Descripción: Plasma temperature and radial density profiles of the plasma species in a high energy density cutting arc have been obtained by using a quantitative schlieren technique. A Z-type two-mirror schlieren system was used in this research. Due to its great sensibility such technique allows measuring plasma composition and temperature from the arc axis to the surrounding medium by processing the gray-level contrast values of digital schlieren images recorded at the observation plane for a given position of a transverse knife located at the exit focal plane of the system. The technique has provided a good visualization of the plasma flow emerging from the nozzle and its interactions with the surrounding medium and the anode. The obtained temperature values are in good agreement with those values previously obtained by the authors on the same torch using Langmuir probes. © 2010 American Institute of Physics.

Descripción: A two-wavelength quantitative Schlieren technique that allows inferring the electron and gas densities of axisymmetric arc plasmas without imposing any assumption regarding statistical equilibrium models is reported. This technique was applied to the study of local thermodynamic equilibrium (LTE) departures within the core of a 30 A high-energy density cutting arc. In order to derive the electron and heavy particle temperatures from the inferred density profiles, a generalized two-temperature Saha equation together with the plasma equation of state and the quasineutrality condition were employed. Factors such as arc fluctuations that influence the accuracy of the measurements and the validity of the assumptions used to derive the plasma species temperature were considered. Significant deviations from chemical equilibrium as well as kinetic equilibrium were found at elevated electron temperatures and gas densities toward the arc core edge. An electron temperature profile nearly constant through the arc core with a value of about 14000-15000 K, well decoupled from the heavy particle temperature of about 1500 K at the arc core edge, was inferred. © 2011 American Institute of Physics.

Descripción: In this work, an experimental study on the influence of the nozzle geometry on the physical properties of a cutting arc is reported. Ion current signals collected by an electrostatic probe sweeping across a 30 A oxygen cutting arc at 3.5 mm from the nozzle exit were registered for different nozzle lengths. The temperature and density radial profiles of the arc plasma were found in each case by an inversion procedure of these signals. A comparison between the obtained results shows that the shorter nozzle (RN = 0.50 mm, L N = 4.5 mm operated at 0.7 MPa and 35 Nl/min) produces a thinner and hotter arc than the larger nozzle (RN = 0.50 mm, LN = 9.0 mm operated at 1.1 MPa and 20 Nl/min). This behavior is attributed to the marked difference of gas flow rate due to the clogging effect. A smaller gas mass flow reduces the convective cooling at the arc border and decreases the power dissipation of the arc column, resulting in small axis temperatures. © 2009 IOP Publishing Ltd.

Descripción: Measurements of plasma velocities in a 30 A high energy density cutting torch are reported. The velocity diagnostic is based on the analysis of the light fluctuations emitted by the arc which are assumed to propagate with the flow velocity. These light fluctuations originate from plasma temperature and plasma density fluctuations mainly due to hydrodynamic instabilities. Fast photodiodes are employed as the light sensors. The arc core velocity was obtained from spectrally filtered light fluctuations measurements using a band-pass filter to detect light emission fluctuations emitted only from the arc axis. Maximum plasma jet velocities of 5000 m s -1 close to the nozzle exit and about 2000 m s -1 close to the anode were found. The obtained velocity values are in good agreement with those values predicted by a numerical code for a similar torch to that employed in this work. © 2009 American Institute of Physics.

Descripción: A stationary, one-dimensional fluid model is presented to describe the interelectrode region of a nonfiltered vacuum arc operated with a background gas. The model includes the electron energy equation and the main elastic and inelastic atomic processes among metallic ions, electrons, and gas particles. To validate the model predictions an experimental study of the plasma-neutral gas structure, using a titanium (Ti) cathode and argon (Ar) as the background gas, is presented. The measured electron temperature and the experimental dependence on the pressure of neutral Ti and Ar spectroscopic emission lines are well reproduced, using a simple atomic model to interpret the plasma radiation emission. © 2006 American Institute of Physics.

