Laser Fusion Ignition

More Out Than In

At 1:03 a.m. on 5 December 2022, NIF delivered 2.05 MJ of ultraviolet laser light to its target and measured 3.15 MJ of fusion energy. For the first time the fusion energy released was larger than the laser energy delivered to the target: a target gain of about 1.5. The peer-reviewed paper gives 3.1 ± 0.16 MJ.

The shot released about 1.1 × 10¹⁸ neutrons, one from each deuterium–tritium fusion. In 1997 the US National Academy of Sciences defined ignition as a gain greater than one, and this is the definition NIF uses. A shot on 8 August 2021 had already reached ignition in the sense of the Lawson criterion, releasing about 1.3 MJ, but its gain was still below one.

Laser 2.05 MJ → fusion 3.15 MJ · target gain about 1.5 · 5 December 2022

Counting From the Grid

The 2.05 MJ is only the laser light that reached the target. For the shot, NIF first stored about 400 MJ of electrical energy in 3,840 capacitors over 60 seconds, then released it in 400 microseconds into 7,680 flashlamps. The flashlamps pump the amplifiers, which make about 4 MJ of infrared light; turned into ultraviolet, about 2 MJ is left. The capsule absorbs about 258 kJ, and about 19.4 kJ goes into the fuel itself.

Those 400 MJ or so all come from the grid: some 200 times the laser energy delivered to the target. The 3.15 MJ released was less than one hundredth of it. NIF fires about 400 shots a year. A power plant would light about ten targets a second, and even a test facility with zero net electric demand would need a target gain approaching 15.

Capacitors ≈ 400 MJ → laser 2.05 MJ → fuel 19.4 kJ → fusion 3.15 MJ · under 1% of the grid’s share

Inward Like a Rocket

The capsule is a hollow ball of diamond with a layer of frozen deuterium–tritium on its inside. The X-rays heat the outer layer of the diamond into a plasma that streams outwards, and the recoil drives the rest of the shell inwards, like a rocket’s exhaust. Most of the diamond is burnt away on the way.

In a few nanoseconds the shell reaches 380 to 400 km a second and squeezes the cold fuel to 1,000 to 2,000 times its density. The radius and speed in the figure come from the page’s thin-shell model: it works by the rocket principle, with a few constants set so that it matches the speed, the timing and the size given in the papers. A tokamak holds a thin fuel with magnetic fields for a long time; here the fuel holds together only by its own inertia, for tens of picoseconds, which is why this is called inertial confinement.

Implosion speed 380–400 km/s · fuel compressed 1,000–2,000 times (N221204) · the page’s model peaks at about 384 km/s

Light Into X-rays

The laser does not shine on the capsule directly. The beams enter a small gold can, the hohlraum, through holes at its top and bottom. They strike its inner wall and heat it to about 300 electronvolts, more than three million degrees. The hot wall gives off X-rays that fill the can and bathe the capsule fairly evenly from every side.

For N221204 the hohlraum was 6.4 mm across and 11.24 mm long, with a wall of depleted uranium lined with gold and 0.3 mg of helium in each cubic centimetre; for the 2021 design its laser entrance holes had been narrowed from 3.65 to 3.1 mm. The inner cones’ light lands near its waist, the outer cones’ near its ends. Offsetting the two sets of beams in wavelength (by 2.75 ångströms on this shot) moves energy from one set to the other where they cross, which evens out the squeeze.

Hohlraum 6.4 × 11.24 mm · about 300 eV (313 eV at peak on N221204) · capsule absorbs about 258 kJ

A Hot Spot That Heats Itself

About 9.5 ns after the laser starts, the shell stops and squeezes the centre into a hot spot a few tens of micrometres across: about 13 keV, or 150 million degrees, at a pressure of about 600 billion atmospheres. Deuterium and tritium fuse there, each fusion giving off a 14.1 MeV neutron and a 3.5 MeV alpha particle.

The neutrons nearly all escape, but the alpha particles cannot go far: the hot spot’s areal density, about 0.44 g per square centimetre, is more than an alpha particle’s range at 10 keV, about 0.3. The alphas leave their energy in the hot spot, which gets hotter and burns faster. In shots from late 2020 to early 2021 this self-heating first exceeded the work done by the compression: a burning plasma. On the December 2022 shot the burn lasted about 75 picoseconds and used up about 4.3% of the fuel.

Hot spot 13.1 keV · 608 Gbar · areal density 0.44 g/cm² · burn about 75 ps · 4.3% burnt (N221204)

Cut Open: Ten Metres

The target chamber is an aluminium sphere 10 m across, assembled from 10 cm aluminium panels and weighing 287,000 pounds, about 130 tonnes. It is covered with 0.3 m of concrete laced with boron to absorb the neutrons from fusion. It was lifted into place on 17 June 1999; during a shot its inside is a vacuum.

Each of the 48 beam ports carries a final optics assembly. Three diagnostic instrument manipulators bring instruments close to the centre from the top and the waist, and a seven-metre positioner holds the target at the centre and keeps it at about 18 K. The model is 1 : 8, with a wedge on this side cut away from the top to just below the equator; the figure is 1.75 m tall.

