Impurity Transport Codes
Impurity transport codes follow non-fuel ions through their charge states, predicting radiation, dilution, and accumulation in the plasma core.
Tracking the contaminants
Impurities, from wall erosion, seeded gases, or helium ash, dilute the fuel, radiate power, and can accumulate in the core. Impurity transport codes evolve the density of each impurity species across its many charge states as it is transported, ionized, and recombined, predicting where impurities go and how much power they radiate.
Because a single element exists in dozens of charge states with different transport and radiation, the problem is a coupled system of transport equations linked by ionization and recombination rates.
Charge-state balance
At each location the local temperature sets the balance of charge states through ionization and recombination coefficients drawn from atomic databases. Line radiation from partially stripped ions is a strong function of charge state, so the radiated power depends sensitively on the coronal or non-coronal balance the code computes.
Neoclassical accumulation
Heavy impurities are prone to neoclassical inward transport that concentrates them on axis, where their radiation can collapse the core temperature. Predicting whether a given impurity accumulates or is flushed out requires combining neoclassical and turbulent transport, tying impurity codes to the broader transport framework.
Design relevance
For the Hyperion breeder, impurity modeling estimates radiated power, fuel dilution, and helium-ash removal, all of which bear on achieving the design fusion power of 88.7 MW at Q 3.424. Seeded-impurity radiation also links to the divertor detachment strategy. These are simulation studies before construction.
- Evolves impurity densities across charge states
- Charge-state balance sets line radiation
- Heavy impurities can accumulate on axis
- Predicts dilution, radiation, and ash removal