# Nonlinear growth of zonal flows by secondary instability in general magnetic geometry

## TL;DR

This paper develops a theory for the nonlinear growth of zonal flows in magnetized plasma turbulence via secondary instability, applicable to tokamaks and stellarators, and validates it against gyrokinetic simulations.

## Contribution

It introduces a general magnetic geometry theory for zonal flow growth, extending previous models and confirming predictions with simulations.

## Key findings

- Predicted growth rates match gyrokinetic simulations.
- Theory applicable to both tokamaks and stellarators.
- Error scales with the small parameter of the theory.

## Abstract

We present a theory of the nonlinear growth of zonal flows in magnetized plasma turbulence, by the mechanism of secondary instability. The theory is derived for general magnetic geometry, and is thus applicable to both tokamaks and stellarators. The predicted growth rate is shown to compare favorably with nonlinear gyrokinetic simulations, with the error scaling as expected with the small parameter of the theory.