10 m across · 10 cm aluminium · 0.3 m borated concrete · about 130 t · model 1 : 8

192 Beams in Four Cones

The 192 beams travel in groups of four, entering the chamber through 48 ports: 24 groups from the upper half, 24 from the lower. The ports lie on four cones at 23.5°, 30°, 44.5° and 50° from the vertical: the first two are the inner cones, 64 beams; the other two the outer cones, 128 beams.

Each beam travels about 1,500 m from where it is made to the target. On the December 2022 shot the beams arrived within 30 picoseconds of each other and were pointed to within 50 micrometres. Ultraviolet light is invisible; the page draws it violet, and leaves out the beams of the wedge that is cut away.

48 groups of 4 · inner cones 23.5° / 30° · outer cones 44.5° / 50°

Amplified a Quadrillion Times

Every shot starts in the master oscillator, a fibre laser that makes pulses of only a few nanojoules; the shape of the pulse is set here. Forty-eight preamplifier modules raise it to about a joule per beam and send it into two laser bays, each 122 m long.

The amplifiers hold 3,072 slabs of neodymium-doped phosphate glass, 16 for each beam, pumped by 7,680 flashlamps. A plasma-electrode Pockels cell acts as an optical switch, sending the light four times through the main amplifier and then twice through the power amplifier. Each beam comes out with more than 20 kJ, amplified more than a quadrillion times in all. Mirrors in the switchyards then send the beams to the upper and lower halves of the chamber.

A few nJ → about 1 J a beam → more than 20 kJ a beam (1053 nm) · 3,072 glass slabs · 7,680 flashlamps · gain over 10¹⁵

Infrared to Ultraviolet

The amplifiers give infrared light at 1053 nm. In the final optics assemblies on the chamber, a first crystal of KDP turns about two thirds of it into green light at 527 nm, and a second crystal of DKDP combines that green with the rest of the infrared into ultraviolet at 351 nm.

The design’s peak conversion efficiency is 85%, but a shaped pulse has long stretches at low power, which convert poorly: over a shot about 4 MJ of infrared becomes about 2 MJ of ultraviolet. In the hohlraum ultraviolet light makes fewer hot electrons, which would heat the fuel too early. After the crystals comes a slightly wedged focusing lens, 7.7 m in focal length, which sets the leftover infrared and green aside from the focus; last is a debris shield.

1053 nm → 527 nm → 351 nm · KDP + DKDP · about 4 MJ infrared → 2 MJ ultraviolet a shot · 48 assemblies

A Small Gold Can

Here the target is drawn 100 times its size. Each laser entrance hole of the hohlraum is closed by a thin film that keeps the helium in. The hohlraum sits in an aluminium thermal–mechanical package with silicon heat sinks; the cryogenic positioner cools it to about 18 K and holds it there to within a thousandth of a degree.

The capsule hangs at the hohlraum’s centre between two Formvar films 45 nm thick, known as the tent. A fill tube 2 µm across runs into the capsule from the side; the deuterium–tritium is filled through it.

Model 100 : 1 · hohlraum 6.4 × 11.24 mm · about 18 K · tent films 45 nm · fill tube 2 μm

Smaller Than a Peppercorn

The capsule’s shell is nanocrystalline diamond, also called high-density carbon: 1,050 µm in inner radius, about 85 µm thick, weighing 4.25 mg. Buried in it is a layer 15 µm thick with about 0.6% of its atoms tungsten, which blocks X-rays that would heat the fuel too early.

On the shell’s inside is a layer of frozen deuterium–tritium 64.5 µm thick, 220 µg in all, with deuterium–tritium vapour in the middle. The ice is kept at about 18 K, a little below the triple point of deuterium–tritium, about 19.8 K. The shell must be smooth to nearly a nanometre; the shells of 2022 had ten times fewer surface pits and inner voids than those of 2021.

Diamond shell about 85 μm · tungsten-doped layer 15 μm · DT ice 64.5 μm (220 μg) · about 18 K

Ten More Since

By June 2026 NIF had reached ignition 11 times. On 30 July 2023 the same 2.05 MJ of laser light gave 3.88 MJ; from October 2023 the laser energy on ignition targets went up to 2.2 MJ. On 7 April 2025, 2.08 MJ gave 8.6 MJ, a target gain of 4.13 and the record so far. The latest was on 20 June 2026: 7.9 MJ, a gain of about 3.8.

In April 2026 a project to raise the laser’s energy from 2.2 to 2.6 MJ passed its conceptual design approval. Every one of these gains counts the laser light delivered to the target, not the losses between the grid and the laser.

11 ignitions (to June 2026) · record 8.6 MJ from 2.08 MJ, gain 4.13 (7 April 2025)

Taken Apart

The final optics and their beam tubes slide out along their own ports, the diagnostic manipulators and the target positioner back along their axes; the aluminium sphere and the column stay.

Point at a part and it lifts out alone, the rest fading; at rest, every part is named.