Short Wavelength Geodesic Acoustic Mode Excitation by Energetic Particles

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.