Neutral-Particle Transport at the Edge
Kinetic neutral codes track atoms and molecules recycling from surfaces, a physics essential to fueling, radiation, and divertor detachment.
Neutrals in a plasma boundary
Ions striking a surface recombine and return as neutral atoms and molecules. These neutrals travel ballistically until they ionize, dissociate, or charge-exchange, redistributing particles and energy. In the cold, dense divertor they dominate the physics, so a faithful neutral model is indispensable for edge simulation.
Why kinetic, not fluid
Neutrals have long mean free paths compared to plasma gradients, so a fluid description is often invalid. Kinetic neutral codes, usually Monte-Carlo, follow individual neutral trajectories through the plasma background, sampling ionization, charge-exchange, and molecular reactions along the way.
Processes tracked
- Recycling and reflection of particles at material surfaces
- Charge-exchange, which cools the plasma and moves energy across field lines
- Molecular dissociation chains that govern the cold divertor
- Radiation from excited neutrals that helps dissipate exhaust power
Coupling to the plasma
The neutral model and the plasma fluid solver are iterated to consistency: the plasma sets the ionization and charge-exchange rates the neutrals experience, while the neutrals set the fueling, cooling, and momentum sinks the plasma feels. This tight coupling is the core of SOLPS-class edge simulation.
Data dependence
Neutral transport relies on extensive atomic and molecular reaction-rate data. The fidelity of a simulation depends on the completeness and accuracy of these databases, especially the molecular processes that control detachment, so the data source is part of any careful result.
Neutral physics is what makes divertor detachment possible, linking this modeling directly to the exhaust-handling strategy of a high-power device.