An Indispensable Truth by Francis F. Chen

An Indispensable Truth by Francis F. Chen

Author:Francis F. Chen
Language: eng
Format: epub
Publisher: Springer New York, New York, NY


Mappings, Chaos, and Magnetic Surfaces

Figure-8 stellarators are hard to make, especially since the coils have to be accurate enough to keep the field lines from wandering out to the walls.8 It was soon realized, however, that the necessary twist of the field lines can be produced without twisting the entire torus. We mentioned that the field lines in a toroidal magnetic bottle are twisted like the stripes on a candy cane. The way to produce such helical field lines can be visualized more easily if we decompose them into toroidal lines, as in Fig. 4.13a, and poloidal lines, as in Fig. 4.13b. Adding these two types of fields together will result in a field with helical field lines. To produce the toroidal part of the field, we can use coils like those in Fig. 4.14. Now we want to add coils that will produce the poloidal field. Figure 4.19 shows how this is done. Let there be a number of toroidal hoops placed all around the torus; two of these are shown in Fig. 4.19. If each hoop carries a current in the toroidal direction, as shown by the horizontal arrows, it will produce a magnetic field around itself in the direction shown by the arrows on the small circles around each hoop. The part of this field that extends into the plasma will be mostly in the poloidal direction. Imagine that there are an infinite number of these hoops covering the surface of the torus. Their fields inside the plasma will add up to give a purely poloidal field, as shown by the dashed arrows.

Fig. 4.19Generation of poloidal fields with coils



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