Fusion energy gain (Q) and target gain

Assembled from 43 blocks · 4 evidence · 23 related

1. Story
2. There are at least three quantities that public writing calls the same thing, and conflating them is the most common error about fusion. Q_plasma is fusion power out divided by heating power delivered into the plasma — JET's 1997 deuterium-tritium peak is usually quoted around Q ≈ 0.6, with lower values under quasi-steady conditions. Target gain, the inertial-confinement analogue, is fusion yield divided by the laser energy delivered to the target: NIF reported 1.5 on 5 December 2022 and 4.13 on 7 April 2025. Q_engineering, or wall-plug gain, is net electricity out divided by all electricity in, including magnets, cryogenics, lasers, pumps and control systems — no fusion device in history has exceeded a Q_engineering of one, and none has ever produced a watt of electricity from fusion. The three numbers can differ by two orders of magnitude for the same shot. A headline reporting one and implying another is not describing a different degree of the same achievement; it is describing a different achievement entirely.
3. Knowledge
4. National Ignition Facility
5. Joint European Torus (JET)
6. Fusion energy gain (Q) and target gain
7. Breakeven
8. Connections
9. National Ignition Facility
10. Joint European Torus (JET)
11. Breakeven
12. JET's DTE1 campaign sets the first deuterium-tritium fusion power record
13. NIF achieves target gain greater than unity for the first time
14. NIF repeats target gain and raises the yield to 3.88 MJ
15. JET sets the single-pulse fusion energy record of 69.26 MJ
16. NIF achieves ignition for the sixth time
17. NIF reaches 8.6 MJ yield and a target gain of 4.13
18. Commonwealth Fusion Systems' declared target for net energy gain at SPARC
19. Breakeven
20. Inertial confinement fusion
21. Fusion energy gain (Q) and target gain
22. NIF exceeds the Lawson criterion for ignition
23. NIF achieves target gain greater than unity for the first time
24. NIF repeats target gain and raises the yield to 3.88 MJ
25. NIF achieves ignition for the sixth time
26. NIF reaches 8.6 MJ yield and a target gain of 4.13
27. ITER
28. Tokamak (toroidal magnetic confinement)
29. Fusion energy gain (Q) and target gain
30. JET's DTE1 campaign sets the first deuterium-tritium fusion power record
31. JET's DTE2 campaign releases 59 MJ in a single pulse
32. JET sets the single-pulse fusion energy record of 69.26 MJ
33. JET ceases plasma operations after 105,842 pulses
34. National Ignition Facility
35. Fusion energy gain (Q) and target gain
36. Plasma-facing materials and first wall
37. NIF achieves target gain greater than unity for the first time
38. NIF reaches 8.6 MJ yield and a target gain of 4.13
39. Evidence
40. On 5 December 2022 a NIF indirect-drive implosion achieved a target gain of 1.5, with 2.05 MJ of 351 nm laser light producing about 3.1 MJ of fusion yield and significantly exceeding the Lawson criterion for ignition
41. Sets out the physics design changes — reduced coast time and maximised hot-spot internal energy — behind the first laser indirect-drive experiment to exceed target gain of unity, and frames the result explicitly as scientific breakeven
42. JET's DTE1 campaign reached 16.1 MW peak fusion power and 22 MJ in a single pulse, with a fusion-power-to-input-power ratio of about Q = 0.62 at the transient peak
43. The energy actually drawn from the grid to fire a NIF ignition shot exceeds the fusion yield by roughly two orders of magnitude, so target gain above unity is not wall-plug breakeven