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Time-resolved laser-Doppler measurements in a high-current arc plasma

A symmetrical laser-Doppler system has been used for the measurement of velocities in a free-burning arc plasma. A self-triggering raster display system has been developed to record high-frequency ( approximately 10 MHz) Doppler signals over extended periods of time (several ms), so that measurements can be made of rapidly changing velocities in highly accelerated flows over prolonged periods of time. An approximate correlation between signal frequency and amplitude has been established and a simplified theoretical description given of the motion of evaporating particles in a plasma flow. The method has yielded not only information about the time evolution of the plasma flow velocity across the arc cross section, but also an estimate of the spatial distribution of globular electrode material throughout the arc as a function of time after arc initiation.