A platform for investigating Alfvén waves with large perpendicular wavenumbers

Large perpendicular wavenumbers cause shear Alfvén waves to develop large parallel wave electric fields . These waves are expected to develop nonlinear effects whern the associated magnetic field fluctuations are on the order of of δΒ_⊥/B₀ ≈ λ_⊥/(τV_A). Here τ is the wave period and δΒ_⊥ is the Alfven wave magnetic field, B₀ is the background field. To create waves in this regime a specialized antenna (“Alfvén tennis racket”) was constructed and will be discussed. The antenna has successfully generated two dimensional Alfvén vortices in planes transverse to B₀ with k_⊥= 0.63 cm^-1. A second antenna has been constructed to generate a different k_⊥ and can be simultaneously used to launch counter-propagating waves. In initial experiments on the Large Plasma Device at UCLA, a pattern of Alfven wave vorticies were generated in a Helium plasma with measured δΒ_⊥/B₀ ≈ 1% in far field of the antenna. ( λ_⊥= 8 cm, density = 1.25X10¹³ cm^-3, λ_|| = 112.5 cm, V_A = 6.2X10⁷ cm^-1 , He) . This is roughly 1/4 of the wave field necessary for nonlinearities. The wave field is diagnosed with 3-axis magnetic probes. A small (1 cm diameter) ion beam using test atoms of Argon will be used to measure test ion trajectories and deduce the wave magnetic field. A test electron beam indicated some cross field electron transport. Data will be utilized to test for chaotic motion. This type of antenna will serve as a new platform to study large amplitude shear waves with high k_⊥.