Laser light scattering in laboratory plasmas

The theory of light scattering from a collection of free electrons is reviewed, and it is shown that the frequency spectrum observed at a detector is precisely that of the density fluctuations of a particular scale length in the scattering medium, the scale length being determined by the wavelength of the incident light and the geometry of the experimental arrangement. The electron density fluctuation in a plasma is calculated, and it is shown that the plasma Debye shielding distance λD is a critical length in the theory, the electrons behaving independently on a scale shorter than λD and collectively on a scale longer than λD. The collective behaviour is characterized by the presence of waves that can give rise to well-defined resonances in the scattered-light spectrum. The effects of differing ion and electron temperatures, current flowing in the plasma, magnetic field, and Coulomb collisions are considered briefly. Technical considerations in planning experiments to test the theory and to apply it to the diagnosis of real laboratory plasmas are discussed, with attention being given to signal-to-noise ratio, stray light, and the dispersing instrument to be used at the detector. Some representative experiments that have been carried out are reviewed.