It would presumably take an order of magnitude less beam energy to run an inertial‐confinement thermonuclear reactor with ion beams than with electron beams. But until last year the technological problems of producing and focusing a sufficiently intense light‐ion beam have kept the emphasis of particle‐beam fusion research on electron‐beam devices. The largest particle‐beam fusion machines, at Sandia in Albuquerque and at the Kurchatov Institute in Moscow (still under construction), were originally designed to implode deuterium‐tritium pellets with beams of 2‐MeV electrons.