Fusion triple product (n·T·τ)

Density × temperature × confinement time — the physics figure-of-merit for how close a plasma is to ignition, and the fairest single axis for comparing different confinement approaches. Breakeven needs ~10²¹; a D-T power plant ~5×10²¹. The magnetic-confinement record (JT-60U) has stood since the 1990s. Inertial-confinement NIF is transient and defined differently — tracked via energy gain Q instead.