A stellarator is a device used to confine hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. The name refers to the possibility of harnessing the power source of the sun, a stellar object.
Invented by Lyman Spitzer of Princeton University in 1951 while on a ski trip, it is one of the earliest controlled fusion devices. Spitzer led a development effort known as Matterhorn, and the first purely experimental model, Model A, was operational in 1953. Larger models followed, but demonstrated generally poor performance, far worse than theoretical predictions. Stellarators were popular in the 1950s and 1960s, but the much better results from tokamak designs led to them falling from favor in the 1970s. The largest of the original Princeton machines, Model C, began operation in 1961 and ran until 1969 when it was converted to a tokamak.
More recently, in the 1990s, problems with the tokamak concept have led to renewed interest in the stellarator design, and a number of new devices have been built. Some important modern stellarator experiments are Wendelstein 7-X in Germany, the Helically Symmetric Experiment (HSX) in the USA, and the Large Helical Device in Japan.
How the plasma is heated
There are several ways to heat the plasma (which must be done before ignition can occur).
- Current heating
- The plasma is electrically conductive, and heats up when a current is passed through it (due to electrical resistance). Only used for initial heating, as the resistance is inversely proportional to the plasma temperature.
- High-frequency electromagnetic waves
- The plasma absorbs energy when electromagnetic waves are applied to it (in the same manner as food in a microwave).
- Heating by neutral particles
- A neutral particle beam injector makes ions and accelerates them with an electric field. To avoid being affected by the Stellarator's magnetic field, the ions must be neutralised. Neutralised ions are then injected into the plasma. Their high kinetic energy is transferred to the plasma particles by collisions, heating them.
Several different configurations of stellarator exist, including:
- A stellarator with continuous helical coils. It can also have the continuous coils replaced by a number of discrete coils producing a similar field.
- A stellarator in which a helical coil is used to confine the plasma, together with a pair of poloidal field coils to provide a vertical field. Toroidal field coils can also be used to control the magnetic surface characteristics. The Large Helical Device in Japan uses this configuration.
- Modular stellarator
- A stellarator with a set of modular (separated) coils and a twisted toroidal coil. e.g. Helically Symmetric Experiment (HSX) (and Helias (below))
- A helical axis stellarator, in which the magnetic axis (and plasma) follows a helical path to form a toroidal helix rather than a simple ring shape. The twisted plasma induces twist in the magnetic field lines to effect drift cancellation, and typically can provide more twist than the Torsatron or Heliotron, especially near the centre of the plasma (magnetic axis). The original Heliac consists only of circular coils, and the flexible heliac (H-1NF, TJ-II, TU-Heliac) adds a small helical coil to allow the twist to be varied by a factor of up to 2.
- A helical advanced stellarator, using an optimized modular coil set designed to simultaneously achieve high plasma, low Pfirsch–Schluter currents and good confinement of energetic particles; i.e., alpha particles for reactor scenarios. The Helias has been proposed to be the most promising stellarator concept for a power plant, with a modular engineering design and optimised plasma, MHD and magnetic field properties. The Wendelstein 7-X device is based on a five field-period Helias configuration.
Optimization to reduce transport losses
The goal of magnetic confinement devices is to minimise energy transport across a magnetic field. Toroidal devices are relatively successful because the magnetic properties seen by the particles are averaged as they travel around the torus. The strength of the field seen by a particle, however, generally varies, so that some particles will be trapped by the mirror effect. These particles will not be able to average the magnetic properties so effectively, which will result in increased energy transport. In most stellarators, these changes in field strength are greater than in tokamaks, which is a major reason that transport in stellarators tends to be higher than in tokamaks.
University of Wisconsin electrical engineering Professor David Anderson and research assistant John Canik proved in 2007 that the Helically Symmetric eXperiment (HSX) can overcome this major barrier in plasma research. The HSX is the first stellarator to use a quasisymmetric magnetic field. The team designed and built the HSX with the prediction that quasisymmetry would reduce energy transport. As the team's latest research showed, that is exactly what it does. "This is the first demonstration that quasisymmetry works, and you can actually measure the reduction in transport that you get," says Canik.  
The newer Wendelstein 7-X in Germany was designed to be close to omnigeneity (a property of the magnetic field such that the mean radial drift is zero), which is a necessary but not sufficient condition for quasisymmetry; that is, all quasisymmetric magnetic fields are omnigenous, but not all omnigenous magnetic fields are quasisymmetric.
- Daniel Clery (2015). "The bizarre reactor that might save nuclear fusion". Science. doi:10.1126/science.aad4746.
- Stix, Thomas (1998). "Highlights in Early Stellarator Research at Princeton" (PDF). Journal of Plasma Fusion: 3–8.
- "After ITER, Many Other Obstacles for Fusion Power". Science. January 17, 2013.
- Wakatani, M. (1998). Stellarator and Heliotron Devices. Oxford University Press. ISBN 0-19-507831-4.
- Harris, J. H.; Cantrell, J. L.; Hender, T. C.; Carreras, B. A.; Morris, R. N. (1985). "A flexible heliac configuration". Nuc. Fusion. 25 (5): 623. doi:10.1088/0029-5515/25/5/005.
- Basics of Helias-type Stellarators at the Wayback Machine (archived 21 June 2013)
- Canik, J. M.; et al. (23 February 2007). "Experimental Demonstration of Improved Neoclassical Transport with Quasihelical Symmetry". Physical Review Letters. 98 (8): 085002. Bibcode:2007PhRvL..98h5002C. doi:10.1103/PhysRevLett.98.085002. PMID 17359105.
- New stellerator a step forward in plasma research (news article on phys.org)
- "Omnigeneity – FusionWiki". fusionwiki.ciemat.es. Retrieved 2016-01-31.
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