MEASUREMENTS OF CONDUCTIVITY NONUNIFORMITIES AND FLUCTUATIONS IN COMBUSTION MHD PLASMAS

Magnetohydrodynamic (MHD) generator performance may be significantly reduced by spatial nonuniformities in the electrical conductivity of the plasma working fluid. Theories have predicted these performance degradations, but sufficient experimental data to thoroughly characterize nonuniformities and test the theories have not been available. This scarcity of data has been partially caused by a lack of suitable nonuniformity diagnostics; consequently, the primary goal of this thesis was to develop diagnostics for the characterization of electrical conductivity nonuniformities in combustion MHD plasmas. Secondary goals were to obtain an initial characterization of nonuniformities in the Stanford M-2 linear generator and to make recommendations concerning the use of the diagnostics in practical MHD generator configurations.; Conductivity nonuniformities are generally characterized from measurements of conductivity fluctuations at several locations in a MHD channel. Local or point measurements are preferred. Since local fluctuation diagnostics had not been satisfactorily demonstrated in combustion plasmas, a large part of the present thesis was devoted to the development of a laser-induced fluorescence (LIF) diagnostic for nonintrusive measurements of local conductivity fluctuations. The LIF diagnostic uses a cw dye laser to pump a transition between two excited states of sodium atoms which are injected into the plasma with the customary potassium seed material. The resulting fluorescence signal is a good relative indicator of the local plasma conductivity. This diagnostic and other line-of-sight averaged optical nonuniformity diagnostics were successfully demonstrated in several experiments in the Stanford M-2 combustion system. Results were used to characterize the nonuniformities in the M-2 system and to compare and evaluate the diagnostics.; Conductivity nonuniformities were found to be predominantly "streamers" which had relatively long length scales of the order of 1 m in the axial flow direction. Shorter transverse length scales of the order of 0.1 m were found perpendicular to the flow direction. Both local and line-of-sight averaged conductivity fluctuation intensities were of the order of ten percent rms, and reasonably strong cross-correlation coefficients were obtained between fluctuation signals of the two types of diagnostics. Some differences between local and line-of-sight averaged measurements were also observed; consequently, local measurements of conductivity fluctuations are recommended whenever possible. Extensions of the LIF diagnostic for multiple-point measurements, however, may be difficult, and a combination of LIF (local) and plasma luminosity (line-of-sight averaged) diagnostics is recommended for future characterizations of conductivity uniformities in combustion MHD plasmas. Optical access requirements of the LIF diagnostic may limit its applicability in some superconducting magnet configurations, but the diagnostic should be useful in the evaluation of commercial MHD combustors in terms of nonuniformity characteristics.