Princeton Plasma Physics Laboratory,Larger version
Princeton, New Jersey
TFTR has been the first large tokamak to produce significant quantities of fusion power (up to 10 Million Watts) from the fusion of DT (Dueterium and Tritium). Although, this has not equalled the input heating power (the breakeven condition), it has been a valuable test-bed for the new physics associated with a DT fusioning plasma. The blue/magenta colorations of the magnetic surface are contours of electrostatic potential from a type of plasma turbulence known as a TAE instability. This instability is driven by the alpha particles which are a by-product of fusion reactions. This type of turbulence was one of the new areas of DT fusion physics which the TFTR experiment was able to access.
ITER is a large tokamak being developed under an international collaborative effort between the United States, Japan, the European Economic Community, and the Former Soviet Union. ITER is designed to achieve an ignited DT plasma. In this image we show a magnetic flux surface along with the magnetic field lines which lie in that surface. The blue/magenta colorations of the surface are again contours of electrostatic potential from the TAE instability. In larger, hotter plasmas, such as ITER, this instability tends to have a shorter scale length (compare to the above TFTR image).Larger version
In this figure we show only the magnetic field lines (for the above ITER device) without the associated magnetic flux surfaces. The inner black colored magnetic field line is the magnetic axis and has a simple circular topology.Larger version
Moving outward from this we next come to the blue and purple color coded field lines. These traverse the torus with a finite angle relative to the toroidal direction (i.e., the long way around the donut). This pitch in these field lines causes them to traverse around the torus the small way (poloidally) as they move toroidally. This property is termed rotational transform and is a consequence of the fact that both toroidal magnetic field (produced by external magnets) and poloidal magnetic field components (produced by the toroidal plasma current) are present. Rotaional transform is essential for good plasma confinement. Comparing the blue and purple field lines one can observe slight differences in their pitch angles. This shearing between adjacent field lines is generally present in tokamaks due to the dependence of the poloidal magnetic field on the distance from the magnetic axis. Magnetic shear is helpful for stabilizing certain classes of plasma instabilites.
The green magnetic field line represents a special case. They are close to a rational magnetic surface (i.e., where the rotational transform is a low order rational number). This field line comes close to closing on itself and does not spread out over the associated magnetic surface the way the blue and purple lines do. This stronger coherence of the rational field lines has the consequence of making rational magnetic surfaces especially susceptible to kink and tearing instabilities. However, with sufficient shear, the spatial extent of these instabilities can be limited.