Alfvén wave collisions, the fundamental building block of plasma turbulence. IV. Laboratory experiment

Abstract

Turbulence is a phenomenon found throughout space and astrophysical plasmas. It plays an important role in solar coronal heating, acceleration of the solar wind, and heating of the interstellar medium. Turbulence in these regimes is dominated by Alfvén waves. Most turbulence theories have been established using ideal plasma models, such as incompressible MHD. However, there has been no experimental evidence to support the use of such models for weakly to moderately collisional plasmas which are relevant to various space and astrophysical plasma environments. We present the first experiment to measure the nonlinear interaction between two counterpropagating Alfvén waves, which is the building block for astrophysical turbulence theories. We present here four distinct tests that demonstrate conclusively that we have indeed measured the daughter Alfvén wave generated nonlinearly by a collision between counterpropagating Alfvén waves.

A dynamical model of plasma turbulence in the solar wind

“…. simulations in the MHD regime [18] and verified experimentally in the laboratory [57][58][59], establishing Alfvén wave collisions as the fundamental building block of astrophysical plasma turbulence. This nonlinear mechanism is likely to be the physics leading to a non-zero thirdmoment of turbulent magnetic field measurements, a statistical measure often used to estimate the turbulent energy cascade rate [60].…”

Measurements of the nonlinear beat wave produced by the interaction of counterpropagating Alfvén waves

“…A similar procedure was used to measure the perpendicular magnetic field components for experiment 1. 48 Note that each experiment actually consisted of three data runs: (1) the ASW antenna only on, (2) Loop antenna only on, (3) both antennas on simultaneously.…”

Wavevector anisotropy of plasma turbulence at ion kinetic scales: solar wind observations and hybrid simulations

“…Examples of wavevector anisotropy can be found in near-Earth solar wind (e.g., Matthaeus et al, 1990;Chen et al, 2012), astrophysical systems such as diffusion of galactic cosmic ray (Bieber et al, 1994(Bieber et al, , 1996Ahlers, 2014) and magnetic field decay process in the neutron star crust (Cumming et al, 2004), as well as in laboratory plasmas (Howes et al, 2012;Drake et al, 2013). All these studies conclude that plasma turbulence is primarily anisotropic such that the energy spectrum is extended preferentially in the perpendicular direction to the mean magnetic field.…”