Study of plasma density fluctuations by the correlation of crossed CO2 laser beams

A technique is described which uses correlation of the forward scattering from two intersecting laser beams to measure the spatial distribution ñ(r) of density fluctuations in a plasma directly. Model calculations are presented for the case of low frequency density fluctuations in a tokamak plasma, and experimental results are described for fluctuations in the Alcator A tokamak. Other potential uses of the crossed-beam correlation technique are discussed such as measurement of the nonequal time correlation function which can yield information about the propagation velocity of the density fluctuations.