sentences:  When they did, a series of problems, typically related to the homogeneity of the collapse, turned out to seriously disrupt the implosion symmetry and lead to much cooler core temperatures than originally expected.Through the 1980s the estimated energy required to reach ignition grew into the megajoule range, which appeared to make ICF impractical for fusion energy production.For instance, the National Ignition Facility (NIF) uses about 420 MJ of electrical power to pump the driver lasers, and in the best case is expected to produce about 20 MJ of fusion power output.