Speaker
Description
The nonlinear dynamics of Toroidal Alfvén Eigenmodes (TAEs) destabilized by the resonant interaction with trapped energetic particles, and to a lesser extent, non-standard copassing particles, are investigated using the HMGC code. Focusing on the low-shear Tokamak configuration adopted in the ITPA benchmark, the analysis employs the Hamiltonian-mapping approach, where the resonant phase-space structure is sampled using test particles evolving in the self-consistent perturbed fields computed by the full-population HMGC simulation. Multiple precession-bounce resonances effectively drive the TAEs unstable, with density mixing occurring around each resonance radius as resonant particles form closed-trajectory islands in the (phase, radius) space. The islands expand with increasing mode amplitude, eventually coalescing into a global density-mixing region. This coalescence causes a sharp reduction in the power transfer from particles to the mode, serving as the main mode-saturation mechanism at higher energetic-particle densities. At lower densities, local density mixing around isolated resonances is effective enough to cause mode saturation before the coalescence occurs. The observed nonlinear dynamics are more complex than previously described scenarios due to the interplay of multiple resonances and broader effective fields from large trapped-particle orbit widths. The study reveals that the scaling of the mode saturation amplitude with the linear growth rate of the mode is slightly weaker than linear for high energetic-particle density, consistent with the radial-decoupling character of the saturation mechanism. At lower densities, a stronger scaling, slightly weaker than quadratic, aligns with a resonance-detuning saturation mechanism. These findings advance the understanding of trapped energetic particle transport and contribute valuable insights for predicting plasma confinement dynamics in fusion devices.
| Presentation type | Oral |
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