Collisionless kinetic-fluid closure and its application to the three-mode ion temperature gradient driven system

A novel closure model is presented to give a set of fluid equations which d escribe a collisionless kinetic system. In order to take account of the tim e reversal symmetry of the collisionless kinetic equation, the new closure model relates the parallel heat flux to the temperature and the parallel fl ow in terms of the real-valued coefficients in the unstable wave number spa ce. Effects of the closure model on turbulence saturation and anomalous tra nsport are investigated based on kinetic and fluid entropy balances. When t he closure model is applied to the three-mode ion temperature gradient (ITG ) driven system, the fluid system of equations reproduces the exact nonline ar kinetic solution found by Watanabe, Sugama, and Sato [Phys. Plasmas 7, 9 84 (2000)]. Oscillatory behaviors and initial amplitude dependence of other numerical kinetic solutions of the three-mode ITG problem can also be accu rately described by the fluid system. (C) 2001 American Institute of Physic s.