  Secondary low frequency mode generation by energetic particle induced
geodesic acoustic mode (EGAM) observed in LHD experiment is studied using
nonlinear gyrokinetic theory. It is found that the EGAM frequency can be
significantly higher than local geodesic acoustic mode (GAM) frequency in low
collisionality plasmas, and it can decay into two GAMs as its frequency
approaches twice GAM frequency, in a process analogous to the well-known two
plasmon decay instability. The condition for this process to occur is also
discussed.
