Many people who know P. Leonardo Mascheroni describe him as a maverick and a technology zealot. Now, the Justice Department will try to prove that he is dangerous, too  a man willing to sell atomic secrets in exchange for a chance to realize his dream.

Dr. Mascheroni, 75, is a nuclear scientist who has spent the 22 years since he left the Los Alamos National Laboratory trying to sell Congress, the scientific community, journalists  anyone who would listen, really, including this reporter  on his plan to build a giant laser for the achievement of nuclear fusion.

His plan earned respect and high-level endorsements, but the government chose a different path. Rather than give up, Dr. Mascheroni redoubled his campaign, sending out lengthy technical documents from his home in New Mexico to try to coax Washington to finance his laser.

“You’d get these fat FedEx packages,” said Steven Aftergood, a security expert at the Federation of American Scientists.