sentences:  If the compression is symmetrical enough the increase in temperature can create conditions close to the Lawson criterion and lead to ignition.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.