Speaker
Description
As a key facility for the China Spallation Neutron Source (CSNS) Phase-II upgrade, the newly proposed Fixed-Field Alternating Gradient (FFA) accelerator faces severe nonlinear beam dynamics challenges due to its large-aperture magnets. Simulation tests based on Opera-3D magnetic field maps indicate that the dynamic aperture (DA) of the initial field map is approximately 550 $\pi\cdot\text{mm}\cdot\text{mrad}$. This represents a significant decrease from the theoretical design value and fails to meet the strict requirement of $>1000 \pi\cdot\text{mm}\cdot\text{mrad}$ for a high-intensity accelerator. To diagnose the physical origin of this DA limitation, harmonic analysis and particle loss mechanism studies were conducted systematically based on full-ring tracking data. The harmonic analysis revealed an excessive octupole field component in the original field map, which is particularly pronounced at the entrance of the focusing (F) magnet. Meanwhile, by analyzing the evolution of particle actions and tunes during motion, an anomalous evolution phenomenon far from resonance lines was observed; in response to this phenomenon, a corresponding nonlinear beam dynamics explanation was provided, and the induced particle loss mechanism was systematically elucidated. Guided by this physical analysis, the field map model was improved by adopting an optimization scheme that increases the magnet gap. Tracking verification demonstrates that the optimized field map successfully reduces the octupole component to the theoretical level. Consequently, the DA is nearly doubled, achieving $>1000 \pi\cdot\text{mm}\cdot\text{mrad}$, which fully satisfies the initial design requirements.