Velocities and emission rates of cathode-produced molybdenum macroparticles in a vacuum arc

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Abstract
Time‐ and space‐resolved measurements of the axial and transverse velocity components of Mo macroparticles produced in a 1‐kA peak current, 30‐ms duration vacuum arc were performed using laser‐Doppler anemometry (LDA). Individual macroparticle velocity components in the range of 10–750 m/s were detected and average velocity components in the range of 100–400 m/s were determined. Using the results of the measurements of individual velocity components we obtain that a typical rms value of the macroparticle speed is in the range 400–700 m/s. Comparing the average values of the radial and axial components indicates that typical emission angles are at approximately 30°±5° with respect to the cathode surface. The relative time‐resolved macroparticle emission rate was measured by using the LDA apparatus as a particle counter. Peak macroparticle emission occurred at a time midway between peak current and peak average cathode surface temperature. This result, together with the observation that the macroparticle emission rate could be increased by increasing the current pulse repetition rate, indicate that the macroparticle emission rate is influenced by the average cathode surface temperature as well as the instantaneous arc current.