Properties of high-β microturbulence and the non-zonal transition

The physics underlying the non-zonal transition [M. J. Pueschel et al., Phys. Rev. Lett. 110, 155005 (2013)] are explored in detail, and various studies are presented which support the theory that critically weakened zonal flows are indeed responsible for the failure of ion-temperature-gradient-driven turbulence at high plasma beta to saturate at typical transport values. Regarding flux-surface-breaking magnetic fluctuations and their impact on zonal flows, numerical approaches to obtaining zonal flow residuals are elaborated on, and simulation results are shown to agree with analytical predictions, corroborating the interpretation that flux-surface-breaking magnetic fluctuations cause the transition. Consistently, the zonal-flows-related energetics of the turbulence are found to change fundamentally when exceeding the threshold. (C) 2013 AIP Publishing LLC.