Nonlinear excitation of acoustic modes by large amplitude Alfv\'en waves in a laboratory plasma

TL;DR

This study investigates how large amplitude Alfvén waves in a laboratory plasma can nonlinearly excite ion acoustic modes through three-wave interactions, confirming theoretical predictions with experimental observations.

Contribution

It provides experimental evidence of nonlinear three-wave interactions driven by Alfvén waves in a plasma, validating theoretical models of nonlinear wave coupling.

Findings

Resonance observed in beat wave response as a damped ion acoustic mode

Spatial profile and amplitude of the beat mode match theoretical predictions

Experimental confirmation of nonlinear three-wave interaction driven by ponderomotive force

Abstract

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 (LAPD). 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 non-linear pondermotive force.