magnetohydrodynamics (MHD) is the theory that treats a plasma as a single electrically conducting fluid coupled to magnetic fields — the framework fusion science uses to describe plasma equilibrium and large-scale stability.
The name compounds magneto, hydro, and dynamics: the dynamics of conducting fluids in magnetic fields. Swedish physicist Hannes Alfvén founded the field in the 1940s, showing that a conducting fluid and a magnetic field move together and support their own family of waves; the work earned him the 1970 Nobel Prize in Physics. MHD's central simplification is to ignore individual particles entirely and follow only bulk quantities — density, velocity, pressure, current — which makes it tractable where a full particle-by-particle description is not.
That simplification defines both its power and its limits, and the distinction against kinetic theory is the one that matters in practice: MHD answers questions about the plasma as a whole — whether an equilibrium exists and whether it is stable — while kinetic theory is required for phenomena that live at particle scales, such as turbulence and wave heating. In a 📝Tokamak, MHD sets the operational boundaries of the machine: it describes the kink, ballooning, and tearing instabilities that limit 📝plasma current and pressure, and the disruptions that follow when those limits are crossed. Designing a 📝plasma that stays inside its MHD stability limits is a first-order constraint on every magnetic confinement device. Both CFS machines have a published MHD analysis: 📝Ryan Sweeney's for 📝SPARC in 📝The SPARC Physics Basis (2020), and 📝Nils Leuthold's Columbia-led paper in 📝The ARC Physics Basis (2026).