Токамак TFTR: История и развитие термоядерных исследований в Принстоне

Former experimental tokamak at Princeton Plasma Physics Laboratory

The Tokamak Fusion Test Reactor (TFTR) was an experimental tokamak built at Princeton Plasma Physics Laboratory (PPPL) circa 1980 and entering service in 1982. TFTR was designed with the explicit goal of reaching scientific breakeven, the point where the heat being released from the fusion reactions in the plasma is equal or greater than the heating being supplied to the plasma by external devices to warm it up. during his time at PPPL in the 1970s.

In nuclear fusion, there are two types of reactors stable enough to conduct fusion: magnetic confinement reactors and inertial confinement reactors. The former method of fusion seeks to lengthen the time that ions spend close together in order to fuse them together, while the latter aims to fuse the ions so fast that they do not have time to move apart. Inertial confinement reactors, unlike magnetic confinement reactors, use laser fusion and ion beam fusion in order to conduct fusion. However, with magnetic confinement reactors you avoid the problem of having to find a material that can withstand the high temperatures of nuclear fusion reactions. The heating current is induced by the changing magnetic fields in central induction coils and exceeds a million amperes. Magnetic fusion devices keep the hot plasma out of contact with the walls of its container by keeping it moving in circular or helical paths by means of the magnetic force on charged particles and by a centripetal force acting on the moving particles.

By the early 1960s, the fusion power field had grown large enough that the researchers began organizing semi annual meetings that rotated around the various research establishments. In 1968, the now annual meeting was held in Novosibirsk, where the Soviet delegation surprised everyone by claiming their tokamak designs had reached performance levels at least an order of magnitude better than any other device. The claims were initially met with skepticism, but when the results were confirmed by a UK team the next year, this huge advance led to a "virtual stampede" of tokamak construction. In the US, one of the major approaches being studied up to this point was the stellarator, whose development was limited almost entirely to the PPPL. Their latest design, the Model C, had recently gone into operation and demonstrated performance well below theoretical calculations, far from useful figures. With the confirmation of the Novosibirsk results, they immediately began converting the Model C to a tokamak layout, known as the Symmetrical Tokamak (ST). This was completed in the short time of only eight months, entering service in May 1970. ST's computerized diagnostics allowed it to quickly match the Soviet results, and from that point, the entire fusion world was increasingly focused on this design over any other.

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 ratios were a key advance over earlier models. The last step before the attack on break even would be to make a reactor that ran on a mixture of deuterium and tritium, as opposed to earlier machines which ran on deuterium alone. This was because tritium was both radioactive and easily absorbed in the body, presenting safety issues that made it expensive to use. It was widely believed that the performance of a machine running on deuterium alone would be basically identical to one running on D T, but this assumption needed to be tested. Looking over the designs presented at the meeting, the DOE team chose the Princeton design. This made it roughly double the size of other large scale machines of the era; the 1978 Joint European Torus and roughly concurrent JT 60 were both about half the diameter. As PLT continued to generate better and better results, in 1978 and 79, additional funding was added and the design amended to reach the long sought goal of "scientific breakeven" when the amount of power produced by the fusion reactions in the plasma was equal to the amount of power being fed into it to heat it to operating temperatures. Also known as Q = 1, this is an important step on the road to useful power producing designs. To meet this requirement, the heating system was upgraded to 50 MW, and finally to 80 MW.

Construction began in 1980 and TFTR began initial operations in 1982. A lengthy period of break in and testing followed. By the mid 1980s, tests with deuterium began in earnest in order to understand its performance. In 1986 it produced the first 'supershots' which produced many fusion neutrons. These demonstrated that the system could reach the goals of the initial 1976 design; the performance when running on deuterium was such that if tritium was introduced it was expected to produce about 3.5 MW of fusion power. Given the energy in the heating systems, this represented a Q of about 0.2, or about only 20% of the requirement for break even. Further testing revealed significant problems, however. To reach break even, the system would have to meet several goals at the same time, a combination of temperature, density and the length of time the fuel is confined. In April 1986, TFTR experiments demonstrated the last two of these requirements when it produced a fusion triple product of 1.5 x e=14 Kelvin seconds per cubic centimeter, which is close to the goal for a practical reactor and five to seven times what is needed for breakeven. However, this occurred at a temperature that was far below what would be required. In July 1986, TFTR achieved a plasma temperature of 200 million kelvin (200 MK), at that time the highest ever reached in a laboratory. The temperature is 10 times greater than the center of the Sun, and more than enough for breakeven. Unfortunately, to reach these temperatures the triple product had been greatly reduced to e=13, two or three times too small for break even. Major efforts to reach these conditions simultaneously continued. Donald Grove, TFTR project manager, said they expected to achieve that goal in 1987. This would be followed with D T tests that would actually produce breakeven, beginning in 1989. Unfortunately, the system was unable to meet any of these goals. The reasons for these problems were intensively studied over the following years, leading to a new understanding of the instabilities of high performance plasmas that had not been seen in smaller machines. A major outcome of TFTR's troubles was the development of highly non uniform plasma cross sections, notably the D shaped plasmas that now dominate the field.

Although it became clear that TFTR would not reach break even, experiments using tritium began in earnest in December 1993, the first such device to move primarily to this fuel. In 1994 it produced a then world record of 10.7 megawatts of fusion power from a 50 50 D T plasma (exceeded at JET in the UK, which generated 16MW from 24MW of injected thermal power input in 1997). The two experiments had emphasized the alpha particles produced in the deuterium tritium reactions, which are important for self heating of the plasma and an important part of any operational design. In 1995, TFTR attained a world record temperature of 510 million °C more than 25 times that at the center of the sun. This later was beaten the following year by the JT 60 Tokamak which achieved an ion temperature of 522 million °C (45 keV). Also In 1995, TFTR scientists explored a new fundamental mode of plasma confinement enhanced reversed shear, to reduce plasma turbulence. TFTR remained in use until 1997. It was dismantled in September 2002, after 15 years of operation. It was followed by the NSTX spherical tokamak.