Analytical Approach to Anode Boundary Layer of Gas Tungsten Arcs

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In order to make clear the heat transfer phenomena at the anode boundary layer of gas-tungsten-arcs, the plasma properties were measured by using laser scattering method and Langmuir probe method. Experiments revealed that physical properties were strongly influenced by the arc current. In case of low current (50A), anode boundary layer was 〜250mm in thickness in which electron temperature was considerably different with that of heavy particle temperature and the space potential in the boundary layer was low with respect to the anode. With increasing in arc current, however, the thickness of boundary layer became narrow and the space potential in the layer changed to higher value with respect to the anode in the case of 150 A in arc current. Numerical analysis of anode region was carried out to understand above experimental results. The anode region was divided into three subzones: the anode boundary layer, the presheath and the sheath. The governing equations of the dominating processes with the boundary conditions taken from the solutions of LTE plasmas, were solved by applying the Runge-Kutta procedure. Two parameters θ and α were introduced into this study. The former is the ratio of the heavy particle temperature at the free-fall edge to the anode surface temperature while the latter is the percentage of the electrons entering the lattice of anode. The results indicated that the parameter θ was an important factor influencing the local plasma properties in the boundary layer, the boundary layer thickness and the sheath potential. An estimate of the potential drop in the boundary layer showed almost negative whereas the sheath potential varied between negative and positive strong dependent on the parameter α which was used to make a difference between an anode conducting electricity and a cool wall isolated. For determining the sheath potential, a formula was established, which took account of the electron temperature, the electron flux and the ion flux at the free-fall edge as well as the parameter α.
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Analytical Approach to Anode Boundary Layer of Gas Tungsten Arcs

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