A Monte Carlo simulation of radiation trapping in electrodeless gas discharge lamps

Radiation trapping and transport are important to the power balance of low pressure non-equilibrium plasma lighting sources. This is particularly the case for radio frequency inductively coupled lamps having complex geometries and where control of radiation trapping is an important design consideration. To investigate these issues, a Monte Carlo radiation transport simulation was developed and integrated into a two-dimensional plasma dynamics model. Investigations were performed on the 254 nm (6 P1–6 S0) and 185 nm (6 P1–6 S0) resonance radiation transitions from Hg in Ar/Hg electrodeless discharges. We found that analytically computed radiation trapping factors are less accurate when there is a non-uniform density of absorbers and emitters, as may occur in low pressure lamps, in our case due primarily to cataphoresis. For typical lamp conditions (hundreds of mTorr fill pressure of argon with the vapour pressure of Hg, a few megahertz driving frequency), the electromagnetic skin depth is much larger than the size of the vessel. Therefore, the frequency of excitation does not appreciably affect the distribution of absorbers and emitters, and so has little effect on radiation trapping. Studies were performed on industrially available lamp geometries. We found that the shape of the plasma cavity influences trapping factors, primarily due to the consequences of transport of Hg ions on the distribution of radiators.