| ISSUE 08/30 · PHYSICS |
~4 MIN |
NOW / FUSION RECORD · WEEK 02 — MACHINES UNDER PRESSURE
What fusion gain actually measures
START HERE
Every atom has a tiny centre called its nucleus. Under extreme conditions, nuclei from low-mass atoms such as hydrogen can join and release energy. That joining process is fusion. Fusion gain is a family of ratios comparing fusion energy with a stated input. Here, NIF target gain counts laser energy delivered to the target—not just energy absorbed by its fuel capsule. The number changes meaning when one calculation counts only the fuel while another counts the entire machine.
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/ WHY NOW
In an April 2025 experiment reported in detail in 2026, the US National Ignition Facility delivered 2.08 megajoules of laser energy to a tiny target and measured 8.6 megajoules of fusion energy. That was a target gain above four. The result is real, but it is not the same as a power station producing four times the electricity it consumes. The boundary around the calculation decides what counts as energy in.
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/ THE IDEA
Fusion joins low-mass atomic nuclei. The joined system has slightly less mass than its starting parts, and that difference leaves as energy. At NIF, lasers compress and heat a tiny fuel capsule until the nuclei can join. Target gain compares fusion energy with laser energy that reached the target. Facility gain would compare useful output with all electricity used by lasers, pumps, cooling and other equipment. A power station must cover that larger boundary and still deliver a surplus reliably.
THE FORMAL IDEA
NIF target gain Q = fusion energy out ÷ laser energy reaching the target
| Q = 1 means those two quantities are equal | | Q > 1 means fusion released more than the counted heating input | | this selected definition applies to NIF target gain; other experiments draw different boundaries |
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RUN THE TINY EXAMPLE
Calculate the record target gain
A megajoule (MJ) is one million joules, a unit of energy Laser energy reaching target = 2.08 MJ Fusion yield = 8.6 MJ → target gain Q = 8.6 ÷ 2.08 ≈ 4.13 Facility electricity was larger than 2.08 MJ, so this is not whole-facility gain
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Change the boundary and the denominator changes. That is why a genuine scientific milestone and practical electricity can remain separated by a large engineering gap.
/ SO WHAT?
When you see a fusion claim, ask three questions: which gain is quoted, for how long, and whether the machine included the energy needed to run itself. Those questions convert hype versus cynicism into a useful engineering conversation.
ONE CAVEAT |
| Different fusion approaches and experiments use several gain definitions. Q alone does not cover repetition rate, component lifetime, fuel supply, maintenance, heat conversion or cost. |
KEEP THIS
A fusion gain number is meaningful only when you know where its energy-accounting boundary was drawn.
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NEXT: How software rebuilds a particle collision
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