Inertial confinement fusion

Inertial Confinement Fusion (ICF) is a method of achieving fusion conditions by rapidly compressing and heating a small quantity of fusion fuel. ICF is based on the same principles as thermonuclear bombs, compressing fuel quickly to achieve fusion conditions before the fuel can escape. The high pressure in the fuel causes it to break down, and the desired fusion burn has to be completed before fast disassembly occurs. The inertia of the fuel keeps it contained, giving the technique its name. ICF is one of the two main research areas aiming to build net positive fusion reactors for energy generation. In December 2023, researchers at Lawrence Livermore National Laboratory (LLNL) announced they had achieved fusion ignition, a major scientific breakthrough toward clean energy generation.

While magnetic confinement extends the time that ions spend close to each other to facilitate fusion, ICF aims to fuse nuclei so fast that they don't have time to move apart. The two main approaches to ICF are laser fusion and ion-beam fusion. The laser or ion beams are focused precisely onto the surface of the fusion fuel, which is a pellet a few millimeters in diameter, typically containing deuterium and tritium. This heats the outer layer of the material, creating an outward explosion and an inward compression front or implosion that compresses and heats the inner layers of material, creating the conditions needed for fusion. The energy released heats the surrounding fuel, creating a chain reaction, or ignition, generating more fusion as the reaction spreads outwards from the core of the fuel.

ICF came out of research into thermonuclear weapons. In 1957, Edward Teller and colleagues at Lawrence Livermore National Laboratory (LLNL) began exploring peaceful applications of nuclear explosives, evaluating the feasibility of producing commercial power from exploding a megaton-yield thermonuclear device in a thousand-foot diameter steam-filled cavity housed in granite. This led to research into finding the smallest possible fusion explosion with energy gain for commercial power production and the minimum size of a fusion explosion that could be ignited without the use of a fission primary.

A researcher at LLNL, John H. Nuckolls, proposed imploding a milligram of deuterium and tritium (DT) to super high densities using radiation implosion in a hohlraum cavity energized by a non-nuclear primary. This scheme compresses fusion fuel to extremely high densities and the ignition of small DT masses. In 1960, the invention of the laser offered a solution to the non-nuclear primary. In 1961, researchers at LLNL had an inertial fusion energy strategy that included developing suitable lasers that scaled to high energies and efficient milligram scale, high-gain fusion capsules. However, LLNL did not take the approach seriously, and many in the field saw magnetic fusion as the more viable option. In 1962, John Foster with Teller started a small experimental laser fusion program, directed by Ray Kidder at LLNL.

ICF was first presented publicly in a 1972 paper by Nuckolls and colleagues at LLNL published in Nature. The paper discussed the idea of compressing and heating a small pellet of deuterium and tritium (the thermonuclear fuel) using high-power lasers. The compressed fuel is in the plasma state, and thermonuclear reactions take place until the fuel disassembles. On May 1, 1974, KMS Fusion demonstrated the world's first successful laser-induced fusion experiment in a DT pellet. KMS Fusion was a private sector company pursuing controlled thermonuclear fusion research using laser technology.

In the 1970s, the LLNL began developing more powerful lasers, including the Shiva laser system completed in November 1977. Shiva achieved 10-kJ performance, with a pulse duration shorter than 1 ns. It provided enough energy for experiments with ablatively compressed, directly driven targets and to investigate the problems associated with long-scale-length laser-plasma interactions. LLNL used Shiva to demonstrate fuel compression to 100 times the density of liquid DT using radiation-driven targets. The system worked continuously until it was decommissioned at the end of 1981. Another high-energy laser developed by LLNL was Nova, capable of concentrating 80 to 120 kJ of energy in 3ns. Among the goals of the experiment was to compress DT fusion fuel to 1000 times the density of liquid DT and determine the energy required for efficient ignition and burn of inertial fusion targets. In 1986, Nova produced the largest laser fusion yield to date (11 trillion fusion neutrons). In 1987, Nova compressed a fusion fuel target to roughly a thirtieth of its original diameter.

LLNL's National Ignition Facility (NIF) began construction in 1997. NIF was described as the "world's largest laser," with 192 beams producing 1.8 MJ or 500 terawatts. The laser fired for the first time in 2009, and in 2010, NIF set records for laser energy delivered to an inertial confinement fusion target (1.3 MJ) and neutron yield (300 trillion neutrons). NIF has gone on to set new ICF records, and in December 2022, the facility achieved the first fusion ignition in a laboratory, producing a yield of over 3 MJ, 50% more than the laser energy required to initiate the fusion reaction.