Fusion ignition

The key difference is that breakeven ignores losses to the surroundings, which do not contribute to heating the fuel, and thus are not able to make the reaction self-sustaining.

In comparison, man-made reactors are far less dense and much smaller, allowing the fusion products to easily escape the fuel.

A thermonuclear weapon uses a conventional fission (U-235 or Pu-239/241) "sparkplug" to generate high pressures and compress a rod of fusion fuel (usually lithium deuteride).

The fuel reaches high enough pressures and densities to ignite, releasing large amounts of energy and neutrons in the process.

[14] NIF estimates that the laser supplied 1.9 megajoules of energy, 230 kilojoules of which reached the fuel capsule.

[14] While the experiment fell short of ignition as defined by the National Academy of Sciences – a total energy gain greater than one – most people working in the field viewed the experiment as the demonstration of ignition as defined by the Lawson criterion.

Schematic of the stages of inertial confinement fusion using lasers. The blue arrows represent radiation; orange is blowoff; yellow is inwardly transported thermal energy.
  1. Laser beams or laser-produced X-rays rapidly heat the surface of the fusion target, forming a surrounding plasma envelope.
  2. Fuel is compressed by the rocket-like blowoff of the hot surface material.
  3. 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.
  4. Thermonuclear burn spreads rapidly through the compressed fuel, yielding many times the input energy.