Stellarator
From Wikipedia, the free encyclopedia
A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. The magnetic field necessary to confine the plasma is completely generated by external coils. It was invented by Lyman Spitzer and the first devices were built at the Princeton Plasma Physics Laboratory in 1951. The name was given to this early fusion concept because of the possibility of harnessing the power source of the sun.
Some important stellarator experiments are Wendelstein, in Germany, and the Large Helical Device, in Japan. A new stellarator, NCSX, is currently being built at the Princeton Plasma Physics Laboratory.
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[edit] Description
Although it would seem at first glance that a magnetic torus could contain plasma, if one examines the windings of an electromagnet's wiring around a torus it becomes clear the windings are less dense on the outside of the loop than on the inside. Plasma particles (ions) on the inner portion of the tube would thus see a greater magnetic force than those at the outside, and only particles near the middle would see the "right amount". Since magnetic forces are generally at right angles to motion, non-centered plasma moving around the toroid would thus be forced up or down until it hit the edges of the tube.
The stellarator avoids this with a simple "trick": the toroid is bent into a figure-eight shape. Now when a particle orbits the tube, it spends half the time on the inside of the tube and half on the outside. This equalizes the forces, at least to some degree, and the particle experiences a much smaller overall drifting force.
The earliest stellarators were literally figure-eights, consisting of two sides of a torus connected together with crossed straight tubes. In order to allow the tubes to cross without hitting, the torus sections on either end were rotated slightly. This arrangement was less than perfect, however, as a particle on the "inner portion" at one end would not end up at the "outer portion" at the other, but at some other point rotated from the perfect location due to the tilt of the two ends.
Various different geometries were tried to address these problems, starting with simple changes to allow the ends to lie flat at different levels and placing symmetrical bends in the arms instead. A later version solved the problem more convincingly by introducing a "peanut" shaped tube instead of a figure-eight, the in-bent sides offsetting the out-bend toroidal sections on either end.
But the real solution turned out to be magnetic instead of mechanical: by rotating the magnetic windings themselves as they were wrapped around the chamber, the plasma would be rotated around a simple torus, slowly moving from inside to outside.
[edit] Configurations of stellarator
Torsatron: A stellarator configuration with continuous helical coils. It is also to have the continuous coils to be replaced by a number of discrete coils producing a similar field.
Heliotron: A stellarator configuration in which a helical coil is used to confine the plasma, together with a pair of PF coils to provide a vertical field. TF coils can also used to control the magnetic surface characteristics.
Helias: A stellarator configuration in which the coils resemble distorted, non-planar TF coils so that the continuous helical coils or tokamak-like PF coils are present. The Helias (HELIcal Advanced Stellarator) 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 VII-X device is based on a five field-period Helias configuration.
[edit] Comparison to tokamaks
The tokamak provides the required twist to the magnetic field lines not by manipulating the field with external currents, but by driving a current through the plasma itself. The field lines around the plasma current combine with the toroidal field to produce helical field lines, which wrap around the torus in both directions.
Although they also have a toroidal magnetic field topology, stellarators are distinct from tokamaks in that they are not azimuthally symmetric. They have instead a discrete rotational symmetry, often five-fold, like a regular pentagon.
It is generally argued that the development of stellarators is less advanced than tokamaks although the intrinsic stability they provide has been sufficient to pursue an active development of this concept. Stellarators, unlike tokamaks, do not require a toroidal current, so that the expense and complexity of current drive and/or the loss of availability and periodic stresses of pulsed operation can be avoided. In addition, there is no risk of current disruptions.
On the downside, the three-dimensional nature of the field, the plasma, and the vessel make it much more difficult to do either theory or experimental diagnostics with stellarators. On the other hand, it might be possible to use the additional degrees of freedom to optimize a stellarator in ways that are not possible with tokamaks. It is much harder to design a divertor (the section of the wall that receives the exhaust power from the plasma) in a stellarator, the out-of-plane magnetic coils (common in many modern stellarators and possibly all future ones) are much harder to manufacture than the simple, planar coils which suffice for a tokamak, and the utilization of the magnetic field volume and strength is generally poorer than in tokamaks.
