Core-Edge Coupling
Core-edge coupling joins a core transport model to a separate edge and scrape-off-layer model so the boundary condition on the core is computed, not guessed.
The boundary-condition gap
Core transport solvers need a boundary condition near the plasma edge: the density and temperature at some inner flux surface, often taken at the top of the pedestal or the separatrix. Historically this was an assumption. Core-edge coupling replaces the assumption by connecting the core model to a dedicated edge and scrape-off-layer model that computes those values from first principles.
The physics differs sharply across the interface. The core is closed flux surfaces with strong confinement; the edge and SOL are open field lines with parallel transport to material surfaces, neutral recycling, and steep gradients. Coupling two codes tuned to these regimes is more robust than forcing one code to span both.
Overlap and handoff
Coupling schemes exchange fluxes and state in an overlap region. The core passes fluxes crossing the coupling surface; the edge returns the density and temperature there. Iterating this exchange to convergence yields a continuous profile from magnetic axis to wall.
Numerical challenges
The two codes run on different grids, timescales, and often different processor counts. Stable coupling requires careful operator splitting and relaxation to avoid oscillation between the domains. Mismatched conserved quantities at the interface must be reconciled to preserve global particle and energy balance.
Why it matters
Pedestal height sets core performance through profile stiffness, so the edge boundary condition strongly controls predicted fusion power. For the Hyperion breeder, credible power predictions in simulation require the edge to be computed rather than assumed, which is exactly what core-edge coupling provides.
- Computes the core boundary condition from an edge model
- Handles the physics change across the separatrix
- Exchanges fluxes and state in an overlap region
- Critical for credible power prediction