Editing Inertial confinement fusion (section)

===Mechanism of action{{Anchor|ICF mechanism of action}} ===<!-- This section is linked from [[HiPER]]. See [[WP:MOS#Section management]] -->
The energy needed to overcome the Coulomb barrier corresponds to the energy of the average particle in a gas heated to 100 million [[Kelvin|K]]. The [[specific heat]] of hydrogen is about 14 [[Joule]] per gram-K, so considering a 1 milligram fuel pellet, the energy needed to raise the mass as a whole to this temperature is 1.4 megajoules (MJ).{{sfn|Emmett|Nuckolls|Wood|1974|p=24}}

In the more widely developed [[magnetic fusion energy]] (MFE) approach, confinement times are on the order of one second. However, plasmas can be sustained for minutes. In this case the confinement time represents the amount of time it takes for the energy from the reaction to be lost to the environment - through a variety of mechanisms. For a one-second confinement, the density needed to meet the Lawson criterion is about 10<sup>14</sup> particles per cubic centimetre (cc).{{sfn|Emmett|Nuckolls|Wood|1974|p=24}} For comparison, air at sea level has about 2.7 x 10<sup>19</sup> particles/cc, so the MFE approach has been described as "a good vacuum".

Considering a 1 milligram drop of D-T fuel in liquid form, the size is about 1&nbsp;mm and the density is about 4 x 10<sup>20</sup>/cc. Nothing holds the fuel together. Heat created by fusion events causes it to expand at the [[speed of sound]], which leads to a confinement time around 2 x 10<sup>−10</sup> seconds. At liquid density the required confinement time is about 2 x 10<sup>−7</sup>s. In this case only about 0.1 percent of the fuel fuses before the drop blows apart.{{sfn|Emmett|Nuckolls|Wood|1974|p=25}}

The rate of fusion reactions is a function of density, and density can be improved through compression. If the drop is compressed from 1&nbsp;mm to 0.1&nbsp;mm in diameter, the confinement time drops by the same factor of 10, because the particles have less distance to travel before they escape. However, the density, which is the cube of the dimensions, increases by 1,000 times. This means the overall rate of fusion increases 1,000 times while the confinement drops by 10 times, a 100-fold improvement. In this case 10% of the fuel undergoes fusion; 10% of 1&nbsp;mg of fuel produces about 30&nbsp;MJ of energy, 30 times the amount needed to compress it to that density.{{sfn|Emmett|Nuckolls|Wood|1974|pp=25-26}}

The other key concept in ICF is that the entire fuel mass does not have to be raised to 100 million K. In a fusion bomb the reaction continues because the alpha particles released in the interior heat the fuel around it. At liquid density the alphas travel about 10&nbsp;mm and thus their energy escapes the fuel. In the 0.1&nbsp;mm compressed fuel, the alphas have a range of about 0.016&nbsp;mm, meaning that they will stop within the fuel and heat it. In this case a "propagating burn" can be caused by heating only the center of the fuel to the needed temperature. This requires far less energy; calculations suggested 1&nbsp;kJ is enough to reach the compression goal.{{sfn|Emmett|Nuckolls|Wood|1974|p=26}}

Some method is needed to heat the interior to fusion temperatures, and do so while when the fuel is compressed and the density is high enough.{{sfn|Emmett|Nuckolls|Wood|1974|p=26}} In modern ICF devices, the density of the compressed fuel mixture is as much as one-thousand times the density of water, or one-hundred times that of lead, around 1000 g/cm<sup>3</sup>.{{sfn|Malik|2021|p=284}} Much of the work since the 1970s has been on ways to create the central hot-spot that starts off the burning, and dealing with the many practical problems in reaching the desired density.

[[Image:Inertial confinement fusion.svg|thumb|center|600px|Schematic of the stages of inertial confinement fusion using lasers. The blue arrows represent radiation; orange is blowoff; purple is inwardly transported thermal energy.
{{ordered list
| Laser beams or laser-produced X-rays rapidly heat the surface of the fusion target, forming a surrounding plasma envelope.
| Fuel is compressed by the rocket-like blowoff of the hot surface material.
| During the final part of the capsule implosion, the fuel core reaches 20 times the density of lead and ignites at 100,000,000 ˚C.
| Thermonuclear burn spreads rapidly through the compressed fuel, yielding many times the input energy.
}}]]