FRVT - Artículos en Revistas Internacionales
Permanent URI for this collectionhttp://48.217.138.120/handle/20.500.12272/396
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Item On the dynamics of the space-charge layer inside the nozzle of a cutting torch and its relation with the “non-destructive” double-arcing phenomenon(2011) Prevosto, Leandro; Kelly, Héctor; Mancinelli, BeatrizExperimental observations on the plasma dynamics inside the nozzle of a 30 A oxygen cutting torch operated at conditions close to the double arcing are reported. It is employed a technique previously developed in our laboratory consisting in using the nozzle as a large-sized Langmuir probe. Based on the behavior of the ion current signal and simple estimations, it is concluded that (1) the non-equilibrium plasma inside the nozzle is far from the steady state in time, in contrast to what is frequently assumed. The power supply ripple was identified as the main fluctuations source and (2) large-scale plasma fluctuations inside the nozzle could cause transient (total duration of the order of 100 ls) Townsend avalanches developing in the space-charge layer located between the arc plasma and the nozzle wall. Such events trigger the so called non-destructive double-arcing phenomena without appealing to the presence of insulating films deposited inside the nozzle orifice, as was previously proposed in the literature.Item On the Use of Sweeping Langmuir Probes in Cutting Arc Plasmas—Part I: Experimental Results.(2008) Prevosto, Leandro; Kelly, Héctor; Mancinelli, BeatrizThe 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.Item An Interpretation of Langmuir Probe Floating Voltage Signals in a Cutting Arc.(2009) Prevosto, Leandro; Kelly, Héctor; Mancinelli, BeatrizAn 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.