resonant magnetic perturbation (RMP) is a small, deliberately applied three-dimensional magnetic field used to suppress or soften edge localized modes in a tokamak.
The idea inverts the usual logic of magnetic confinement. A 📝Tokamak is built to be axisymmetric — the same at every angle around the ring — and stray non-axisymmetric "error fields" are normally hunted down and corrected. An RMP is an error field applied on purpose: dedicated coils produce a perturbation, thousands of times weaker than the main field, whose helical structure resonates with the field-line pitch — the 📝safety factor (q) — at the plasma edge. The resonant perturbation degrades confinement in a thin edge layer just enough to bleed pressure from the 📝pedestal, holding it below the threshold at which an 📝edge localized mode (ELM) erupts. The core 📝plasma is left essentially undisturbed: the technique trades a small, continuous confinement tax for the elimination of damaging transient bursts.
ELM suppression by RMP was first demonstrated at the DIII-D tokamak in the early 2000s and has since been reproduced on machines worldwide; ITER's design includes a dedicated set of in-vessel ELM control coils for the purpose. The term should not be confused with the resonant instabilities it manages — an RMP is an applied control field, not a mode the plasma generates itself.