At this point, you may wonder how the complicated picture of banana orbits and magnetic islands jibes with the seemingly unrelated picture of convective cells and zonal flows in turbulence. These phenomena have different time scales. Hot electrons move almost at the speed of light, which is about one foot per nanoscecond. One trip around a large tokamak may be 20 feet, taking 20 ns. If it takes 100 trips to describe a banana orbit, that amounts to 2,000 ns or 2 ms. These individual particle motions therefore occur on microsecond time scales. Microinstabilities, on the other hand, have typical frequencies of 10 kHz, corresponding to a wave period of 100 ps. Growing into turbulence takes several periods, so the time scale is of the order of 1 ms. On this time scale, the plasma can be described as a fluid, but the fluid is not like water or air, in which the particles move randomly. The fluid that participates in microinstabilities and turbulence in a tokamak consists of particles moving in the very peculiar orbits existing in toruses.

There are two longer time scales. With a steady level of turbulence, the plasma settles into a steady state, arranging the distribution of toroidal current to give a stable q profile, possibly with magnetic islands. The radial distributions of density and of ion and electron temperatures arrange themselves so that everything is con­sistent. If these profiles become untenable, there are sawtooth crashes once in a while to rearrange them. All this happens in many milliseconds. Meanwhile, the plasma and its energy are leaking out slowly at rates described by the particle and energy confinement times. As discussed in Chap. 5, this time scale is of the order of seconds, possibly longer in a reactor-size machine.

To keep a discharge going in a reactor, DT fuel in the form of pellets is injected, and the helium “ash” is removed by the divertors. Before it is removed, the helium deposits its energy in the plasma to keep it hot. The length of a pulse in current tokamaks is determined by the transformer action needed to drive the toroidal current, since transformers cannot run DC-wise. Pulse lengths of the order of an hour are already possible. Reactors have to operate continuously, so the part of their current not generated by “bootstrap” will have to be driven “noninductively,” by waves, for instance. Or else, reactors will have to be stellarators, which do not need a current to produce the twist of the field lines. A practical power plant will have to be designed to run continuously for months or years between maintenance shut­downs. That is the longest time scale.