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 (Er) shear exceeds some critical value. These unstable Er 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 backgroundEr 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.