Temperature measurements at thoriated tungsten electrodes in a model lamp and their interpretation by numerical simulation**The paper is dedicated to the memory of the late Dr G M J F Luijks in appreciation of his outstanding pioneering work on high pressure discharge lamps, particularly his contribution to an understanding of the gas phase emitter effect in these lamps in cooperation

Abstract

An atmospheric pressure argon arc is operated with dc currents of different amplitudes in a model lamp between electrodes made of pure and thoriated tungsten. Temperature measurements are performed at these electrodes with a CCD camera being calibrated at λ = 890 nm in absolute units of surface radiance and an interference filter for this wavelength. Temperature distributions are deduced from the CCD camera records of the electrodes assuming that they are grey body radiators. The records show a diffuse mode of attachment at the cathode. Doping the electrode with ThO2 causes a reduction in the cathode temperature by an amount of the order of 1000 K. On the other hand the anode temperature is weakly increased by a doping with ThO2. A reduction in the work function ϕ of the cathode from 4.55 to 3 eV is found by a comparison with cathode temperatures obtained by a numerical simulation of the diffuse mode of arc attachment with a well established cathode boundary layer model. Moreover, it is noted that the reduction is independent of the amount of ThO2 by which the electrode material is doped indicating that the work function of thoriated cathodes is the result of a self adjustment to the work function minimum at a thorium coverage of Θ ≈ 0.5. The weak influence of ThO2 on the anode temperature shows that the average work function of the anode does not depend on the thorium content of the electrode. The results are explained by a thorium ion current, by which evaporated thorium is repatriated to the cathode surface.