A new laser scattering technique permitting the registration of the total scattered spectrum during one shot

There are two possibilities to facilitate the scattering technique thereby avoiding the disadvantages of the ruby laser. The first idea is to use a chopped CW laser. The small scattered signal is separated from the dc plasma light by applying a lock-in amplifier. This method has been accomplished successfully in a low pressure discharge. Preliminary experiments in a hydrogen arc plasma at atmospheric pressure showed that this treatment would require an extremely high time constant to average out the statistical fluctuations of the background plasma shift. Therefore, the second concept has been pursued. A dye is used and tuned electro-optically over the half width of the symmetrical scattered spectrum (about 2 nm) during one shot (about 2μs). Thus, the whole scattering information in picked up with one shot which reduces the number of shots required to decrease the error limits by nearly a factor of hundred as compared with a ruby laser. Modern dye lasers have an output power up to about 1 MW which is just enough to give a detectable scattered signal without appreciably heating the arc plasma. Moreover, their angular divergence is very small; a spatial resolution of 0.2 x 0.2 mm2 may be reached without difficulty. This resolution is about the same as is obtainable in spectroscopy. The experiments are performed in a H2 cascaded arc chamber at atmospheric pressure for which very precise spectroscopic measurements are available permitting a check of the reliabilility of the scattering experiments. (Auth.)