18.08.2015   An Indispensable Truth

Spheromaks

A spheromak (Fig. 10.17) is a toroidal plasma in a chamber with no hole in the middle. There can be toroidal coils to generate a poloidal B-field, but there cannot be any coils to generate a toroidal B-field since that would require a conductor going down the middle. Plasma with imbedded fields is injected into the chamber from external sources, and then the plasma self-organizes into a toroidal shape with both toroidal and poloidal fields (Fig. 10.18). Contrary to stellarators, which do away with the self-organization of tokamaks, spheromaks depend completely on self-organization. The classic method of injection with “plasma guns” is shown in Fig. 10.19.

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Fig. 10.16 Design of the ARIES-ST spherical tokamak reactor (http://www-ferp. ucsd. edu/ ARIES/Docs/ARIES-ST/.)

 

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Fig. 10.17 Artist rendition of a spheromak [18]

 

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Fig. 10.18 Toroidal and poloidal magnetic fields in a spheromak [19]

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Fig. 10.19 Injection of a toroidal plasma into a spheromak with a plasma gun [20]

The stable configuration that results from a period of instability and rearrangement of fields and currents is predicted by the classic theory of J. B. Taylor [9]. The main point is that the force exerted by a current in a B-field is perpendicular to B. These forces will move the plasma around until there are no more forces. That happens when the current J is parallel to B everywhere, so that there is no perpendicular force. Then the lines of B are the same as the lines of J. The B-field created by each element of J is just that which the neighboring elements of J follow. This means that the field is purely poloidal on the outside and purely toroidal on the minor axis, but the fields do not have to be neatly arranged as in Fig. 10.18. There can be a jumble of field lines that satisfy the minimum-force condition. The plasma will organize itself. Also needed is a conducting shell that keeps the whole plasma from expanding. When it tries to expand, image currents in the shell will push the plasma back.

These force-free configurations are interesting to physicists because they occur in many places, including outer space. However, spheromaks are unlikely candi­dates for fusion reactors. So far the confinement times have been short, and the plasmas have to be pulsed. Experiments have been aimed mostly at the problem of magnetic reconnection, which is important in space physics.