Axial mercury segregation in direct current operated low-pressure argon–mercury gas discharges: Part I. Experimental

Abstract

Due to cataphoresis, axial segregation of mercury will occur when the gas discharge of a fluorescent lamp is operated by means of a direct current. A consequence of this is a non-uniform axial luminance distribution along the lamp. To determine the degree of axial mercury segregation experimentally, axial luminance distributions have been measured which are converted into axial mercury vapour pressure distributions by an appropriate calibration method. The mercury segregation has been investigated for variations in lamp tube radius (3.6–4.8 mm), argon buffer gas pressure (200–600 Pa) and lamp current (100–250 mA) at mercury vapour pressures set at the anode in the range from 0.2 to 9.0 Pa. From the experiments it has been concluded that the mercury vapour pressure gradient at any axial position for a certain lamp tube diameter, argon pressure and lamp current depends on the local mercury vapour pressure. This observation is in contrast to assumptions made in earlier modelling publications in which one mercury vapour pressure gradient is used for all axial positions. By applying a full factorial design, an empirical relation of the mercury segregation is found for any set of parameters inside the investigated parameter ranges.

Axial mercury segregation in direct current operated low-pressure argon–mercury gas discharge: Part II. Model

“…To describe our experimental results [1], none of the models mentioned earlier proved to be applicable. From our experiment it has been concluded that the mercury vapour pressure gradient is a function of the local mercury vapour pressure.…”

“…The existing (theoretical and experimental) literature on cataphoresis in low-pressure gas discharges will be reviewed according to the following subdivision: (1) experiments in gas discharges without mercury, (2) experiments in mercury-noble gas discharges, (3) existing models and their applicability.…”

“…As stated before, in our case there are mercury atoms (n), mercury ions (i), argon atoms (o) and electrons (e). For these species equation (3) can be written as (1) ∂p…”

“…In a previous paper [1] we discussed our experimental investigations into the axial mercury segregation due to cataphoresis in direct current (dc) operated low-pressure argon-mercury gas discharges. Axial mercury vapour pressure distributions were determined by measuring axial luminance distributions.…”

“…Then, the description of our model (section 3) and its results (section 4) are presented. Finally, a comparison of the model to our experimental data of [1] is made (section 5) and our conclusions are given (section 6).…”