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, HgI
2, I
2 H
2O and H
2 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 HgI
2 vapor less than-5.9×10
-6 Torr at-15°C do not change η in Penning mixtures, however, H
2 leads to increase in
Vg due to decrease in η, while variation of
Vg caused by the mixed H
2O depends on change in both η and φ.
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