Effective Charge (Z_eff) & Plasma Purity

Z_eff measures how much impurity contaminates a plasma. Keeping it low protects fusion rate and limits radiation — a constant operational goal.

What

Charge-weighted measure of impurity content

Pure hydrogen plasma

Z_eff = 1

Impurities

Raise Z_eff, dilute fuel, boost bremsstrahlung

Goal

Keep Z_eff low for performance

Effective charge (Z_eff) is a single number summarizing how contaminated a plasma is by heavier impurity ions: a pure hydrogen-isotope plasma has Z_eff = 1, and every impurity — eroded wall material, seeded gas, helium ash — pushes it higher. Impurities hurt in two ways: they displace fuel ions (dilution) and they sharply increase bremsstrahlung radiation, which scales with charge squared.

Keeping plasma purity high (Z_eff low) is therefore a constant operational goal, balanced against the deliberate impurity seeding used to radiate power in the divertor — the two must be managed together. For Kronos MetroVolt, purity control protects the fusion rate and keeps the radiation budget within the Lawson accounting, aided by the reflective, low-erosion HfC first wall.