Formation of a plane scattering wave in diagnostics of plasma streams

It is shown that a nonlinear interaction of laser radiation and an additional slow electromagnetic wave with a homogeneous electron stream may give rise to a plane wave with the sum frequency and a high degree of coherence. This wave travels along a direction different from the direction of propagation of the primary waves. The intensity of the new wave is proportional to the product of the intensities of the primary waves and to the square of the electron density. It is shown that a wave of this kind can be used to measure the electron density in a plasma.