The Family of Magnetic Confinement Configurations
The family of configurations may be characterized at one extreme as being externally controlled and at the other extreme as being self-organized. Externally controlled configurations generally have strong, externally applied magnetic fields, minimizing the need for internal plasma currents and imposing favorable stability. However, such systems tend to be large, with relatively low values of Î². Self-organized systems generally employ only a weak, external magnetic field but require strong, internally generated plasma current. The absence of a strong external field tends to reduce the stability and confinement. However, such systems tend to be smaller and simpler. There is a nearly continuous spectrum of configurations, from the most externally controlled (such as the stellarator) to the most self-organized (such as the field-reversed configuration). Externally controlled configurations tend to be the most advanced, and self-organized configurations tend to be at the exploratory stage.
The vocabulary of fusion concepts is rather unfortunate—each variation of the magnetic configuration is labeled with a distinct and rather nondescriptive name. While it provides a useful taxonomy for fusion scientists, it can give the wrong impression—that is, that the family of concepts is a disjointed set of trial-and-error experiments, when it is, in fact, a set of variations that are connected by common plasma physics. The ultimate fusion concept may emerge from the present list of concepts, it may evolve as a hybrid of these concepts, or it may be an as-yet-undiscovered approach.
Elaboration of the features of several representative configurations serves to illustrate the breadth of the family. This description begins at the externally controlled extreme and proceeds toward self-organized systems.
The stellarator is a configuration in which no currents within the plasma are required for confinement. The stellarator was invented by U.S. scientists but has been most extensively explored by German and, more recently, Japanese scientists. Strong external magnets of very complicated structure produce a three-dimensional plasma equilibrium in which there is no direction of symmetry. Stellarators would
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.
THE SPHERICAL TORUS
As one reduces the aspect ratio of the tokamak so that the hole in the center of the torus becomes very small, the Î² 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.
THE REVERSED-FIELD PINCH
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.
THE SPHEROMAK AND THE FIELD-REVERSED CONFIGURATION
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).
There are concepts under study that cannot be categorized in the externally controlled/self-organized scheme. Two examples are magnetized target fusion and the dipole configuration. Magnetized target fusion compresses a compact toroid to densities intermediate between those of magnetically and inertially confined plasmas. It offers a new, perhaps simpler approach to fusion, as well as access to plasma physics regimes. The dipole configuration mimics confinement of plasma in planetary magnetospheres, again offering special advantages and physics insights.