Observation of an Alfv\'en Wave Parametric Instability in a Laboratory Plasma

TL;DR

This paper reports the first laboratory observation of a shear Alfvén wave parametric instability, where a kinetic Alfvén wave generates daughter modes through nonlinear interactions, advancing understanding of plasma wave dynamics.

Contribution

It demonstrates the first experimental detection of shear Alfvén wave parametric instability and analyzes the role of perpendicular nonlinear forces in the process.

Findings

Observation of three daughter modes consistent with KAW sidebands

Daughter modes' frequencies vary with pump wave amplitude

Spatial localization suggests importance of perpendicular nonlinear forces

Abstract

A shear Alfv\'en wave parametric instability is observed for the first time in the laboratory. When a single finite $\omega/\Omega_i$ kinetic Alfv\'en wave (KAW) is launched in the Large Plasma Device above a threshold amplitude, three daughter modes are produced. These daughter modes have frequencies and parallel wave numbers that are consistent with copropagating KAW sidebands and a low frequency nonresonant mode. The observed process is parametric in nature, with the frequency of the daughter modes varying as a function of pump wave amplitude. The daughter modes are spatially localized on a gradient of the pump wave magnetic field amplitude in the plane perpendicular to the background field, suggesting that perpendicular nonlinear forces (and therefore $k_{\perp}$ of the pump wave) play an important role in the instability process. Despite this, modulational instability theory with $k_{\perp}=0$ has several features in common with the observed nonresonant mode and Alfv\'en wave sidebands.