Energy out divided by energy in. Q=1 is scientific breakeven; a power plant needs Q well above 1. The single number for how close fusion is.
Records are falling fast — gain 4 at NIF, 1,000 s-plus plasmas in China and France — but engineering breakeven and a grid-connected plant are still distinct, harder steps. Our read on the gap. (Our opinion, not investment advice.)
NIF crushes a fuel pellet with lasers; EAST, JET and ITER hold plasma in magnetic fields. They post different numbers (gain vs duration) because they're different machines chasing the same goal.
NIF's climb to 8.6 MJ and target gain above 4 came less from more laser power than from better fuel capsules. We explain how high-density carbon (diamond) capsules and hohlraum quality turned a one-off ignition into a repeatable record.
NIF passed scientific breakeven in 2022 — but the lasers drew ~300 MJ from the wall to deliver 2 MJ to the target. Engineering breakeven, where the whole plant nets energy, is the bar that matters for the grid.
Q is fusion energy out divided by energy in. Q=1 is scientific breakeven; a power plant needs Q well into the tens. We explain why NIF's “gain 4” and a plant's Q aren't the same number.
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NIF / LLNL 
WEST (CEA)