Thee-wave coupling between shear Alfvén waves and kink waves in the Large Plasma Device*

Both kink waves and shear Alfvén waves occur simultaneously in plasmas 

ranging from solar prominences, the solar wind, magnetic fusion devices, to Earth's magnetotail. The

potential energy exchange between these low-frequency plasma waves is thus of broad interest.

Experiments aimed at elucidating three-wave interactions between kink and shear waves are performed in the Large 

Plasma Device at UCLA. Flux ropes are generated using a LaB₆ cathode 

discharge (with L=18 m and 0.01< β < 0.1.)

The flux rope (r=8cm) is embedded larger (r=30cm) 

ambient plasma produced by a second, BaO cathode. Shear Alfvn waves

are launched using an internal antenna. When the flux rope is driven kink-unstable, the shear wave develops sidebands separated by the kink 

frequency. Data are presented that confirm that the sidebands satisfy the three-wave

matching conditions from the parent waves: frequency matching along with radial, azimuthal, and parallel wavenumber matching.

Additionally, bispectral analysis of the fluctuating magnetic field time series

demonstrates a strong phase coupling (b²>0.9) between the three wave pairs; here b² is the squared bicoherence.

The perpendicular wavenumber spectra, show that the sidebands are driven at decreasing spatial scales, approaching

dissipation scales (k_r ~ρ_s^-1~ω_pe/c~ ρ_i^-1).