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.Comment: New J. Phys. 201
