Non-linear Alfv en wave interaction leading to resonant excitation of an acoustic mode in the laboratory

The nonlinear three-wave interaction process at the heart of the parametric decay process is studied by launching counter-propagating Alfv en waves from antennas placed at either end of the Large Plasma Device [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)]. A resonance in the beat wave response produced by the two launched Alfv en waves is observed and is identified as a damped ion acoustic mode based on the measured dispersion relation. Other properties of the interaction including the spatial profile of the beat mode and response amplitude are also consistent with theoretical predictions for a three-wave interaction driven by a nonlinear ponderomotive force. A simple damped, driven oscillator model making use of the MHD equations well-predicts most of the observations, but the width of the resonance curve is still under investigation. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4919275]