  A general criterion is proposed and found to successfully predict the
emergence of chirping oscillations of unstable Alfv\'enic eigenmodes in tokamak
plasma experiments. The model includes realistic eigenfunction structure,
detailed phase-space dependences of the instability drive, stochastic
scattering and the Coulomb drag. The stochastic scattering combines the effects
of collisional pitch angle scattering and micro-turbulence spatial diffusion.
The latter mechanism is essential to accurately identify the transition between
the fixed-frequency mode behavior and rapid chirping in tokamaks and to resolve
the disparity with respect to chirping observation in spherical and
conventional tokamaks.
