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Why must inertial confinement fusion compress fuel to extremely high densities?

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3D illustration of the interior reaction chamber of a tokamak fusion reactor, showing a large, cylindrical, metallic structure with curved walls and bright light panels. — fusion reactor A tokamak is a device used in nuclear-fusion research for magnetic confinement of plasma.

In inertial confinement fusion (ICF), fuel must be compressed to extremely high densities—typically 1,000 to 10,000 times that of normal solids—to overcome the inherent limitations of the confinement method.12

Unlike magnetic confinement, which holds plasma for extended periods, ICF does not attempt to contain the plasma beyond the time it takes for the fuel to disassemble. Because the plasma is confined only by its own inertia, it survives for only a fraction of a second (roughly one nanosecond). To achieve a practical energy yield within this extremely brief window, the reaction rate must be significantly accelerated.312

Since the power released by fusion reactions is proportional to the square of the ion density, compressing the fuel to extreme densities ensures that a sufficient number of fusion reactions occur before the pellet blows apart. Furthermore, high mass density helps maintain the fuel's integrity during the burn, while a larger radius helps slow the disassembly time, allowing the fusion process to occur efficiently before the target expands and cools.132