Report
- Complex Global Simulation
- Report
Nonlinear excitation of energetic particle driven geodesic acoustic mode by Alfvén instability in ASDEX-Upgrade Tokamak
Recently, the coexistence of multiple energetic particle driven instabilities was observed in experiments on the ASDEX-Upgrade tokamak. A hybrid simulation using the MEGA code was performed to investigate the properties of those instabilities. It is found that the energetic particle driven geodesic acoustic mode (EGAM) is initially stable, then zonal flow gradually occurs with the growth of the Alfvén instability, and finally, the EGAM is nonlinearly excited and the amplitude exceeds that of the Alfvén instability. The mode properties of the nonlinearly excited EGAM are quite different from linearly excited EGAM. For the linearly excited EGAM, the frequency can be higher than conventional geodesic acoustic mode frequency, but only under the condition of bump-on-tail energetic particle distribution. Interestingly, the higher frequency holds true even under the condition of a slowing-down energetic particle distribution for the nonlinearly excited EGAM. This difference is because when the EGAM is nonlinearly excited, the energetic particle distribution function changes from its initial distribution due to the Alfvén instability. This change in the distribution function affects the properties of the EGAM.
The numerical simulations were performed on Plasma Simulator of NIFS, JFRS-1 of IFERC-CSC, and Fugaku of R-CCS.
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