Charging of Macroparticles in a Pulsed Vacuum Arc Discharge

A pulsed vacuum arc discharge emits a plasma as well as macroparticles (MPs) in the form of micrometer-sized molten droplets of cathode material. Due to their direction of flight and submicrometer to 100-mum diameter, these MPs often pose a contamination threat for both spacecraft-based thrusters and thin-film deposition systems. The velocity, mass, and charge of copper MPs emitted by a 100-A arc was experimentally measured and compared to a model based on thermionic electron emission. The MP velocity was determined by using a time-of-flight velocity filter. The charge was calculated by measuring particle deflection in a transverse electric field. The model predicts, and the experimental results verify, that the charge on the MPs becomes positive once the plasma is extinguished, and the MP travels in a vacuum, as would occur in a pulsed vacuum arc, versus a dc arc. Experimental results show a roughly quadratic dependence of particle charge on the particle diameter (q ~ D²), with a 1-mum particle having a positive charge of ~1000 electronic charges (1.6 times 10^-16 C), and a 5-mum particle having a charge of ~25 000 electronic charges. The model is particle temperature dependent, and gives q ~ D² at 1750 K and q ~ D^1.7 at 2200 K. Arguments are also made for limitations on particle temperature due to radiative and evaporative cooling.