Power balance in the positive column of narrow bore (T2) fluorescent lamps

Abstract

Experimental measurements of the electric field and absolute radiance of 254 nm radiation in the positive column of Hg–Ar discharges, corresponding to T2 (outer diameter 7 mm) fluorescent lamps (FLs) are presented. The discharges were operated with 5 Torr argon filling pressure, for discharge currents 20–200 mA and cold spot temperatures 20–80 °C. These narrow bore discharges are ‘highly loaded’, with current densities up to an order of magnitude higher than in larger radius standard FL, and mercury depletion at the axis due to radial cataphoresis can be severe. The cold spot temperature for a maximum production of 254 nm radiation is found to be around 50 °C, compared with ∼40 °C for standard FL. The measurements are compared with the results of a numerical model. The model results are also used to provide a power balance for the positive column, showing the relative importance of all power dissipation mechanisms (radiation, gas heating and particle diffusion to the wall of the discharge).