Temperature measurements at thoriated tungsten electrodes in a model lamp and their interpretation by numerical simulation

Andre Bergner, Michael Westermeier, Cornelia Ruhrmann, Peter Awakowicz, Jürgen Mentel

JOURNAL OF PHYSICS D-APPLIED PHYSICS, Volume: 44, Issue: 50, Article Number: 505203, DOI: 10.1088/0022-3727/44/50/505203, Published: DEC 21 2011


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 lambda = 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 ThO(2) 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 ThO(2). A reduction in the work function phi 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 ThO(2) 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 Theta approximate to 0.5. The weak influence of ThO(2) 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.

Tags: plasma