Far-forward scattering of CO2-laser radiation by density fluctuations in a hollow cathode arc

Far-forward scattering measurements from a cylindrical magnetized arc plasma are reported. A well-stabilized CW CO2 laser serves as a light source. Self-excited electron density fluctuations with frequencies in the range 40 kHz to 1 MHz have been observed. Three frequency peaks can be distinguished from a broad background signal. The fluctuation level is maximal near the cathode, where the mean electron density approaches Nc=2*1014 cm-3 on axis. In this region a relative fluctuation amplitude of 5% has been measured. In addition to the beam profile measurements, which are necessary in far-forward scattering experiments, a spatial profile of the modulated light intensity has been obtained. It is found that the scattered power is maximal in the region of the largest electron density gradient. The fluctuations are described as azimuthal propagating waves. In contrast to the plane-wave model used in other publications this model takes the spatial limits of the fluctuation field into account. The spatial distribution of the modulated light intensity at the detection plane is calculated by using the well-known diffraction theory. The theoretical model is in good agreement with the experimental data.