Neutral Particle Transport Codes
Neutral transport codes track un-ionized atoms and molecules through the plasma, supplying the fueling, recycling, and charge-exchange sources the plasma models need.
Neutrals matter
Fusion plasmas are surrounded by neutral gas from recycling at surfaces, gas puffing, and pellet injection. Neutrals are invisible to magnetic confinement, so they penetrate the plasma along straight lines until they are ionized or charge-exchange. Neutral transport codes compute where neutrals deposit particles and where they remove momentum and energy, providing the source terms that plasma transport and edge codes require.
Because neutrals are collisionless between reactions and their mean free path spans the geometry, the natural method is kinetic: track particles through the domain rather than solving a diffusion equation.
Monte Carlo dominance
The workhorse method is Monte Carlo: launch representative neutrals from surfaces and sources, follow them through ionization and charge-exchange collisions using cross-section data, and tally the resulting sources on the plasma grid. This handles complex geometry and molecular chemistry that a fluid neutral model cannot.
Coupling and iteration
Neutral sources depend on the plasma, and the plasma depends on the neutral sources, so neutral codes are iterated with edge fluid or core transport codes until both converge. The statistical noise of Monte Carlo neutrals must be controlled so it does not destabilize the coupled iteration.
Design relevance
For the Hyperion breeder, neutral transport sets the fueling efficiency and the recycling that feeds the divertor, both simulated during design. Accurate neutral modeling is a prerequisite for credible density-control and exhaust predictions ahead of the Q2 2027 construction start.
- Tracks un-ionized atoms and molecules
- Supplies fueling, recycling, charge-exchange sources
- Kinetic Monte Carlo is the standard method
- Iterated to convergence with plasma codes