Properties of Droplets Emitted from Vacuum Arc Cathodes Made of Copper, Tungsten and Titanium

Molten droplets emitted from the cathode of vacuum arcs have been investigated by using a time-of-flight setup along a vertical flight tube equipped with two light barriers at different distances from the cathode surface. The light scattered by particles passing the light barriers was registered. The velocity was determined by the flight time. Simultaneously, from the amount of the scattered light, the size of the passing particle was inferred by applying the Mie-scattering theory. Because the arrangement of the cathode could be oriented concerning the flight tube, a coarse angular distribution of the parameters was obtained. Due to a large amount of data, computerized processing by newly developed algorithms was required to identify and characterize the several tens of thousands of particle peaks. The number of particles per discharge depends strongly on the cathode material and varies remarkably with the angle between flight direction and cathode surface. Generally, smaller particles were found to have higher velocities and vice versa. The distribution of the particle sizes follows a power law. The distribution of the emission velocities exhibits more than one local maximum pointing to distinct particle populations having distinct typical velocities. These populations contribute differently to the total amount of particles at various angles.