sentences:  The amount of laser energy needed to effectively compress the targets to ignition conditions has grown rapidly from early estimates.In the "early days" of ICF research in the 1970s it was believed that as little as 1 kilojoules (kJ) would suffice, and a number of experimental lasers were built to reach these power levels.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.