  We propose a new mechanism for the generation of zonal flows in magnetically
confined plasmas, complementing previous theories based on a modulational
instability. We derive a new conservation law that operates in the regime of
weakly nonlinear dynamics, and show that it serves to focus the inverse cascade
of turbulent drift wave energy into zonal flows. This mechanism continues to
operate in the absence of the separation of dynamical scales typically assumed
in instability calculations.

Plasma Physics;Fluid Dynamics