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result in a relatively large reactor with superconducting coils. However, they are inherently steady-state and are free of the instabilities or disruptions that would be driven by the plasma current. Stellarators offer physics advances by enabling the study of plasma stability in the absence of plasma current and the investigation of new symmetry principles. For example, in the new quasi-symmetric stellarator configuration, a complicated three-dimensional magnetic structure would appear as nearly two-dimensional to an orbiting particle.


Tokamaks are also externally controlled, but less so than the stellarator. The tokamak was invented by scientists from the former Soviet Union. A strong toroidal magnetic field is applied externally, but plasma current is required to produce a weaker magnetic field, which is directed along the shorter (poloidal) direction. The tokamak is two-dimensional (there is symmetry in the toroidal direction). The most highly studied configuration, the tokamak has contributed enormously to numerous areas of plasma physics, and it serves as an informal standard against which other configurations can be compared in reactor attributes.


As one reduces the aspect ratio of the tokamak so that the hole in the center of the torus becomes very small, the β stability limit increases. This relatively compact, high-pressure fusion reactor concept is known as the spherical torus. The configuration can uncover tokamak physics at the geometric extreme of small aspect ratio, where the pressure limit and the pressure-driven self-current (the bootstrap current) are expected to be very large. The virtues of the configuration were extolled by U.S. scientists, but a spherical torus was first successfully built and tested in the United Kingdom.


As the externally applied toroidal magnetic field of the tokamak is reduced by a factor of 10, the plasma becomes more self-organized. This configuration, known as the reversed-field pinch (because the toroidal magnetic field reverses direction with radius), offers possible reactor advantages by eliminating the need for a strong toroidal field. However, the weaker magnetic field reduces the stability and confinement of the plasma. The reversed-field pinch provides an experimental vehicle with which to investigate the behavior of magnetic field turbulence and relaxation relevant to a range of natural and fusion plasmas.


At the extreme of self-organized plasmas are toroidal plasmas, which are taken to the limit of unity aspect ratio—the central hole is eliminated. Such a plasma is potentially very attractive as a fusion energy source; it is extremely compact (nearly a sphere) and requires no external magnets. However, the macroscopic stability and confinement of such configurations may be degraded. Two examples of such compact toroids are the spheromak (which contains both poloidal and toroidal magnetic fields generated by plasma currents) and the field-reversed configuration (the simplest geometry, containing only poloidal fields


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