Fusion triple product (n·T·τ_E)

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The triple product is the plasma density n multiplied by the ion temperature T multiplied by the energy confinement time τ_E, and it is the single number that expresses why fusion is hard: all three quantities must be high at the same time, and improving one usually degrades another. It is conventionally expressed in units of keV·s·m⁻³. For a self-sustaining deuterium-tritium burn the threshold is commonly quoted at roughly 3×10²¹ keV·s·m⁻³, though published values range up to about 5–6×10²¹ depending on which criterion is being applied, what profile shapes are assumed, and whether ignition or merely breakeven is the target. The magnetic and inertial approaches attack the same product from opposite ends — seconds at 10²⁰ m⁻³ versus fractions of a nanosecond at some 10³¹ m⁻³ — which is why their results are not directly comparable.

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