Editing Tokamak Fusion Test Reactor (section)

===Princeton Large Torus===
During the early 1970s, [[Shoichi Yoshikawa]] was looking over the tokamak concept. He noted that as the size of the reactor's minor axis (the diameter of the tube) increased compared to its major axis (the diameter of the entire system) the system became more efficient. An added benefit was that as the minor axis increased, confinement time improved for the simple reason that it took longer for the fuel ions to reach the outside of the reactor. This led to widespread acceptance that designs with lower [[aspect ratio]]s were a key advance over earlier models.<ref name=princeton/>

This led to the [[Princeton Large Torus]] (PLT), which was completed in 1975. This system was successful to the point where it quickly reached the limits of its Ohmic heating system, the system that passed current through the plasma to heat it. Among the many ideas proposed for further heating, in cooperation with [[Oak Ridge National Laboratory]], PPPL developed the idea of [[neutral beam injection]]. This used small [[particle accelerator]]s to inject fuel atoms directly into the plasma, both heating it and providing fresh fuel.<ref name=princeton/>

After a number of modifications to the beam injection system, the newly equipped PLT began setting records and eventually made several test runs at 60&nbsp;million&nbsp;K, more than enough for a fusion reactor. To reach the [[Lawson criterion]] for ignition, all that was needed was higher plasma density, and there seemed to be no reason this would not be possible in a larger machine. There was widespread belief that break-even would be reached during the 1970s.<ref name=cancer/><ref name=princeton/>