Fusion gain (Q)

The ratio of fusion energy released to the energy used to heat the fuel, written Q; above 1 means net energy from the fuel.

• Q=1 is 'scientific breakeven'. The US National Ignition Facility passed it in December 2022: 3.15 MJ out from 2.05 MJ of laser light.
• Its lasers drew roughly 300 MJ from the grid, so the facility as a whole was far from breakeven; that is the gap to 'engineering' gain.
• No magnetic-confinement device has reached Q=1; the record is about 0.67 at JET in 1997, and ITER aims for Q=10.
• A power plant likely needs plasma gain well above 10 to cover its heating systems, magnets, pumps and conversion losses.