Laser ignition of deuterium/tritium mixture for a fusion explosion rather than a critical mass fission explosion.
And Wikipedia (yes, I know the warnings) lists Californium-252 as having critical mass of 2.73 kg (6.02 lbs) with a diameter of 6.9 cm.
Before I posted I saw the Californium listing on wikipedia but really - which country has the ability to make sufficient amounts of Californium:)