Inertial confinement fusion (ICF) is a method of achieving nuclear fusion by quickly compressing and heating a material. This process is usually done with high-powered lasers, which are all focused onto a small pellet to rapidly heat it up. The intense heating vaporizes the material inside the pellet, creating a shock wave which is hot and dense enough to cause the material to fuse. Although inertial confinement fusion has yet to produce more useful energy than it consumes, research on how to build a commercially viable power source is still in progress.
The basic ingredients of an inertial confinement fusion pellet are deuterium and tritium, both hydrogen isotopes. The fusion reaction between deuterium and tritium is much easier to achieve than any other reaction, and so a power-producing deuterium/tritium reactor is the primary goal of modern fusion research. These pellets are very small, weighing much less than a gram, and are inserted one at a time into the inertial confinement fusion reactor.
Once the pellet is loaded, very large lasers are used to rapidly heat the pellet up to fusion temperature, at millions of degrees Fahrenheit (Celsius). The rapid heating of the outer layer of the pellet causes it to vaporize and rapidly expand, putting pressure on the interior of the pellet. If the lasers supply enough energy, the interior of the pellet will be compressed quickly enough to induce nuclear fusion, which in turn makes the pellet hotter. This condition is called "ignition," and it is the goal of most modern-day inertial confinement fusion experiments.
The primary difficulty with inertial confinement fusion is delivering enough power to the pellet to heat it to fusion temperature before the pellet disperses into space. In order to produce power from fusion, the reaction must exceed a value called the Lawson criterion, which gives the minimum confinement time necessary for any given volume of fuel. This requires many megajoules of energy to be passed through the laser system in a matter of microseconds; doing this reliably, without consuming too much power, presents a huge technical challenge. A new approach to the confinement problem called "fast ignition" has been proposed, where a single quick laser burst ignites the pellet after it has already been compressed. Although this approach looks promising in theory, it has not yet been successfully tested.