  This paper describes a conceptual framework for understanding kinetic plasma
turbulence as a generalized form of energy cascade in phase space. It is
emphasized that conversion of turbulent energy into thermodynamic heat is only
achievable in the presence of some (however small) degree of collisionality.
The smallness of the collision rate is compensated by the emergence of
small-scale structure in the velocity space. For gyrokinetic turbulence, a
nonlinear perpendicular phase mixing mechanism is identified and described as a
turbulent cascade of entropy fluctuations simultaneously occurring at spatial
scales smaller than the ion gyroscale and in velocity space. Scaling relations
for the resulting fluctuation spectra are derived. An estimate for the
collisional cutoff is provided. The importance of adequately modeling and
resolving collisions in gyrokinetic simulations is biefly discussed, as well as
the relevance of these results to understanding the dissipation-range
turbulence in the solar wind and the electrostatic microturbulence in fusion
plasmas.
