ExB-Shear Effects on Magnetic-Flutter Diffusion of Electron-Drift Trajectories in ITG Turbulence

Magnetic-field stochasticity arises due to microtearing perturbations, which can be driven linearly [1] or nonlinearly [2], even at very modest values of the plasma beta. The resulting magnetic-flutter contribution may or may not be a significant component of the overall electron (particle and thermal) transport. Initial investigations [3] of the effect of ExB shear on electron-drift magnetic-flutter diffusion coefficient $D_{\mathrm{edr}}(r$,v$_{\vert \vert}$) using perturbed magnetic fields from GYRO simulations of ITG turbulence show two interesting results: 1) an absence of any peak in $D_{\mathrm{edr}}(r$,v$_{\vert \vert }$) at values of the ``resonant'' parallel velocity, v$_{\vert \vert}$, at which the ExB shear negates the magnetic shear, and 2) a significant increase in $D_{\mathrm{edr}}(r$,v$_{\vert \vert }$) for electrons with v$_{\vert \vert}$ surprisingly far from the resonant velocity. We explore these effects both through a more detailed quantification of the displacement and decorrelation rates of the orbits, as a function of parallel distance, and through a simplified model of electron drift motion in a poloidally localized turbulent magnetic field. Furthermore, we argue that a correct model will have ExB shearing of the perturbed magnetic field structures themselves, and we extend our investigations to include this effect. \\[4pt] [1] W. Guttenfelder, et al., Phys. Plasmas \textbf{19}, 056119 (2012);\\[0pt] [2] E. Wang, et al., Phys. Plasmas \textbf{18}, 056111 (2011);\\[0pt] [3] A. M. Dimits, et al., 2014 TTF Meeting.