One of the most powerful methods of diagnosis is to use the scattering of electromagnetic radiation from the plasma. The attractiveness of this diagnostic derives from two main features. First, it is, for all practical purposes, a nonperturbing method, requiring only access of radiation to the plasma. Second, it offers the potential of determining detailed information about the distribution function of electrons and sometimes even of the ions too. These advantages are sufficient to offset the great technical difficulty of the measurements. Electromagnetic wave scattering diagnostics are now widespread, especially in hot plasma experiments.

The process of electromagnetic wave scattering by charged (elementary) particles may be thought of as follows. An incident electromagnetic wave impinges on the particle. As a result of the electric and magnetic fields of the wave, the particle is accelerated. The charged particle undergoing acceleration emits electromagnetic radiation in all directions. This emitted radiation is the scattered wave.

Of course, this description is purely classical. From a quantummechanical viewpoint we might have described the process in terms of photons colliding with the particle and hence “bouncing off” in different directions. This would lead to an identical mathematical formulation provided there is negligible change in the mean particle momentum during collision with the photon. This will be the case provided that the photon mass is much smaller than the particle mass: ħω « mc2. This classical limit of scattering by free charges is called Thomson scattering. On the other hand, when the photons are sufficiently energetic that their momentum cannot be ignored, the quantum-mechanical modifications lead to different results and the situation is called Compton scattering.