  The set of equations describing nonlinear evolution of a single toroidal
Alfven eigenmode are derived, including both zero frequency zonal structure
(ZFZS) generation and wave-particle phase space nonlinearities. The simplified
case of neglecting wave-particle phase space nonlinearity is then investigated
to focus on different roles of energetic particles and bulk plasmas on ZFZS
generation. It is shown that energetic particles and bulk plasma play dominant
roles in ZFZS generation in different nonlinear stages, and the corresponding
processes are qualitatively different. Several properties of ZFZS generation,
e.g., fine- vs. meso-scale, forced driven vs. spontaneous excitation, are
clarified by the present analysis.
