Spectroscopic study of the wall corrosion in super high-pressure sodium discharge lamps

Abstract

In 30-W sodium discharge lamps with Na pressures up to 1 atm a chemical reaction takes place between Na and the vessel, consisting of sintered polycrystalline Al2O3 (PCA), leading to the formation of β-aluminates and Al. From emission measurements at the 396-nm line the Al pressure in operating lamps is derived. The transmission of the PCA wall and the plasma temperature, which are necessary in the Al pressure derivation, are determined by additional measurements on the 436- and 818-nm emission lines of Hg and Na, respectively. The Al pressure is axially dependent, with a maximum in the center being about 10−6 atm at a lamp power of 32 W. When the lamp power is increased by 25% the central Al pressure increases by a factor of 5. The central Al pressure depends exponentially upon lamp power. The linear shape of the axial pressure profiles near the ends of the lamp indicates that Al production takes place mainly in the center of the lamp. Using a diffusion model the production rate of Al atoms is estimated to be 4×1011 atoms/s at 32 W. The experimentally determined pressures are higher than pressures calculated by equilibrium thermodynamics.