  Taking the collisionless damping of geodesic acoustic mode (GAM) as an
example, the physics processes underlying wave particle resonances in the short
wavelength limit are clarified. As illus- trative application, GAM excitation
by energetic particles in short wavelength limit is investigated assuming a
single pitch angle slowing-down fast ion equilibrium distribution function.
Conditions for this energetic particle-induced GAM (EGAM) to be unstable are
discussed.
