Producing structures at the Larmor-radius scale via the interaction of shear Alfvén waves and magnetic flux ropes*

Understanding the transfer of energy from larger to smaller scales remains an active subject of study in plasma physics. Two types of system-sized structures are magnetic flux ropes and shear Alfvén waves; and, one of the dissipation scales is the ion Larmor radius.  These experiments are carried out in the Large Plasma Device (LAPD) at UCLA. Flux ropes are generated using a LaB₆ cathode-anode discharge ( L = 18 m, n=3x10¹²cm^-3, T_e=6eV, 10^-3 < β < 0.1) within a larger, r = 30 cm, plasma. Alfvén waves are launched with antennas inside the ropes.  Magnetic field frequency spectra reveal multiple sidebands centered on
the Alfvén wave frequency, separated by multiples of the (much lower) kink frequency. Using moving probes, the cross-field mode structure of the sidebands is revealed by frequency
filtering for the desired mode. Magnetic field, density, and potential data are presented which clearly show higher m numbers and smaller spatial scales for increasing sideband separation frequency. The   spatial scales approach k_⊥ρ_i=1. Bi-coherence calculations along with the m-number and frequency
matching point to a nonlinear, three-wave mechanism for the energy transfer.