Impurity and Radiation Control

Impurity control balances the radiation that protects walls against the core dilution and radiative collapse that too many impurities cause.

The dual role of impurities

Impurities, atoms heavier than the hydrogen fuel, both help and hurt. In the edge and divertor, they radiate power over a large area and protect surfaces from concentrated heat. In the core, they radiate energy that should have heated the fuel and they dilute the fuel by taking up charge, lowering fusion output. Control keeps impurities where they help and out of where they hurt.

Sources and sinks

Impurities enter from wall sputtering, from deliberate seeding gases, and from the fusion process itself as helium ash. They leave through the divertor pumping and through core transport events like sawtooth crashes. Control acts on the deliberate levers: seeding rate, fueling, and the transport-modifying actuators.

Radiation as a controlled quantity

The total and spatially resolved radiated power is a controlled variable. Bolometers measure it in real time. A radiation controller adjusts impurity seeding to hold edge radiation at a set fraction of the input power, enough to protect the divertor but not so much that the radiation front moves into the confined plasma and cools it.

Core accumulation

Heavy impurities can accumulate in the core, where central heating and sawteeth help expel them. A radiation controller watches the central radiation signal; a rising core signal calls for more central heating or sawtooth pacing to break up the accumulation before it collapses the temperature.

In the Kronos program

The Hyperion breeder must manage helium ash from deuterium-tritium fusion alongside seeded impurities used for divertor protection. Its radiation control couples to divertor detachment control, since both act on the same edge radiation, and to sawtooth control, which helps keep the core clean. These interactions are studied in simulation ahead of operation.