Non-linear phenomena in tokamak plasmas

A tokamak is the simplest magnetic configuration for the confinement of high temperature plasma bearing in mind its subsequent use for nuclear fusion reactors. A tokamak looks like a vacuum toroidal chamber with magnetic coils. This chamber is filled by hydrogen (or hydrogen isotopes) gas, at low pressure. Then with the help of an induced azimuthal electric field a toroidal discharge is excited which heats the plasma up to very high temperature. Additional heating can be used to further increase the plasma temperature and to reach hundreds of millions degrees C. A toroidal magnetic field is used to suppress plasma instabilities. The concept itself is a simple one but the behaviour of high temperature plasmas in tokamaks has turned out to be very complicated. A lot of non-linear processes develop in the plasma. Some of them are similar to the usual hydromagnetic turbulence. But this turbulence coexists with many coherent structures strongly coupled with each other. Step by step the physics of many cooperative effects in tokamak plasmas have been successfully clarified. They are discussed in this review both from a experimental and a theoretical point of view.