A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. It is one of the earliest controlled fusion devices, first invented by Lyman Spitzer in 1950 and built the next year at what would later become the Princeton Plasma Physics Laboratory. The name refers to the possibility of harnessing the power source of the sun, a stellar object.
Stellarators were popular in the 1950s and 60s, but the much better results from tokamak designs led to them falling from favor in the 1970s. More recently, in the 1990s, problems with the tokamak concept has led to renewed interest in the stellarator design, and a number of new devices have been built. Some important modern stellarator experiments are Wendelstein, in Germany, and the Large Helical Device, in Japan. Princeton Plasma Physics Laboratory started building a new stellarator, NCSX, but as of 2008, work was abandoned  due to high costs.
Early fusion research generally followed two major lines of study; devices that were based on momentary compression of the fusion fuel to high densities, like the pinch devices being studied primarily in the UK, and devices that used lower densities but longer confinement times, like the magnetic mirror and stellarator. In the later systems, the key problem was confining the plasma for long times without the hottest, most valuable, particles escaping from the device.
As plasma is electrically charged, and thus magnetic, it can be confined by an appropriate arrangement of magnetic fields. The simplest to understand is a solenoid, consisting of a helix of wire wrapped around a cylindrical support. A plasma inside the solenoid will experience an inward force that would confine it in the center of the helix. However, in this case the plasma would see no force along the long axis, and would rapidly flow out the ends of the solenoid and escape.
One solution to that problem is to simply bend the solenoid around into a ring, closing the ends. However in this case the magnetic field is no longer uniform. The electrical windings the inside edge of the toroid the are closer together, and further apart on the outside edge. This leads to a weaker field on the outside than the inside. A particle circulating the torus at the exact center of the torus will see a balanced force, but one circulating closer to the inside edge will see a downward force, while one circulating closer to the outside will see an upward force. These particles will eventually drift out of the confinement area.
Spitzer's innovation was a change in geometry. He suggested extending the torus with straight sections to form a racetrack shape, and then twisting one end by 180 degrees to produce a figure-8 shaped device. When a particle is on the outside of the center on one of the curved sections, by the time it flows through the straight area and into the other curved section it is now on the inside of center. This means that the upward drift on one side is counteracted by the downward drift on the other.
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