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NIMROD calculations of energetic particle driven toroidal Alfv\'en eigenmodes


Toroidal Alfv\'en eigenmodes (TAEs) are gap modes induced by the toroidicity of tokamak plasmas in absence of continuum damping. They can be excited by energetic particles (EPs) when the EP drive exceeds other dampings. A TAE benchmark case, which was proposed by the International Tokamak Physics Activity (ITPA) group, is studied in this work. Numerical calculations of linear growth of TAEs driven by EPs in a circular-shaped, large aspect ratio tokamak have been performed using the Hybrid Kinetic-MHD (HK-MHD) model implemented in the NIMROD code. This HK-MHD model couples a delta f particle-in-cell (PIC) representation of EPs with the 3D MHD representation of the bulk plasma through moment closure for the momentum conservation equation. Both the excitation of TAEs and their transition to energetic particle modes (EPMs) have been observed. The influence of EP density, temperature, density gradient and position of the maximum relative density gradient, on the frequency and the growth rate of TAEs are obtained, which are consistent with those from eigen-analysis calculations and gyrokinetic simulations for an initial Maxwellian distribution of EPs. The relative pressure gradient of EP at the radial location of TAE gap, which represents the drive strength of EPs, can strongly affect the growth rate of TAEs. When the density and temperature of EP distribution are above certain threshold, the transition from TAE to EPM occurs and the mode structure changes.