1 MJ is the optimal energy scale for inertial confinement fusion. Going larger makes the fuel harder to compress, going smaller means there is less fuel that ge

1 MJ is the optimal energy scale for inertial confinement fusion. Going larger makes the fuel harder to compress, going smaller means there is less fuel that gets compressed; either way your ratio of input energy to output energy goes down. Also while these energies are low, they are also for very short durations. 1.8 MJ in 1 ns is 1.8 PW.