(TH/P8-32) Non-perturbative Models of Intermittency in ITG Drift Wave Turbulence with Zonal Flows

J. Anderson1), E. Kim1)
1) Applied Mathematics, University of Sheffield, Sheffield, United Kingdom of Great Britain and Northern Ireland

Abstract.  There has been overwhelming evidence that coherent structures play a critical role in determining the overall transport in magnetically confined plasmas. These coherent structures cause intermittent, bursty events, which can mediate significant transport of heat and particles, for instance, imposing a large heat load on container walls. A crucial question in plasma confinement is thus the prediction of the PDFs of the transport due to these structures and of their formation. Here, we report on a first analytical result on these two closely related problems by using a novel non-perturbative method. We first compute the PDF tails of global momentum flux and heat flux in the ion-temperature-gradient (ITG) turbulence, by assuming that a short-lived modon (a bipolar vortex soliton) is a coherent structure responsible for bursty and intermittent events, contributing to the PDF tails. The tails of PDF of global momentum flux and heat flux are shown to be stretched exponential, which is broader than a Gaussian. This result suggests that rare events of large amplitude due to coherent structure are crucial in transport (similarly to what was found in the previous local studies), offering a novel explanation for exponential PDF tails of momentum flux found in recent experiments at CSDX at USCD. We show the crucial dependence of the overall amplitude of the PDFs on the temperature and density scale lengths. We then present a consistent theory of the PDFs of momentum flux and of the formation of shear flow, by incorporating the interplay between ITG turbulence and shear flow. While the PDF tails of momentum flux have similar stretched exponential, the amplitude of the PDF tails are significantly reduced by shear flows. Furthermore, the PDF tails of shear flow formation have different exponential behaviour, with the overall amplitude severely quenched by strong flow shear. These results highlight the key role of structures on intermittent transport through the feedback on turbulence. Implications for transport in tokamaks will be discussed.

Full paper available (PDF)