Nonlinear instability in simulations of Large Plasma Device turbulence

TL;DR

This paper compares various turbulence simulations of the Large Plasma Device, revealing a dominant nonlinear instability that drives turbulence regardless of boundary conditions and linear stability differences.

Contribution

It demonstrates that a robust nonlinear instability governs turbulence in LAPD simulations, overriding linear stability differences caused by boundary conditions.

Findings

Turbulence is primarily driven by a nonlinear instability.

Linear eigenmode structures are destroyed by the nonlinear instability.

Different boundary conditions lead to similar turbulence behavior.

Abstract

Several simulations of turbulence in the Large Plasma Device (LAPD) [W. Gekelman et al., Rev. Sci. Inst. 62, 2875 (1991)] are energetically analyzed and compared with each other and with the experiment. The simulations use the same model, but different axial boundary conditions. They employ either periodic, zero-value, zero-derivative, or sheath axial boundaries. The linear stability physics is different between the scenarios because the various boundary conditions allow the drift wave instability to access different axial structures, and the sheath boundary simulation contains a conducting wall mode instability which is just as unstable as the drift waves. Nevertheless, the turbulence in all the simulations is relatively similar because it is primarily driven by a robust nonlinear instability that is the same for all cases. The nonlinear instability preferentially drives $k_\parallel = 0$ potential energy fluctuations, which then three-wave couple to $k_\parallel \ne 0$ potential energy fluctuations in order to access the adiabatic response to transfer their energy to kinetic energy fluctuations. The turbulence self-organizes to drive this nonlinear instability, which destroys the linear eigenmode structures, making the linear instabilities ineffective.