The first noncircular stellarator may have been the Heliac in Canberra, Australia, but the granddaddy of them all is the LHD in Toki, Japan, shown in Fig. 10.6. Envisioned by Koji Uo while on sabbatical at Princeton in the 1960s and completed in 1998, this machine showed that large superconducting coils producing 30-T magnetic fields could be manufactured and operated reliably for years. The most amazing accom­plishment, however, was the demonstration that the weird, twisting vacuum chamber and similarly complicated magnet coils could actually be manufactured to the required tolerances. Figure 10.7 shows an artistic photograph of the vacuum chamber.

In operation, the LHD has outperformed tokamaks in several aspects. The plasma density has reached 1021 m-3 (1015 cm-3), which is many times larger than the Greenwald limit (Chap. 8). This shows that this unexplained, empirical limit may apply only to tokamaks and can be exceeded in stellarators. The maximum ion and electron temperatures achieved were 13.5 and 10 keV, respectively, though not at the same time. Nonetheless, T exceeds Te in normal operation, as is desirable since it is Г that causes fusion. Beta, the ratio of plasma pressure to magnetic-field pressure (Chap. 8), is an important measure of the quality of a fusion plasma. The beta value of 5% achieved in LHD is higher than is normal for tokamaks. Not all these record-breaking numbers can be obtained at the same time, of course. What counts is the triple product Tut, the simultaneous ion temperature, density, and confinement time plotted in Fig. 8.1. On that scale, the LHD would be at 0.44, at about the middle of the plot. With fueling by pellet injection, discharges an hour long can be produced in LHD when the power is lowered so that Tut is at 80% of its maximum value.