Laser Doppler anemometry: A tool for studying macroparticle dynamics in a vacuum arc

A method, based on laser Doppler techniques, for measuring macroparticle velocities and emission rates in vacuum (metal vapour) arcs is presented. A real-fringe forward-scattering system was employed, excited by a He-Ne laser, producing an ellipsoidal probe volume. Time domain analysis of individual scattering events was used to determine the macroparticle velocity components, while counting of the individual events was used to yield macroparticle emission rates. The experimental method enabled a study of macroparticle dynamics as a function of space and time in pulsed vacuum arcs of 1 and 2 kA peak current, 5 and 30 m s^-1 duration, sustained between 12-14 mm electrodes, spaced 4 mm apart and fabricated from either Cd, Zn, Al, Cu, or Mo. Broad velocity component distributions, ranging from about 10 m s^-1 up to about 800 m s^-1, and dependent on cathode metal, arc current waveform, time after arc initiation and spatial location in the discharge volume, were measured. The macroparticle emission rate was found to reach its peak a few milliseconds after the occurrence of peak current.