Descripción: Langmuir probe measurements in an atmospheric pressure direct current (dc) plasma jet are reported. Sweeping probes were used. The experiment was carried out using a dc non-transferred arc torch with a rod-type cathode and an anode of 5 mm diameter. The torch was operated at a nominal power level of 15 kW with a nitrogen flow rate of 25 Nl min -1. A flat ion saturation region was found in the current-voltage curve of the probe. The ion saturation current to a cylindrical probe in a high-pressure non local thermal equilibrium (LTE) plasma was modeled. Thermal effects and ionization/recombination processes inside the probe perturbed region were taken into account. Averaged radial profiles of the electron and heavy particle temperatures as well as the electron density were obtained. An electron temperature around 11 000 K, a heavy particle temperature around 9500 K and an electron density of about 4 × 10 22 m -3, were found at the jet centre at 3.5 mm downstream from the torch exit. Large deviations from kinetic equilibrium were found throughout the plasma jet. The electron and heavy particle temperature profiles showed good agreement with those reported in the literature by using spectroscopic techniques. It was also found that the temperature radial profile based on LTE was very close to that of the electrons. The calculations have shown that this method is particularly useful for studying spraying-type plasma jets characterized by electron temperatures in the range 9000-14 000 K. © 2012 American Institute of Physics.

Descripción: Double-arcing is a phenomenon that occurs when a transferred arc, flowing inside an electrically insulated nozzle, breaks into two separate arcs: one that connects the cathode with the nozzle, and another that connects the nozzle with the anode. Experimental evidence suggests that the reason for double-arcing is a Townsend like breakdown occurring in the thin space-charge layer, which separates the plasma from the metallic nozzle, due to the high voltage drop across it. Breakdown phenomena in a gas between metallic electrodes have been extensively studied; however the present case involves breakdown of a high-temperature gas between one electrode (the nozzle) and a plasma boundary. A 1-D model of the gas breakdown development in the space-charge layer contiguous to the nozzle of a cutting arc torch operated with oxygen is reported. The dynamics of the discharge is analyzed. The kinetic scheme includes processes of ionization of heavy particles by electron impact, electron attachment, electron-ion recombination and ion-ion recombination.

Descripción: A sequence of plutonic mafic rocks inter-stratified with both mafic- and metasedimentary-derived migmatites is found along the San Juan valley in the center of the Sierra de Valle Fértil. This natural example shows the transition from igneous to metamorphic petrologic processes which occurred during the crystallization of mafic magmas and the subsequent partial melting of crystallized gabbroic rocks. This work studies the mineralogical changes associate to this petrologic transition. Thermobarometric estimates based on amphibole-plagioclase indicate that the mafic magmas crystallized at around 1100°C and 5 ± 0.5 kbar. The conditions under which gabbroic rocks were partially melted are estimated using two pyroxenes ther-mometry and amphibole-plagioclase thermobarometry. Similar physical conditions in the range between 740 and 840°C and 5 to 6.5 kbar are recovery from mineral assemblages in the mesosomes and leucosomes of mafic migmatites. The main mineral compositional changes that accompanied the partial process of the gabbroic rocks are: 1) depletion of aluminium content and Mg# ratio in pyroxenes; 2) depletion of anorthite mole fraction of the plagioclases; and 3) depletion of the Mg# ratio in amphiboles. These mineral compositional variations are consistent with those found by experimentally melting mafic protoliths. Experimental results showed that the temperature for promoting amphibole-dehydration melting in mafic proto-liths is of around 850°C, whereas the addition of an aqueous fluid would lower the melting temperature to around 800°C. Combining field relationships, textural features, mineral compositional variations and thermo-barometric estimates with experimental results, we demonstrated that the transition from igneous mafic rocks to mafic migmatites was due to isobaric cooling. Moreover, repetitive intrusion of mafic magmas sustained the sequence at granulite-facies conditions, so avoiding rapid cooling. The natural case from the centre of the Sierra de Valle Fértil is interpreted to expose an example of the plutonic-me-tamorphic core of the Famatinian magmatic arc, where the abnormally high metamorphic gradient provide evidence that large volume of mafic magmas reached and emplaced at paleodepths of between 16 and 20 kilometres.

Descripción: This is the first characterization of the structure and temporal variation of the plankton communities comprising the complete food web in five peat bog pools related to environmental factors over two consecutive ice-free periods in Tierra del Fuego (548S). Remarkably, picophytoplankton was composed solely of eukaryotic cells, surpassing the dominance expectations for these acidic water bodies, whereas testaceans were virtually absent, even as tychoplankters. Abundances of the different planktonic communities were slightly higher than those reported for Northern Hemisphere peat bogs and humic lakes. Mixotrophic nutrition prevailed among nano- and microphytoplankters, a strategy also common in humic lakes. The structures in spring of the planktonic communities were similar. In contrast, in late summer there were differences in the abundance and biomass of the different trophic compartments among small, shallow water bodies and large ones. These seem to be dictated by distinct pool size-driven patterns of water temperature variation. A general shift in the control of heterotrophic flagellates abundance in the pools occurred, changing from bottom-up regulation in spring to top-down control in late summer related to © The Author 2013.