Thee-wave coupling between shear Alfvén waves and kink waves in the Large Plasma Device
POSTER
Abstract
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 (kr ~ρs-1~ωpe/c~ ρi-1).
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 (kr ~ρs-1~ωpe/c~ ρi-1).
*Work performed at the Basic Plasma Science Facility which is funded by the DoE FES (DE-FC02-07ER54918) and the NSF (PHY-1561912)
Presenters
-
Steve T Vincena
- University of California, Los Angeles
- University of California, Los Angeles, US