The demonstration of break-even fusion power production for brief pulses is a realistic prospect for the large deuterium–tritium tokamaks scheduled for completion in the early 1980's. But they are not likely to achieve plasma ignition. Before one can begin building practical fusion reactors, it will be necessary to build at least one significantly larger experimental tokamak—one that can achieve and control an ignited plasma for minutes at a time, with fusion neutrons bombarding the walls at power levels exceeding a megawatt per square meter.