A Special Kind of Torus

The name tokamak comes from the Russian words toroidalnaya kamera magnitnaya katushka meaning “toroidal chamber magnetic coils,” though it might have been appropriate to name it after the Russian word tok, meaning current. As mentioned in Chap. 4, this device was unveiled at the 1958 Geneva Conference. In those days, the Russians had the lead in space satellites, but their fusion research was done with poor equipment and considered primitive. The Americans and Britons, by contrast, had shiny, expensive, and well-engineered machines which they proudly displayed. The tokamak, however, turned out to be the one that worked the best and is the leading type of magnetic plasma container today. It was developed by a team led by Academician Lev Artsimovich on an idea of Andrei Sakharov and Igor Tamm and has been adopted by all nations working on magnetic fusion energy.

In Chap. 5, we showed that a magnetic bottle had to be a topological torus and that it had to have helically twisting magnetic field lines in order to compensate for the vertical particle drifts caused by the toroidal shape. The field lines also had to be sheared to stabilize the Rayleigh-Taylor interchange instability. In a stellarator, the proper magnetic field shape can be created with external helical windings car­rying current. In a tokamak, this is simplified by driving a large amount of current through the plasma itself. The current flows in the toroidal direction (the long way around the torus), and it generates a poloidal magnetic field (the short way around the cross section). When this poloidal field is added to the main toroidal field from the large outside coils, the magnetic field inside the plasma is twisted into helices. Moreover, since the poloidal field is not the same on every magnetic surface, the helical field also has shear. This is illustrated in Fig. 6.1. A strong field in the tor­oidal direction is created by external coils, of which only three are shown for clarity. Inside the plasma, one magnetic surface is shown. A toroidal current is driven through the plasma inside this surface, and this creates a poloidal field, which adds to the toroidal field to form a twisted helical field. Depending on how much current

there is inside each magnetic surface, the amount of twist differs from one surface to the next, and so the field is also sheared to prevent instabilities.

Using the plasma itself as a current-carrying coil to generate the twisting field would seem to be a great simplification, but we have not yet shown the hardware needed to drive this current. The advantage of the tokamak is more subtle. The cur­rent path for the poloidal field is not fixed by an external coil but can be varied by the plasma; and, fortuitously, the plasma has a self-curing property that distributes the current in a beneficial way. We explain this more fully later on.