Direct current cataphoretic effects on Hg 2537-A radiation from Hg + Ar discharges (dc fluorescent lamps)

The Hg 2537-A intensity from Hg + Ar discharges was measured as an independent function of mercury vapor pressure (as influenced by a cold spot temperature) at various constant currents and tube radii with ~4-Torr argon. As a function of mercury pressure, the intensity rises to a peak (which defines an optimum mercury cold spot temperature) and then decreases with further increase in mercury pressure due to the combination of self-absorption and electron deexcitation. The behavior of the optimum mercury cold spot temperature is dependent upon ac or dc conditions. For ac, the optimum mercury pressure is ~7 mTorr (corresponding to a Hg cold spot temperature of ~40 degrees C) and comparatively insensitive to current. By contrast, the optimum mercury cold spot temperature for the dc case is dependent upon whether the anode end or cathode end is cooled. The dc lamp with anode end cooled yields an optimum mercury cold spot temperature less than 40 degrees C and decreases with increasing current, while the optimum with cathode cooled is greater than 40 degrees C and increases with increasing current. We believe that the peak intensity always occurs at the same real mercury density (because that determines the self-absorption), but the Hg cold spot temperature required to achieve this density is affected by dc cataphoretic pumping phenomena.