Effective Charge and Fuel Dilution

The effective charge Z_eff summarizes impurity content; higher Z_eff means more dilution and radiation, both subtracting from fusion power.

One number for cleanliness

The effective charge, Z_eff, is a density-weighted average of the ion charges in the plasma. A pure hydrogen-isotope plasma has Z_eff of 1; any impurity raises it. Z_eff is the compact way to state how clean a plasma is, and it enters the physics twice: radiation losses scale with it, and fuel dilution follows from it because impurity ions displace fuel ions at fixed electron density.

For a foundry the stakes are direct. Fusion power depends on the square of the fuel-ion density, so a rise in Z_eff that displaces fuel ions cuts power more than proportionally. Holding Z_eff low protects the fusion rate that gives Q_sci 3.076, while a modest, edge-localized impurity contribution can still be tolerated or even wanted for exhaust.

Sources and control

Z_eff is set by wall material choices, the first-wall condition, any seeded impurity for radiation, and the accumulation of helium ash from the fusion reactions themselves. Managing it means choosing low-Z or well-behaved wall materials, controlling seeding, and exhausting helium ash before it builds up. The achievable Z_eff for Hyperion is a design-and-simulation input to the gain calculation, and its value carries real leverage over the result.

• Z_eff = 1 for pure fuel; impurities raise it
• Higher Z_eff → more radiation and more dilution
• Set by wall materials, seeding, and helium-ash accumulation

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.