Thermonuclear Bomb

NIF chamber during construction.

The first large-scale demonstration of fusion through rapid inertial compression came with the thermonuclear weapon Ivy Mike in 1952. In a staged thermonuclear device, a fission primary produces intense X-radiation that fills the surrounding radiation case and drives the compression and heating of a separate secondary containing fusion fuel. This radiation-driven implosion anticipated some of the physical principles later used in inertial confinement fusion.

Project Plowshare

Research into peaceful applications of nuclear explosions also helped stimulate ideas for producing much smaller fusion micro-explosions. In the late 1950s, John Nuckolls at Lawrence Livermore began investigating whether tiny quantities of deuterium–tritium fuel could be compressed and ignited without using a fission explosion as the driver. The invention of the laser in 1960 soon provided a promising way of delivering the required energy.

Drivers

Several possible drivers were considered, including pulsed-power systems, charged-particle beams, plasma jets and hypervelocity projectiles. After the invention of the laser in 1960, high-power pulsed lasers rapidly became the leading driver for inertial confinement fusion research.

Indirect Drive

Early inertial-fusion concepts drew directly on the radiation-driven implosion physics of thermonuclear weapons, leading to what later became known as indirect drive. In parallel, researchers including Ray Kidder developed direct-drive concepts in which laser beams irradiate the fuel capsule itself. Inertial fusion research has therefore been important both for fusion-energy science and for studying high-energy-density physics relevant to nuclear weapons without full-scale nuclear testing.

Laser Enhancement

Early laser experiments in the Soviet Union, France and the United States produced neutron emission from irradiated deuterium-containing targets, although the interpretation of some early results was uncertain. In 1974, experiments using the Janus laser at Lawrence Livermore and a similar system at KMS Fusion demonstrated thermonuclear conditions in laser-driven targets and produced fusion neutrons.

Successive Livermore laser systems such as Long Path, Cyclops and Argus were used to develop high-energy neodymium-glass lasers, improve beam quality and study laser–plasma interactions. These experiments provided the technical foundations for increasingly powerful multi-beam fusion lasers.

The 20-beam Shiva laser, completed in 1977, delivered about 10 kJ of infrared laser energy in sub-nanosecond pulses. Although it did not achieve ignition, Shiva provided crucial information about target compression and laser–plasma interactions. In particular, experiments showed that infrared laser light generated energetic electrons that prematurely heated the fuel and degraded the implosion.

One important solution was to use shorter-wavelength laser light, which couples more favourably to the target and reduces the production of damaging hot electrons. In the early 1980s, frequency-conversion crystals were introduced to convert the infrared output of neodymium-glass lasers into ultraviolet light, a technique that became standard in later large ICF facilities.

Fast Ignition

OMEGA laser beams compressing a fusion target.

In 1994, the fast-ignition concept was proposed. In this approach, the fuel is first compressed by a nanosecond-scale laser pulse and is then rapidly heated by a separate ultra-intense picosecond pulse. In some target designs, a metal cone provides access close to the compressed fuel and helps deliver energetic particles to the ignition region. The proposed European HiPER project later investigated fast ignition as a possible route to high gain with lower driver energy. In Japan, construction of the petawatt-class LFEX (Laser for Fast Ignition Experiment) began in 2003 to investigate fast-ignition physics.

Ignition Achieved

The need for much greater and more precisely controlled compression led to the construction of increasingly powerful laser facilities. Construction of the National Ignition Facility (NIF) at Lawrence Livermore began in 1997. Its 192 laser beams were designed to deliver megajoule-scale ultraviolet pulses to a tiny target, enabling experiments far beyond the capabilities of earlier systems.

Other major high-energy-density laser facilities followed, including Orion in the United Kingdom, Laser Mégajoule in France and Shenguang III in China, broadening international research into inertial fusion and related high-energy-density physics.

On 5 December 2022, NIF achieved 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 corresponded to a target gain of about 1.5 and marked the first laboratory fusion experiment in which the fusion yield exceeded the laser energy reaching the target. NIF has since repeated ignition many times. On 7 April 2025 it achieved a record yield of 8.6 MJ from 2.08 MJ of laser energy, corresponding to a target gain of 4.13; by June 2026, ignition had been achieved eleven times.