Zonal flow generation in the improved confinement mode plasma and its role in confinement bifurcations

Unstable fluctuations develop in the initially quiescent plasma in the improved confinement mode of the H-1 heliac when the radial electric field ( E r ) shear exceeds some critical value. These unstable E r shear-driven modes are shown to generate zonal-flow-like poloidally symmetric potential structures, similar to those generated in the low confinement mode (Shats M G and Solomon W M 2002 Phys. Rev. Lett. 88 045001). The structures modulate their parent waves, the background E r shear and the fluctuation-driven radial transport. The onset of zonal flows is observed as a precursor to the plasma confinement bifurcation to an even higher confinement regime.