| Summary: | Spatially resolved measurements of the absorption by H(n ≤ 2) atoms on the Balmer-α transition in the plume of a dc arc jet reactor, operating at an input power of 6.4 kW and with an Ar/H 2 feedstock gas mixture, are used to extract radially dependent H(n ≤ 2) atom number densities at flows of H 2 of 0.2, 0.5, 0.8 and 1.0 slm (standard litres per minute). The analysis to obtain number densities employs inverse Abel transformation of measured column densities and assumes cylindrical symmetry within the gas plume. The measured number density distributions are compared with the outcomes of a computer model of the arc jet and show quantitative agreement. Annular structure in the H(n ≤ 2) distributions, evident at the lower added H 2 flows, is a consequence of Ar + reaction with H 2 molecules diffusing radially into the plume from the cooler periphery of the reactor, followed by a dissociative electron attachment to ArH + ions. At the higher H 2 flows, the H(n ≥ 2) distribution retracts towards the nozzle through which the plasma enters the reactor and peaks on the central axis, giving a conical structure to the luminous gas plume. These variations of plume structure with added H 2 flow are well described by the sophisticated two-dimensional model, which includes heat, mass and radiation transfer and chemical kinetics of the expanding argon-hydrogen plasma, gas-surface processes at the substrate and reactor walls and carefully characterized initial conditions for the gas expansion from the nozzle orifice into the reactor chamber. © 2006 IOP Publishing Ltd.
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