Diameters and Velocities of Droplets Emitted from the Cu Cathode of a Vacuum Arc

Cathode spots of vacuum arcs emit cathode material in the form of plasma as well as droplets. Generated by arcs burning at the first wall of fusion devices the droplets may effectively contaminate the fusion plasma. Essential characteristics of the droplets (diameter, velocity, and emission direction) and their interrelations are poorly known so far. In the present paper, a new approach of optical droplet investigation is presented. Generated by a pulsed vacuum arc, the droplets fly vertically inside a drift tube against gravity finally passing a light beam. The time-of-flight and the detected scattered light allow a simultaneous determination of droplet velocity and size. Different solid angle directions have been realized by rotating the cathode with respect to the flight tube axis. Using this technique the sizes and velocities of droplets emitted from a Cu cathode into different directions were obtained.