Effects of Metal Halides and Impurities on Discharge Parameters

Abstract

Substantial effects of metal halides and impurities on discharge parameters contributing to starting voltage in metal halide lamps were investigated.Townsend's first ionization coefficients α in Ar-Hg Penning mixtures containing NaI, HgI2, I2 H2O and H2 were measured at room temperature and the ionization coefficients per volt η were calculated. Glow discharge break-down voltage Vg as a function-of Pd (P is argon pressure, d is inter-electrode distance) for various combinations of gas composition and coated electrode was measured using an experimental discharge tube. Relation between Vg and discharge parameters (η and work function φ of the electrode) was discussed.As a result, NaI vapor at room temperature and HgI2 vapor less than-5.9×10-6 Torr at-15°C do not change η in Penning mixtures, however, H2 leads to increase in Vg due to decrease in η, while variation of Vg caused by the mixed H2O depends on change in both η and φ.