FRVT - Artículos en Revistas Internacionales

Permanent URI for this collectionhttp://48.217.138.120/handle/20.500.12272/396

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Author: search.filters.author.Kelly, HéctorSubject: search.filters.subject.Plasma diagnostic.

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    On the Use of Sweeping Langmuir Probes in Cutting-Arc Plasmas—Part II: Interpretation of the Results.
    (2008) Prevosto, Leandro; Kelly, Héctor; Minotti, Fernando
    A semiempirical Langmuir probe model is intro duced that is particularly adapted to high-energy-density cutting arcs, for which, as we have shown in Part I, the ion current collected by negatively biased probes shows no plateau in the ion branch of the current–voltage (I–V ) probe characteristic, and the signal amplitude is independent of the probe radius. According to the model, the ion drag due to the high-velocity plasma flow around the probe limits the effectively collecting area to a small fraction of the probe surface. If, according to the experimental evidence, this fraction is made independent of the probe radius, then its value results proportional to the probe bias, and so no plateau is found, at least as long as the collecting area is less than (half) the probe surface, which happens only at rather high probe bias. The model requires the determination of the function relating the electric field (in the region between the unperturbed plasma and the space-charge sheath close to the probe) to the parameters of the problem. Dimensional analysis together with empirical information allow to restrict the form of this function to leave only an auxiliary dimensionless function, which can be argued to be practically constant and whose value can be determined between rather tight bounds. As an example, radial profiles of plasma temperature and density are obtained by applying the proposed model to the experimental values of a I–V probe characteristic obtained in Part I. The derived temperature profile is in good agreement with a previous published numerical simulation for a similar cutting torch.
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    On the Use of Sweeping Langmuir Probes in Cutting Arc Plasmas—Part I: Experimental Results.
    (2008) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    The first study of Langmuir probes applied to cut ting arcs using a sweeping-probe system is presented. It is found that, under a relatively broad range of experimental conditions (changes in the probe material, in the probe radii, or in the sweeping frequency of the probes), no probe damage is registered, notwithstanding the large value of the power flux present with these arcs. In practice, probes with radii down to 63 µm and with sweeping rotation frequencies down to 8.7 s−1 (probe transit time of ≈140 µs through the arc) were used without noticeable alterations. In the measurements of the ion current collected by negatively biased probes, the following two unexpected features are found: the lack of a current plateau in the ion branch of the I–V probe characteristic and the independence of the signal amplitude on the probe radius. According to the experimental evidence, as well as several estimations, we have neglected electron emission of the probe surface as a relevant mechanism in modi fying the ion branch of the characteristic. On the contrary, some arguments on which a collection model will be based are presented.
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    An Interpretation of Langmuir Probe Floating Voltage Signals in a Cutting Arc.
    (2009) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    An experimental study of the electrostatic probe floating voltage signals in a cutting arc and its physical interpre tation in terms of the arc plasma structure is reported. Sweeping electrostatic probes have been used to register the local floating potential and ion current at 3.5 mm from the nozzle exit in a 30-A arc generated by a high energy density cutting torch with a nozzle bore radius of 0.5 mm and an oxygen mass flow rate of 0.71 g · s−1. It is found that the floating potential signal presented a central hump with duration almost similar to that corresponding to the ion current signal but having also lateral wings with much larger duration. Capacitive coupling between the probe and the conducting body of the nozzle and arc as a source for the float ing potential signal was discarded. It is assumed that the hump in these probe voltage signals results from the presence of an electrostatic field directed in the radial direction outward the arc axis that is caused by thermoelectric effects. The probe floating voltage signal is inverted using the generalized Ohm’s law together with the Saha equation, thus obtaining the radial profiles of the temperature, particle densities, radial electric field, and potential of the plasma at the studied section of the arc. The resulting temperature and density profiles derived from our interpretation are in good agreement with the data published elsewhere in this kind of high-pressure arcs. There is not a straightforward connec tion between the measured hump amplitude in the floating signal (≈4 V) and the derived increase in the plasma potential between the arc edge and the arc center (≈10 V), due to the global zero cur rent balance condition established by the finite size of the probe. It is shown, however, that the probe takes a floating potential value close to that corresponding to the plasma temperature at the probe center.