Characteristics of nonlinear interaction in entropy cascade

Two-dimensional electrostatic turbulence in weakly-collisional magnetized plasmas can be described as a turbulent cascade of entropy in phase space [1]. Nonlinear phase mixing [2] introduces small structures of distribution function in both position and velocity space. By invoking the Kolmogorov-type phenomenology applied in the phase space, it is argued that entropy cascades to smaller scales as energy or enstrophy does in the Navier-Stokes turbulence. In this presentation, we report the detailed characteristics of nonlinear interaction in the gyrokinetic turbulence. We diagnose the triad interaction in the wave-number as well as in the velocity dual space using the AstroGK code [3], and see how quantitatively the theoretical assumptions are satisfied, namely, (a) the scale locality of nonlinear intercation, and (b) constancy of entropy flux both in position and in velocity space. \\[4pt] [1] T. Tatsuno \textit{et al}., Phys.\ Rev.\ Lett.\ \textbf{103}, 015003 (2009).\\[0pt] [2] W. Dorland and G. Hammett, Phys.\ Fluids B \textbf{5}, 812 (1993).\\[0pt] [3] http://www.physics.uiowa.edu/\~{}ghowes/astrogk/