Edge and Scrape-Off-Layer Fluid Codes
Edge fluid codes such as the SOLPS class model plasma and neutrals in the boundary region, predicting the heat and particle loads reaching the divertor.
The boundary region
Outside the last closed flux surface, plasma flows along open field lines into the scrape-off layer and onto divertor targets. This edge region governs how exhaust power and particles are handled, how impurities enter the core, and whether plasma-facing components survive. Edge fluid codes model it as coupled plasma and neutral fluids.
Coupled plasma and neutrals
SOLPS-class codes couple a multi-fluid plasma solver, evolving density, momentum, and energy for ions and electrons along and across field lines, to a neutral-particle model that tracks recycling neutrals released at surfaces. The two are solved together because neutrals fuel and cool the plasma while the plasma ionizes the neutrals.
What they predict
- Heat flux and its width at the divertor targets
- Particle and impurity fluxes to surfaces
- Plasma temperature and density profiles across the scrape-off layer
- Conditions for detachment, where the plasma cools before reaching the target
Detachment
A central goal is predicting detachment, in which volumetric radiation and neutral interactions dissipate the exhaust power before it reaches the target, spreading the load and lowering peak heat flux to survivable levels. Edge codes model the impurity radiation and neutral physics that make detachment possible.
Cost and complexity
Edge codes are notoriously demanding: the geometry is intricate, the physics spans many processes, and convergence to a steady state can be slow. Runs are computationally heavy and require careful setup, but they are the principal tool for divertor design and exhaust planning.
For any high-power device, edge modeling is how the exhaust challenge is quantified and mitigation strategies are tested before hardware is built.