Thermonuclear Bomb

The first large-scale demonstrations of inertially confined thermonuclear fusion occurred in thermonuclear weapons. The United States carried out its first thermonuclear test in 1952, followed by the Soviet Union in 1953. In these devices, energy released by a fission primary rapidly compresses and heats fusion fuel to thermonuclear conditions. Various peaceful applications of nuclear explosions were later investigated, including excavation, underground engineering and stimulation of gas reservoirs, but none developed into a practical civilian energy technology.

Inertial-confinement fusion research has been driven by both energy and defence applications. Small laboratory fusion experiments can reproduce some of the extreme temperatures, pressures and radiation environments relevant to thermonuclear weapons without nuclear explosive testing. This connection has provided substantial government support for ICF research, alongside its long-term potential as an energy source.

On 5 December 2022, an experiment at the National Ignition Facility released 3.15 MJ of fusion energy, achieving laboratory fusion ignition for the first time.

Lasers

A variety of drivers for compressing fusion targets were considered, including pulsed-power machines, charged-particle beams, plasma guns and hypervelocity projectiles. After the invention of the laser in 1960, high-power lasers became the leading driver for inertial-confinement fusion research. In the early 1970s, experiments with systems such as the Soviet Kalmar Nd:glass laser demonstrated that multiple laser beams could compress D–T targets and initiate fusion reactions, helping establish laser-driven fusion as a viable experimental approach.

Indirect Drive

Lawrence Livermore National Laboratory developed a series of increasingly powerful lasers, including Janus, Cyclops and Argus, and began exploring indirect-drive implosions in the mid-1970s. In this approach, laser beams heat the inner walls of a hohlraum, creating an X-ray radiation field that irradiates the fuel capsule more uniformly than direct laser illumination.

The 20-beam Shiva laser at Lawrence Livermore National Laboratory reached full power in 1977, delivering about 10.2 kJ of infrared laser energy. Although far from ignition, Shiva revealed important limitations caused by hydrodynamic instabilities and laser-plasma interactions, including energetic electrons that could preheat the fuel and reduce its compressibility. A major advance was the conversion of infrared laser light to shorter-wavelength ultraviolet light, which improved laser-target coupling and reduced unwanted hot-electron preheating. This approach was incorporated into later systems such as the Nova laser, which produced 40 kJ of ultraviolet energy in 1985.

Fast Ignition Approach

The fast-ignition concept was proposed in 1994. Unlike conventional central hot-spot ignition, it separates fuel compression from ignition: a nanosecond-scale driver pulse first compresses the fuel, after which an intense picosecond-scale pulse generates energetic particles that rapidly heat a small region of the compressed fuel and initiate fusion burn.

Ignition

Construction of the National Ignition Facility began at Lawrence Livermore National Laboratory in 1997. By 2010, its 192 laser beams were delivering more than 1 MJ of ultraviolet energy to inertial-fusion targets. A major breakthrough came on 8 August 2021, when NIF produced more than 1.35 MJ of fusion energy and demonstrated a burning-plasma regime, with alpha-particle self-heating playing a major role in sustaining the fusion reactions.

On 5 December 2022, NIF achieved laboratory fusion ignition for the first time, producing 3.15 MJ of fusion energy from 2.05 MJ of laser energy delivered to the target. This was the first controlled fusion experiment in which the fusion yield exceeded the laser energy delivered to the target, although the total energy consumed by the facility remained much greater. Ignition has since been reproduced repeatedly. On 7 April 2025, NIF produced a record 8.6 MJ from 2.08 MJ of laser energy, corresponding to a target gain of 4.13. By June 2026, NIF had achieved ignition eleven times.