On the radial propagation of turbulence in gyro-kinetic toroidal systems
  In this paper a conservation equation is derived for the radially dependent entropy in toroidal geometry using the local approximation of the gyro-kinetic framework. This equation naturally leads to an operative definition for the turbulence intensity. It is shown that the conservation equation can be split in two separate conservation equations, one describing the dynamics of the zonal modes and one for the non-zonal modes. In essence the paper provides an operative tool for both analytic as well as numeric studies of the radial propagation of turbulence in tokamak plasmas. 
P. Migliano, R. Buchholz, S.R. Grosshauser, W.A. Hornsby, A.G. Peeters
physics.plasm-ph