A stellarator is a fusion power device that confines plasma using external magnets. It is one of many types of magnetic confinement fusion devices, and among the first to be invented. The name "stellarator" refers to stars, because fusion mostly occurs in stars such as the Sun. It is one of the earliest human-designed fusion power devices.
The stellarator was invented by American scientist Lyman Spitzer in 1951. Much of its early development was carried out by Spitzer's team at what became the Princeton Plasma Physics Laboratory (PPPL). Spitzer's Model A began operation in 1953 and demonstrated plasma confinement. Larger models followed, but demonstrated poor performance, losing plasma at rates far worse than theoretical predictions. By the early 1960s, attention turned to fundamental theory. By the mid-1960s, Spitzer was convinced that the stellarator was matching the Bohm diffusion rate, which suggested it would never be a practical fusion device.
The USSR's T-3 tokamak demonstrated a leap in performance in 1968. PPPL converted the Model C stellarator to the Symmetric Tokamak (ST) to confirm or deny its results. ST surpassed them. Large-scale stellarator work in the US was replaced by tokamaks. Research continued in Germany and Japan, addressing many of the original problems, and began to approach the performance of early tokamaks.
The tokamak ultimately proved to have problems similar to the stellarators (for different reasons). Since the 1990s, stellarator interest rekindled. New techniques increased field quality and power, improving performance.
History Previous work In 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford were the first to create fusion, using a particle accelerator to shoot deuterium nuclei into a metal foil containing deuterium, lithium or other elements. These experiments allowed them to measure the nuclear cross section of various reactions of fusion between nuclei. They determined that the tritium–deuterium reaction occurred at a lower energy than any other fuel, peaking at about 100,000 electronvolts (100 keV).
100 keV corresponds to a temperature of about one billion kelvin. As described by the Maxwell–Boltzmann statistics, a bulk gas at a much lower temperature will still contain some particles at these energies. Because fusion reactions release so much energy, even a small number of such reactions can release enough energy to maintain the gas at the required temperature. In 1944, Enrico Fermi demonstrated that this would occur at a bulk temperature of about 50 million Celsius, within the range of existing experimental systems. The key problem was confining the plasma; no material container could withstand those temperatures. However, plasmas are electrically conductive, subjecting them to electric and magnetic fields.
In a magnetic field, the plasma's electrons and nuclei circle the magnetic lines of force. One confinement approach is to place a tube of fuel inside the open core of a solenoid. A solenoid creates magnetic lines running down its center, and fuel would be held away from the walls by orbiting these lines of force. But such an arrangement does not confine the plasma along the length of the tube. The obvious solution is to bend the tube around into a torus (donut) shape, so that any one line forms a circle, and the particles can circle forever.
However, for purely geometric reasons, the magnets ringing the torus are closer together on the inside curve, inside the "donut hole". Fermi noted that this would cause the electrons to drift away from the nuclei, eventually causing large voltages to develop. The resulting electric field would cause the plasma ring inside the torus to expand until it hit the reactor walls.
Stellarator After World War II, researchers began considering ways to confine a plasma. George Paget Thomson of Imperial College London proposed a system now known as z-pinch, which runs a current through the plasma. Due to the Lorentz force, this current creates a magnetic field that pulls the plasma in on itself, keeping it away from the walls. This eliminates the need for external magnets, avoiding Fermi's problem. Various teams in the UK built a number of small experimental devices using this technique by the late 1940s.
Ronald Richter was a German scientist who emigrated to Argentina. His thermotron used electrical arcs and mechanical compression (sound waves) for heating and confinement. He convinced Juan Perón to fund development of an experimental reactor. Known as the Huemul Project, this was completed in 1951. Richter convinced himself fusion had been achieved despite disagreements with other researchers.
While preparing for a ski trip to Aspen, Spitzer received a telephone call from his father, who mentioned an article on Huemul in The New York Times. Spitzer concluded it could not possibly work; the system could not provide enough energy. He then began considering alternatives. The stellarator concept came while riding a ski lift.
His approach was to modify the torus' geometric layout to address Fermi's concerns. By twisting one end of the torus compared to the other, forming a figure-8 layout instead of a circle, the magnetic lines moved closer and further from the torus' center. A particle orbiting these lines constantly moves in and out across the minor axis of the torus, drifting upward through half of one orbit and reversing in the other. The cancellation is not perfect, but it appeared this would sufficiently reduce net drift that the fuel would remain trapped long enough to reach the required temperatures.
Matterhorn A secret research lab at Princeton University carried on theoretical work on H-bombs after 1951. Spitzer was invited to join this program, given his previous research in interstellar plasmas.
