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Review of the Department of Energy's Inertial Confinement Fusion Program: The National Ignition Facility
Figure 1 shows the predicted nominal fusion energy output as a function of the laser energy incident on the NIF point design target presented to the committee. It indicates that the predicted gain (ratio of fusion yield to laser energy) at the nominal laser energy is about 6. The shape of the yield curve is cliff-like, in that fusion yield increases very rapidly, from near zero to its full value, over a relatively small range of incident laser energy. A plot of fusion yield as a function of other relevant drive parameters (such as laseruniformity or capsule surface finish) would exhibit a similar structure. This curve leads to the operative definition of ignition adopted by the committee: gain greater than unity.
However, there are two diagnosable milestones on the yield curve. At a gain of about 0.1, energy deposited by fusion alpha particles is sufficient to double the central temperature. At a gain of about 0.3, fusion reactions occur over a sufficient region to induce propagation of the thermonuclear burn into the denser, colder, outer fuel.