The Princeton Large Torus (or PLT), was an early tokamak built at the Princeton Plasma Physics Laboratory (PPPL). It was one of the first large scale tokamak machines and among the most powerful in terms of current and magnetic fields. Originally built to demonstrate that larger devices would have better confinement times, it was later modified to perform heating of the plasma fuel, a requirement of any practical fusion power device.
The tokamak became a topic of serious discussion in 1968, when the Soviets published new data showing them to be far better than any other fusion device. This generated significant skepticism among other researchers and it was some time before the PPPL was convinced to convert their Model C stellarator to the tokamak configuration. It immediately validated the Soviet results and then surpassed them. The next step in developing the system would be to build a larger machine to test whether the confinement time of the plasma scaled as expected. PLT was designed to not only be larger, but also have dramatically higher internal plasma currents on the order of 1 MA.Fusion: The Energy of the Universe
Another problem with the tokamak approach is that it does not directly heat its fuel to the required temperatures over 50 million Kelvin. Around the time that PLT was being built, Oak Ridge National Laboratory had successfully introduced the neutral beam injection heating concept, or NBI. NBI was added to PLT and it began setting record after record, eventually reaching 75 million K, well beyond the minimum needed for a practical fusion device. Its success was a matter of some controversy within the newly formed Department of Energy (DOE), who was at the same time looking to cut the fusion budget. This resulted in what was known as "the PLT weekend" when the press learned of the success and the DOE attempted to downplay it.
PLT's success led the way for plans to build an even larger machine capable of reaching breakeven, a long-sought goal in fusion power. This system emerged as the Tokamak Fusion Test Reactor, or TFTR. Originally slated to be built at Oak Ridge, PLT's success led to it winning the TFTR contest as well.
History The doldrums When the physics of nuclear fusion was first put on a firm footing during the early 1950s, a string of proposed devices to harness that energy was quickly created. These all aimed to solve the problem of containing a plasma fuel that was heated to at least 50 million Kelvin, which would melt any substance. The trick used by most of these devices was to manipulate the plasma with magnetic fields; as the plasma consisted of free electrons and ions, it could carry an electrical current and was subject to magnetic forces.
Based on simple plasma diffusion theory, the amount of time it takes for an ion to escape from a magnetic bottle depends on the size of the bottle and the square of the power of its magnets. This means larger machines will be inherently better at confining their fuel, both because it has farther to go to get out, and because larger machines can host larger and more powerful magnets. The corollary is that small machines can tell you only so much about the performance of a design at the scale needed for a practical reactor; one would have to build an intermediate-sized machine and compare the plasma leakage rate to ensure it followed the expected scaling. There was some early doubt on this point; the only direct experience with plasmas, from the Manhattan Project, suggested the leakage rate was linear with the magnetic field. If this Bohm diffusion were true, a practical fusion reactor would likely be impossible.
Of the many early concepts for reactor designs, three systems came to the fore, the magnetic mirror, the z-pinch and the stellarator. Early examples demonstrated that they could confine plasma to the level expected of a small machine. The outlier was the pinch, which demonstrated obvious instabilities that were addressed with new magnets. These early devices led to larger and more powerful versions of these same concepts. These invariably failed to improve the plasma confinement, leaking fuel at unsustainable rates. Investigations led to several newly discovered instabilities that seemed to be an inherent part of all of these designs.
At the first international meeting on fusion in 1958, it was clear all of the devices were suffering from these issues. By the early 1960s, the entire field had descended into what became known as "the doldrums". Even Lyman Spitzer, one of fusion's greatest proponents, concluded Bohm diffusion appeared to be a fundamental limit.
Tokamak At a similar meeting in 1965 at the recently opened Culham Centre for Fusion Energy in the UK, the Soviet team introduced preliminary results on a device they called the tokamak. Physically, it was very similar to the z-pinch concept, which had been extensively developed by the UK in the ZETA device and proved to be no more useful than other early systems, beset with instabilities. In contrast, the Soviets were claiming that their seemingly minor variation on ZETA was producing dramatically better results, about 10 times the Bohm limit. Their claims were dismissed out of hand, especially by Spitzer.
At the next meeting in 1968, in Novosibirsk, the Soviets presented much more data and all of it demonstrated their machines were producing confinement times from 10 to 100 times better than any other device. Once again, these results were met by skepticism. This time, however, the Soviets were prepared. The UK team working on ZETA had introduced a new diagnostic technique using lasers that Lev Artsimovich had called "brilliant". He invited the UK team to their lab, the heart of the Soviet bomb-making establishment, to make their own measurements. By the summer of 1969, the laser showed the tokamak was even better than the Soviet results suggested. They phoned into a meeting of US fusion researchers in August and told them the news before it was released publicly in November.
