Development of Far-Infrared Laser Diagnostics for Simultaneous measurements of MHD Plasma Electron Density and Conductivity

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In a large scale MHD channel, it will become important to get information on the fluctuations of the electron density and electrical conductivity, and on the correlation between them. The far-infrared (FIR) laser has offered a powerful diagnostic tool for this purpose. However, the optical setup for measuring the electron density and conductivity has been different ; the electron density is measured with the FIR laser interferometry and the conductivity with its beam transmissivity. Therefore, it has not been possible to measure electron density and conductivity simultaneously. Also, the main limitation of this diagnostic method is the slow response time of the pyroelectric detector that is used to detect the fringe shifts of the FIR laser interferometer. In this research project, we proposed a modified Michelson interferometer that enabled the simultaneous measurement of these two parameters. This modified interferometer was tested with a potassium-seeded flame, and also the pyroelectric detector was replaced by the Schottky-bonier diode that had a response time of the order of ten nanoseconds. With these modifications, the following results were obtained.1. To enable the simultaneous measurements of the electron density and conductivity, one beam splitter was added in the signal arm of the conventional Michelson interferometer. The results from this new system were quite satisfactory, and this system was proved very effective as an MHD diagnostic tool.2. To measure the fluctuation, the Schottky-banier-diode was used. The fluctuation of the electron density was measured by using this diode, and the density fluctuation of up to 5OkHz was measured. The upper limit of the measurable frequency depended on the speed of the analog-to-digital converter of the present system, and it was possible to measure the higher frequency by using a fast analog-to-digital converter.