Laboratory Observations Consistent with Non-linear Decay of a Kinetic Alfvén Wave

Abstract:

Alfvén waves, a fundamental mode of magnetized plasmas, are ubiquitous in space plasmas. For example, a cascade of non-linearly interacting Alfvén waves is believed to play a key role in solar wind turbulence. At perpendicular length scales below the ion gyroradius, this takes the form of Kinetic Alfvén Waves (KAWs). Theoretical predictions show that these Alfvén waves may be unstable to various decay instabilities (e.g. [1,2]). In particular, theory predicts that a KAW may decay into two daughter KAWs even at very low amplitude (δB/B<10^-3). Given the turbulent nature of solar wind observations and limited data-points, laboratory experiments may play a vital role in exploring the key physics responsible.

The present work, conducted at UCLA's Large Plasma Device (LAPD) represents the first fundamental laboratory study of the non-linear Alfvén wave interactions responsible for Alfvén wave decay instabilities. These experiments include the first laboratory observation of the Alfvén-acoustic mode coupling at the heart of the Parametric Decay Instability [3]. More recently, laboratory efforts have focused on the predicted non-linear decay of one KAW into two daughter KAWs. In these experiments, a single high-frequency ω/Ω_i~0.7 Alfvén wave is launched, resulting in two daughter modes with frequencies and wave numbers that suggest co-propagating KAWs produced by decay of the pump wave. The observed process is parametric in nature, with the frequency of the daughter modes varying as a function of pump mode amplitude. Efforts are underway to fully characterize this set of experiments and compare with decay instabilities predicted by theory and simulations.

[1] JV Hollweg, J. Geophys. Res. 99, 23 431 (1994).
[2] YM Voitenko, Journal of plasma physics 60.03 (1998).
[3] S Dorfman and T Carter, Phys. Rev. Lett. 110, 195001 (2013).

Supported by DOE, NSF, and DOE FES and NASA Eddy Postdoctoral Fellowships