At first, there was little movement in the US, as each of the labs had its own design that they felt was more interesting. The directors of the fusion program within the Atomic Energy Commission (AEC) were interested in at least confirming or denying the Soviet results, but found the labs uninterested in such work. In particular, the AEC felt it would be easy to convert Princeton's Model C stellarator to a tokamak, but the lab's director, Harold Furth, refused to even consider it, dismissing the Soviet claims out of hand. Only Oak Ridge National Laboratory showed any interest; they had no other large-scale devices in planning and were open to giving the tokamak a try. As soon as plans were announced to this effect, Furth's boss Melvin B. Gottlieb had a lunchtime talk with Furth. The two returned from lunch to describe how to convert the Model C.
The conversion started in September 1969 and was completed eight months later as the rechristened the Symmetrical Tokamak. It immediately confirmed, then bested, the Soviet results. It appeared at last that a stable plasma configuration was possible and the road to practical fusion power was suddenly opened.
Tokamak rush
The success in confining plasma in smaller machines left many questions that would have to be answered. One was whether the tokamak scaled as expected; to test this, a larger machine with higher internal currents and magnetic fields would be needed. Another issue was how to heat the plasma; the tokamak lacked any significant self-heating so some form of external heating would be needed. Finally, some system would be needed to extract impurities from the plasma, both from the initial non-pure fuel as well as removing "fusion ash", the results of successful reactions (typically helium).
Of the three issues, the most obvious one was the extraction of impurities. It had long been understood that using a modified mass spectrometer would allow heavier ions to be removed. These were known as divertors, and Princeton's stellarator devices were among the first machines to use them. The style of diverter on the stellarators was not ideal for the tokamak, but Princeton had already solved that problem as part of their Floating Multipole-1 machine, which, before the tokamak, was one of the few other devices to demonstrate confinement times beyond the Bohm limit. To test whether their poloidal diverter would work in a tokamak configuration, plans began for a new small machine, the Poloidal Diverter Experiment, or PDX.
Heating was another issue, and there were many different ideas on how to do this. The stellarator also lacked self-heating, and to address this, Princeton had been carrying out experiments using ion cyclotron resonance heating. This uses powerful radio transmitters tuned to the rotational frequency of the ions, heating them in a fashion similar to the way a microwave oven heats water molecules. As this technology was already well understood, Princeton proposed a small test machine to try a different heating approach using plasma compression, like the earlier pinch machines, in a system known as the Adiabatic Toroidal Compressor (ATC). Other concepts included using turbulence in the plasma and the injection of hot ions into the fuel using small particle accelerators.
Finally, to test scaling, a larger machine with much more powerful magnets and internal currents would be needed. Initially, this was the main goal of the Princeton Large Torus, but allowances were made that new forms of heating could be added to the machine without serious disruption. The design was finalized in early 1971 and construction began later that year.
NBI Oak Ridge did not have their own fusion design during the early days of the program, and concentrated instead on ways to keep fusion machines fueled. This led to the development of a series of small particle accelerators that shot fuel atoms into the plasma one at a time. This turned out to be an excellent way to heat the plasma as well, and Oak Ridge continued work on these lines using mirror reactors through the 1960s. When the Soviet results were announced, they began considering how to do the same with a tokamak. Their early calculations were not promising, but a visit by Bas Pease from the UK Culham fusion laboratory urged them to continue with this approach.
While PPPL had been debating whether to convert Model C to a stellarator, Oak Ridge had proposed building a new tokamak, ORMAK. This used a novel way to generate the magnetic field to make it more uniform, to meet or beat the performance of the Soviet TM-3 machine. In the second stage of development, they would add neutral beam heating. It was at this point, in June 1970, that the Symmetrical Tokamak began reporting initial results. Worried about being redundant, the Oak Ridge team decided to adapt the ORMAK's transformer cage as the basis for a much larger tokamak and add NBI immediately. Their machine was complete by late 1970, but bringing it into operation took most of 1971 and the first physics results were not being returned until early 1972. By 1973 the machine was operating sufficiently well that the lab began plans to turn on the NBI injectors.
PPPL was not willing to give up its leadership position and quickly hatched a plan to "scoop" Oak Ridge. They abandoned the compression heating technique in ATC and quickly fit low-power NBI to it. These demonstrated clear heating effects in 1973, before the NBI systems on ORMAK were operational. With this success, Oak Ridge began to fall from favour within the Washington Steering Committee.
Initial operations