Symplectic methods for optimization of wall heat loads by fusion alpha particles in stellarator reactors

Predicting alpha particle heat loads on first-wall components is essential for stellarator reactor design. We present a symplectic guiding-center integration method that traces fusion-born alpha particles from the plasma core to physical wall surfaces defined by CAD geometry. The approach uses Meiss–Hazeltine coordinates, which extend beyond the last closed flux surface, combined with segment–triangle intersection using the CGAL library. We compare the SIMPLE code with the LSODE-based BEAMS3D tracer for W7-X reactor conditions. Applying the method to the Proxima 5C quasi-isodynamic stellarator, we compare four wall representations at 250 mm offset: VMEC-extended coordinates, VMEC-to-wall interpolation, harmonic (map2disc) coordinates, and explicit STL mesh intersection. While total prompt-loss fractions are similar across methods (approximately 5% within 500 ms), the first-hit distributions depend on wall representation and mesh resolution. This sensitivity arises from the near-tangent incidence of prompt-loss particles, where small differences in effective wall orientation produce large changes in intersection location. Accurate prediction of divertor heat loads therefore requires explicit treatment of wall geometry rather than parameterized coordinate approximations. The collisionless guiding-center model does not resolve full-orbit, sheath, or material-response effects at impact.