Editing Inertial confinement fusion (section)

===Heating concepts===
Early calculations suggested that the amount of energy needed to ignite the fuel was very small, but this does not match subsequent experience.

==== Hot spot ignition ====
[[File:NIF output over 11 years without legend.png|upright=1.5|thumb|alt=Plot of NIF results from 2012 to 2022|Plot of NIF target gain from 2012 to 2022, on a logarithmic scale. Note the 10× increase in gain in 2021 due to the achievement of ignition, followed by the achievement of target gain greater than 1 in 2022.]]

The initial solution to the heating problem involved deliberate "shaping" of the energy delivery. The idea was to use an initial lower-energy pulse to vaporize the capsule and cause compression, and then a very short, very powerful pulse near the end of the compression cycle. The goal is to launch shock waves into the compressed fuel that travel inward to the center. When they reach the center they meet the waves coming in from other sides. This causes a brief period where the density in the center reaches much higher values, over 800&nbsp;g/cm<sup>3</sup>.{{sfn|Pfalzner|2006|p=15}}

The central hot spot ignition concept was the first to suggest ICF was not only a practical route to fusion, but relatively simple. This led to numerous efforts to build working systems in the early 1970s. These experiments revealed unexpected loss mechanisms. Early calculations suggested about 4.5x10<sup>7</sup>&nbsp;J/g would be needed, but modern calculations place it closer to 10<sup>8</sup>&nbsp;J/g. Greater understanding led to complex shaping of the pulse into multiple time intervals.<ref>{{cite web |url=https://nifuserguide.llnl.gov/home/4-laser-system/44-pulse-shape-timing-and-prepulse/441-pulse-shaping |title=Pulse Shaping |website=LLNL}}</ref>