from an ”O” to an ”X” point in the wave pattern and thereafter undergo stochastic motion.