Two electromagnetic waves in an inhomogeneous plasma are coupled through the intermediary of longitudinal waves. A light of frequency ω is converted to a longitudinal wave at an inhomogeneity in a plasma, say at a plasma-vacuum boundary, by which the second light is scattered with frequency shifted by ±ω. An expression is given for the intensity of the scattered light when the intensities of the two light waves incident on the plasma are known.
The preliminary experiment of the microwave transmission propagating along the external magnetic field was performed for the θ-pinch plasma. From the beginning of the discharge, till the electron density become cut-off, the accumulation effect of the electron density and Faraday effect are observed. Faraday effect means that the magnetic field is trapped at least 100 gauss at the beginning time of the cut-off condition.The duration time of the cut-off condition can also be measured.Lastly in the decaying process of the plasma, the abnormal phenomenon of the microwave amplification was found. These phenomena are discussed.