Impurities and Radiative Losses

Impurity ions radiate energy and dilute the fuel; a little is useful for exhaust, too much extinguishes the burn.

Not everything in the plasma is fuel

Real plasmas contain impurities — ions of elements heavier than the hydrogen fuel, sputtered from walls or deliberately seeded. Impurities have two effects. They radiate energy efficiently, especially heavier species, cooling the plasma; and they take up space and electrons, diluting the D-T fuel and lowering the fusion rate at fixed pressure.

Both effects cut against gain, so impurity control is a confinement issue. Yet a controlled amount of radiation is useful at the edge, where it helps dissipate exhaust power and induce detachment. The design wants radiation concentrated at the edge for exhaust benefit and minimized in the core where it would sap the burn.

Dilution and the burn

Fuel dilution is captured by the effective charge of the plasma: the more impurity, the higher the effective charge and the fewer fuel ions per electron. Because fusion power depends on the fuel-ion density squared, even modest dilution noticeably reduces power. Keeping impurities low enough to protect the core burn while allowing edge radiation for exhaust is a balance quantified in the design-and-simulation model.

• Impurities radiate energy and dilute the fuel
• Edge radiation aids exhaust; core radiation saps the burn
• Dilution reduces fusion power, which scales with fuel density squared

This page describes a design-and-simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. No hardware net-gain is claimed before FOAK.