Zeff (effective charge) is the density-weighted average electric charge of the ions in a plasma — the standard single-number measure of its impurity contamination.
A 📝plasma of pure hydrogen isotopes has a Zeff of exactly 1, because every ion carries a single charge. Every non-fuel ion pulls the average upward: helium ash nudges it, and heavy atoms sputtered from the wall raise it disproportionately, since Zeff weights each species by the square of its charge. A trace of a high-Z metal such as 📝tungsten can therefore move Zeff more than a far larger quantity of a light gas. Working machines typically run with Zeff between about 1.5 and 2, and reactor designs strain to keep it low.
The number matters because a plasma pays for contamination twice. Bremsstrahlung losses scale directly with Zeff, so a dirtier plasma radiates its heat away faster as 📝impurity radiation; and since each high-charge ion displaces many fuel ions at fixed pressure, high Zeff means fuel dilution — fewer 📝fusion-capable nuclei in the same volume. Zeff is routinely inferred from the brightness of the plasma's visible bremsstrahlung, making it one of the standard quantities a control room watches to judge how clean its plasma is.