Energy dynamics in a simulation of LAPD turbulence

TL;DR

This paper investigates energy transfer processes in LAPD turbulence simulations, revealing a dominant nonlinear instability that drives turbulence and aligns well with experimental observations.

Contribution

It identifies a nonlinear instability as the primary energy source in LAPD turbulence, surpassing the linear drift wave instability, and demonstrates its role in reproducing experimental turbulence characteristics.

Findings

Nonlinear instability dominates energy injection in turbulence.

Suppression of $k_ ext{parallel}=0$ modes alters turbulence spectrum.

Simulation results agree with experimental data.

Abstract

Energy dynamics calculations in a 3D fluid simulation of drift wave turbulence in the linear Large Plasma Device (LAPD) [W. Gekelman et al., Rev. Sci. Inst. 62, 2875 (1991)] illuminate processes that drive and dissipate the turbulence. These calculations reveal that a nonlinear instability dominates the injection of energy into the turbulence by overtaking the linear drift wave instability that dominates when fluctuations about the equilibrium are small. The nonlinear instability drives flute-like ($k_\parallel = 0$) density fluctuations using free energy from the background density gradient. Through nonlinear axial wavenumber transfer to $k_\parallel \ne 0$ fluctuations, the nonlinear instability accesses the adiabatic response, which provides the requisite energy transfer channel from density to potential fluctuations as well as the phase shift that causes instability. The turbulence characteristics in the simulations agree remarkably well with experiment. When the nonlinear instability is artificially removed from the system through suppressing $k_\parallel=0$ modes, the turbulence develops a coherent frequency spectrum which is inconsistent with experimental data.