The momentum flux probability distribution function for ion-temperature-gradient turbulence

TL;DR

This paper analyzes the probability distribution function tails of momentum and heat flux in ion-temperature-gradient turbulence, highlighting the role of coherent structures called modons in contributing to bursty, intermittent transport events.

Contribution

It introduces a model incorporating modons to compute the PDF tails of fluxes, revealing an exponential tail form and its dependence on physical parameters.

Findings

PDF tails of momentum flux are exponential with a specific form.

The PDF tails are broader than Gaussian, indicating bursty transport.

Numerical analysis shows how PDF tails depend on temperature, density scale lengths, and other parameters.

Abstract

There has been overwhelming evidence that coherent structures play a critical role in determining the overall transport in a variety of systems. We compute the probability distribution function (PDF) tails of momentum flux and heat flux in ion-temperature-gradient turbulence, by taking into account the interaction among modons, which are assumed to be coherent structures responsible for bursty and intermittent events, contributing to the PDF tails. The tail of PDF of momentum flux $R = < v_x v_y>$ is shown to be exponential with the form $\exp{\{-\xi R^{3/2}\}}$, which is broader than a Gaussian, similarly to what was found in the previous local studies. An analogous expression with the same functional dependence is found for the PDF tails of heat flux. Furthermore, we present a detailed numerical study of the dependence of the PDF tail on the temperature and density scale lengths and other physical parameters through the coefficient $\xi$.