Interferometric measurement of electron and vapor densities in a high-current vacuum arc

Abstract

Electron and vapor densities in a 0.4–4.0-kA copper vapor (vacuum) arc were investigated by means of optical interferometry. Nonsimultaneous measurements were made with a visible wavelength (0.63 μm) and an infrared wavelength (10.6 μm) to separate the electron and vapor contributions. The electron contribution to the index of refraction was found to dominate at both wavelengths. An upper limit to the ratio of copper vapor density to electron density of 2.8 could be set as a result of the 0.63-μm measurement. The integral of the electron density over the optical pathlength was measured at various lateral positions of the arc by means of the 10.6-μm interferometer; the Abel transformation was applied to these results to obtain the radial profile of the electron density. A diffuse electron density profile is observed, with the axial densities ranging from 1 × 1014/cm3 at 0.4 kA to 2 × 1015/cm3 at 4 kA. At the transition from the diffuse or quiescent mode to the high-voltage or constricted mode, which occurred at about 3 kA with the electrode geometry used in this study, a sudden decrease in electron density from 1.7 × 1015/cm3 to 0.7 × 1015/cm3 was observed in the center of the discharge. The measured densities were compared with existing theoretical predictions.