Mie scattering based on-line measurement of droplet from vacuum arc

Abstract

Metal droplet is produced accompanied with vacuum arc discharge, which is important to the research of cathode spot and the application of vacuum arc. The droplet comes from the cathode spot crater and can reflect the physical process of the cathode spot. However, it will destroy the uniformity of surface deposition in engineering and should be avoided as much as possible. The measurement of metal droplet usually adopts off-line collector, which could not obtain the signal of the whole space and singe arc. In order to on-line measure the droplet, a new method by the Mie scattering is introduced in this paper, and its feasibility is investigated. The characteristic of the scattering light of titanium droplet is computed by the simulation code. The results indicate that the scattering light of the small droplet is distributed at all angle. With the increase of the diameter, the scattered light is more and more concentrated in the forward direction, which provides the possibility for the inversion of the signals of the droplets with different diameters. Then the detector is designed with different annuluses. When the detector is divided into 35 annuluses, the light energy coefficient matrix is easy to solve and the measurement system has a good resolution. The experimental setup was built and the preliminary experiment was carried out. The results indicate that the diameters of titanium droplets is mainly around 9.8 μm, which verifies the effectiveness of the Mie scattering method for measuring vacuum arc droplets. However, the small droplet information is not detected, so the droplet diameter distribution is quite different from the off-line measurement. The reason is that the signal-to-noise ratio of the measurement system is poor, which cannot effectively obtain the scattered signals of the small droplet. The experimental setup need be optimized further.