Stellarator

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 tokamak design augured a leap in performance. PPPL converted the Model C stellarator to the Symmetrical 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.

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. Due to 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.