SOL and Edge Transport Codes
Scrape-off-layer codes model the open-field-line plasma between the separatrix and the wall, where parallel transport, recycling, and radiation set the heat load on surfaces.
The open-field-line region
Outside the last closed flux surface, field lines strike material surfaces at the divertor or limiter. Plasma flows rapidly along these open lines while diffusing slowly across them. Scrape-off-layer (SOL) codes solve for this region, where the central design question is how to spread and dissipate the exhaust power so surfaces survive.
The dominant balance is parallel transport to the divertor targets against cross-field transport and volumetric losses from radiation and charge exchange. Neutral gas near the targets is a major player, so SOL codes are typically coupled to a neutral transport model.
Fluid and kinetic branches
Two modeling branches exist. Fluid edge codes solve Braginskii-like equations for density, momentum, and energy along and across field lines, coupled to a Monte Carlo neutral model. Kinetic edge codes solve the distribution function directly, capturing non-Maxwellian effects the fluid closure misses.
Detachment and radiation
A key operating regime is divertor detachment, where the plasma cools enough near the target that pressure and heat flux drop before reaching the surface. Seeded impurities enhance radiation to promote detachment. SOL codes predict the impurity levels and geometries needed and are the main tool for divertor design.
Design relevance
For the Hyperion breeder, SOL and edge modeling estimates divertor heat loads and the radiative fraction required to keep them tolerable, in simulation ahead of construction. The spherical-tokamak geometry, with its tight aspect ratio, makes exhaust handling a first-order design constraint.
- Models open field lines to the wall
- Parallel transport vs cross-field diffusion
- Coupled to neutral gas physics
- Predicts divertor heat loads and detachment