Cw Colin Johnston

A self-consistent LTE model of a microwave-driven, high-pressure sulfur lamp

A one-dimensional LTE model of a microwave-driven sulfur lamp is presented to aid our understanding of the discharge. The energy balance of the lamp is determined by Ohmic input on one hand and transport due to conductive heat transfer and molecular radiation on the other. We discuss the origin of operational trends in the spectrum, present the model and discuss how the material properties of the plasma are determined. Not only are temperature profiles and electric field strengths simulated but also the spectrum of the lamp from 300 to 900 nm under various conditions of input power and lamp filling pressure. We show that simulated spectra demonstrate observed trends and that radiated power increases linearly with input power as is also found from experiment.

An improved LTE model of a high pressure sulfur discharge

An existing LTE model (Johnston C W et al 2002 J. Phys. D: Appl. Phys. 35 342) of a high pressure sulfur discharge is improved upon by more accurate and complete treatment of each term in the energy balance. The simulation program PLASIMO (Janssen G M et al 1999 Plasma Sources Sci. Technol. 8 1, van Dijk J 2001 Modelling of plasma light sources: an object-oriented approach PhD Thesis Eindhoven University of Technology, The Netherlands, ISBN 90-386-1819-0), which is an integrated environment for construction and execution of plasma models, has been used to define and solve all aspects of the model. The electric field is treated as being dc, and the temperature dependent nature of species interactions is incorporated in determination of transport coefficients. In addition to the main radiative transition, , several others in S2 are included. These are , and . The S3 molecule is also included in the composition as an absorbing particle. Furthermore, radiation production is treated quantum mechanically. The principle improvement over the previous work is that both the position of the spectral maximum and the pressure shift are quantitatively described by the current model. Both are chiefly due to the presence of S3.

Operational trends in the temperature of a high-pressure microwave powered sulfur lamp

Temperatures have been measured in a high-pressure microwave sulfur lamp using sulfur atomic lines found in the spectrum at 867, 921 and 1045 nm. The absolute intensities were determined for 3, 5 and 7 bar lamps at several input powers, ranging from 400 to 600 W. On average, temperatures are found to be 4.1 ± 0.15 kK and increase slightly with increasing pressure and input power. These values and trends agree well with our simulations. However, the power trend is reversed to that demonstrated by the model, which might be an indication that the skin-depth model for the electric field may be incomplete.

Measured and simulated response of a high pressure sulfur spectrum to power interruption

The power supplied to a high pressure, microwave sulfur plasma sustained by microwaves is interrupted on short timescales. The response of the molecular spectrum between 400 and 900 nm is measured during this interruption. Characteristic decay times of around 20 ms are found and are due chiefly to the large heat capacity of the reacting mixture. The results of a time dependent local thermal equilibrium model of the interruption, including radiation transport of the entire molecular spectrum and heat conduction, are also presented. The simulated signal decay agrees with experiment not only in magnitude but also as a function of wavelength.

A scaling rule for molecular electronic transition dipole moments: Application to asymptotically allowed and forbidden transitions

Guided by the work of Woerdman and Monyakin, we propose rules that allow the electronic transition dipole moment for a transition in one molecule to be determined from that of a similar one in an isovalent species. The rule can be applied to asymptotically allowed and forbidden transitions. We have tested it by applying it in two specific cases: the moments for the A 1Σ→X 1Σ and X 1Σ→B 1Π transitions in Na2 are found from those in Li2, which are asymptotically allowed and the moments for the B→X transition in O2, Se2, and Te2 which are asymptotically forbidden, are found from moment data for S2. Transition moments calculated with this rule are within 15% of the available literature values and behavior as a function of internuclear separation is well described.