On the Use of Sweeping Langmuir Probes in Cutting-Arc Plasmas?Part II: Interpretation of the Results

LEANDRO PREVOSTO; HÉCTOR KELLY; FERNANDO MINOTTI

IEEE TRANSACTIONS ON PLASMA SCIENCE

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

Año: 2008

0093-3813

A semiempirical Langmuir probe model is introducedthat is particularly adapted to high-energy-density cuttingarcs, for which, as we have shown in Part I, the ion currentcollected by negatively biased probes shows no plateau in the ionbranch of the current?voltage (I?V ) probe characteristic, and thesignal amplitude is independent of the probe radius. Accordingto the model, the ion drag due to the high-velocity plasma flowaround the probe limits the effectively collecting area to a smallfraction of the probe surface. If, according to the experimentalevidence, this fraction is made independent of the probe radius,then its value results proportional to the probe bias, and so noplateau is found, at least as long as the collecting area is less than(half) the probe surface, which happens only at rather high probebias. Themodel requires the determination of the function relatingthe electric field (in the region between the unperturbed plasmaand the space-charge sheath close to the probe) to the parametersof the problem. Dimensional analysis together with empiricalinformation allow to restrict the form of this function to leave onlyan auxiliary dimensionless function, which can be argued to bepractically constant and whose value can be determined betweenrather tight bounds. As an example, radial profiles of plasmatemperature and density are obtained by applying the proposedmodel to the experimental values of a I?V probe characteristicobtained in Part I. The derived temperature profile is in goodagreement with a previous published numerical simulation for asimilar cutting torch.