Spitzer then lost interest in bomb design, and turned his attention to fusion as a power source. Spitzer produced a series of reports outlining the conceptual basis for the stellarator, as well as potential problems. The series is notable for its depth; it included a detailed analysis of the mathematics of the plasma and stability along with heating the plasma and dealing with impurities.
Spitzer began to lobby the United States Atomic Energy Commission (AEC) for funding. His plan involved three stages, each relying on the success of the prior stage over the course of a decade:
Model A was tasked to demonstrate that a plasma could be created and that its confinement time was better than a torus. Model B would heat the plasma to fusion temperatures. Model C would attempt to create fusion reactions at a large scale.
Around the same time, Jim Tuck had been introduced to the pinch concept while working at Clarendon Laboratory at Oxford University. He eventually ended up at Los Alamos, where he acquainted the other researchers with the concept. When he heard Spitzer was promoting the stellarator, he travelled to Washington to propose building a pinch device. He considered Spitzer's plans "incredibly ambitious". Nevertheless, Spitzer was funded with $50,000, while Tuck received nothing.
Spitzer, an avid mountain climber, proposed the name "Project Matterhorn" because he felt that "the work at hand seemed difficult, like the ascent of a mountain". Two sections were initially set up, S Section working on the stellarator under Spitzer, and B Section working on bomb design under Wheeler. Spitzer set up the top-secret S Section in a former rabbit hutch.
The other labs then began agitating for their own funding. Tuck managed to arrange some funding for his Perhapsatron through some discretionary budgets at LANL, but other teams at LANL, Berkeley and Oak Ridge (ORNL) also sought funds. The AEC eventually organized Project Sherwood, a new department for these projects.
Early devices Spitzer invited James Van Allen to join the group and set up an experimental program. Allen suggested starting with a "tabletop" device. This led to the Model A design, which began construction in 1952. It was made from 5 cm pyrex tubes about 350 cm in total length, and magnets capable of about 1,000 gauss. The machine began operation in early 1953 and clearly demonstrated improved confinement over the simple torus.
This led to Model B, whose magnets were not well mounted and tended to move when powered to 50,000 gauss. A second design failed for the same reason, but this machine demonstrated several-hundred-kilovolt X-rays that suggested good confinement.
Next came the B-1, which used ohmic heating to reach around 100,000 degrees. This machine demonstrated that impurities in the plasma emitted large x-rays that cooled the plasma. In 1956, B-1 was rebuilt with an ultra-high vacuum system to reduce impurities, but found that even at smaller quantities they were still problematic. Another effect was that during the heating process, the particles would remain confined for only a few tenths of a millisecond, while once the field was turned off, any remaining particles were confined for as long as 10 milliseconds. This appeared to be due to "cooperative effects" within the plasma.
B-2 was similar to B-1, but used pulsed power to allow it to reach higher magnetic energy and included a second heating system known as magnetic pumping. This machine was modified to add an ultra-high vacuum system. Unfortunately, B-2 demonstrated little heating from the magnetic pumping, given its longer required confinement times. It was displayed at the Atoms for Peace show. However, heating system modifications increased the coupling, demonstrating temperatures within the heating section as high as , around 12 million K.
B-64 was completed in 1955, essentially a larger B-1, but powered by pulses that produced up to 15,000 gauss. This machine included a divertor, which removed impurities from the plasma, greatly reducing the x-ray cooling effect. B-64 included straight sections in the curved ends which gave it a squared-off appearance. This appearance led to its nickname, "figure-8, squared", "8 squared", or "64". In 1956 the machine was re-assembled without the twist in the tubes, allowing the particles to travel without rotation.
B-65, completed in 1957, was built using the "racetrack" layout, following the observation that adding helical coils to the curved portions of the device produced a field that introduced the rotation purely through the resulting magnetic fields. This had the added advantage that the magnetic field included shear, which was known to improve stability.
B-3, also completed in 1957, was an enlarged B-2 with ultra-high vacuum and pulsed confinement up to 50,000 gauss and projected confinement times as long as 0.01 second.
The last B-series was the B-66, completed in 1958, essentially a combination of the racetrack layout with the larger size and energy of the B-3.
Unfortunately, these larger machines demonstrated "pump out". This effect caused plasma drift rates higher than classical theory suggested and much higher than the Bohm rates. B-3's drift rate was a full three times that of the worst-case Bohm predictions, and failed to maintain confinement for more than a few tens of microseconds.
Model C
As early as 1954, design of Model C was taking shape. It emerged as a large racetrack with multiple heating sources and a divertor, essentially a larger B-66. Construction began in 1958 and was completed in 1961. It could be adjusted to allow a plasma minor axis between and was in length. The toroidal field coils normally operated at 35,000 gauss.