Sun in a Bottle - Charles Seife [44]
The asymmetry provided a crucial clue. The scientists pinched the plasma by running a current through it. Neutrons that were flying out of the machine in the direction of the current had more energy than those that flew out against it. This revealed that the neutrons were the work of another instability. Just as a pinched filament is unstable when kinked slightly—because the kink grows and grows—it is unstable when a small section gets pinched a little bit more than the rest of the plasma. In this case, the small pinch grows progressively more pronounced; the plasma gets wasp-waisted and pinches itself off. The plasma begins to look like a pair of sausages. This is a sausage instability, and it creates some strong electrical fields near the pinch point. These fields accelerate a small handful of nuclei in the direction of the pinch current. These nuclei then strike the relatively chilly cloud of plasma and fuse, releasing neutrons.
From fusion scientists’ point of view, this kind of fusion was worthless. Scientists were hoping to get a hot cloud of nuclei fusing with itself, a thermonuclear fusion reaction. Instead, Columbus had made a small handful of very hot nuclei interact with cooler ones. This was roughly equivalent to shooting nuclei at a stationary target, and doing that, scientists had concluded, would always consume more energy than it produced. The neutrons produced by the instability, dubbed instability neutrons or false neutrons, weren’t a sign of energy production—just the opposite. Columbus’s neutrons were the sign of energy consumption, not energy production. The false neutrons had given the Los Alamos scientists false hope. Even so, the pinch technique still seemed within striking distance of igniting fusion.
SAUSAGE INSTABILITY: If a pinching plasma is slightly narrower in one place, that narrowing will get more and more severe and eventually squeeze the plasma to make it look like a pair of sausages.
By this time, the Americans knew they had competition from both the Russians and the British. Project Sherwood poured increasing amounts of money into ever-larger machines of all types. The most expensive one in the Sherwood portfolio was the model-C Stellarator proposed by Spitzer, which would cost roughly $16 million to design and build. So it was a humiliation when it appeared that the British had won the fusion race with a much smaller and less-expensive machine: ZETA.
ZETA, which had cost less than $1 million to build, was a powerful pinch machine. Its name reflected the optimism of its designers; ZETA was an acronym for Zero-Energy Thermonuclear Assembly, thermonuclear because it would achieve fusion and zero-energy because it would produce as much energy as it consumed. It was a very bold claim.
ZETA began operation in mid-August 1957 at the Harwell laboratory near Oxford. It wasn’t long before the machine made a big splash. Late in the evening of August 30, the ZETA device started producing neutrons. The scientists did hasty checks to make sure there wasn’t an equipment failure of any sort; the neutrons were real. Pinch. Neutrons. Pinch. Neutrons. Like their American counterparts before them, the British physicists thought the neutrons were the signature of fusion; after all, neutrons were the smoking gun that everybody had been seeking for so long. There were a few skeptics on the ZETA team—some doubted that ZETA had actually achieved fusion—but the joyful chorus of self-congratulation drowned