Linear Simulation Of Energetic Particle Induced Geodesic Acoustic Mode With Bump-on-tail Distribution

Geodesic acoustic mode(GAM),which is a high frequency branch of zonal flows,is symmetric in toroidal direction and nearly symmetric in poloidal direction.Energetic particle-induced geodesic acoustic mode(EGAM)is a GAM-like mode excited by energetic particles.In this paper,we simulated the linear properties of EGAM by using a hybrid kinetic-fluid code.A systematic study of Energetic particle-induced Geodesic Acoustic Mode(EGAM)has been carried out for bump-on-tail energetic distribution in tokamak plasmas.We simulated the stability threshold of EGAM in energetic particle pressure under different energetic particle distributions.It is found that under the bump-on-tail distribution,the EGAM stability threshold is very low.The eignemode structure becomes more and more radially extended as the beam ion distribution evolves from bump-on-tail distribution to fully slowing down distribution.We also simulated the relation between EGAM and energetic particle velocity.According to the simulation result,the frequency of EGAM increase as velocity increases,while the EGAM growth rate has a peak at v/vA=0.3,When the energetic particle gyroradius(or orbit width)increases,the radial width of EGAM eigenmode structure under bump-on-tail distribution becomes wider.In the nonlinear evolution of EGAM,we found that the mode frequency mainly chirps up for slowing-down distribution.