F. O. Minotti

Discharge characteristics of plasma sheet actuators

The electrical characteristics of a plasma sheet device used for subsonic airflow control are studied in this paper. Experiments are undertaken with a two-wire asymmetrical (different diameters, opposite polarity) electrode configuration connected to dc high voltage sources in the presence of a dielectric plate and under different gases (dry air, nitrogen and oxygen). For large distances electrode-plates it has been found that the discharge current consists of a purely dc component. The proximity of the plate reduces notably this dc current component until a limit situation for which the electrodes practically lay on the plate and a current pulsed regime is superimposed on the dc (small) component, thus establishing a plasma sheet regime. This regime could be reached only when the small wire was positive. This work establishes that the pulsed regime may be associated with a succession of positive streamers (cathode directed) which formation is promoted by different parameters of the gas and surface characteristics (thresholds of photoionization and photoemission, charge deposition,...). The dc component seems to be produced by a small number of electrons originated in the ionization region of the negative corona that are amplified in the ionization region of the positive corona. The charged particles produced during the streamer propagation could contribute appreciably to the ion momentum transfer to the gas. This transfer should be due very likely to the drift of the charged species present in the streamer channel during the streamer collapsing phase. The source of momentum transfer associated with the dc current would always persist with a magnitude that depends on the intensity of this current. (Some figures in this article are in colour only in the electronic version)

Electrical studies and plasma characterization of an atmospheric pressure plasma jet operated at low frequency

Low-temperature, high-pressure plasma jets have an extensive use in medical and biological applications. Much work has been devoted to study these applications while comparatively fewer studies appear to be directed to the discharge itself. In this work, in order to better understand the kind of electrical discharge and the plasma states existing in those devices, a study of the electrical characteristics of a typical plasma jet, operated at atmospheric pressure, using either air or argon, is reported. It is found that the experimentally determined electrical characteristics are consistent with the model of a thermal arc discharge, with a highly collisional cathode sheet. The only exception is the case of argon at the smallest electrode separation studied, around 1 mm in which case the discharge is better modeled as either a non-thermal arc or a high-pressure glow. Also, variations of the electrical behavior at different gas flow rates are interpreted, consistently with the arc model, in terms of the development of fluid turbulence in the external jet.

Differences in the metallic plasma-neutral gas structure in a vacuum arc operated with nitrogen and argon

Ion current and electron temperature are measured using electrostatic probes in a dc, nonfiltered vacuum arc operated with argon and nitrogen as filling gases in the pressure range 0.001–1mbar. It is found that the measured ion current for argon is between two and six times larger than for nitrogen, for similar operating conditions. Also, the electron temperature is smaller for Ar. These differences can be satisfactorily explained with a simple one-dimensional model, which includes the most relevant elastic and inelastic processes that take place in the interelectrodic plasma: elastic scattering of metallic ions by neutral gas, charge exchange, electron impact ionization of gas, dissociative recombination of gas ions, and conversion of atomic ions into molecular ions. The observed differences between nitrogen and argon are attributed to the low rate of conversion of atomic argon into molecular argon, that makes inoperative the channel of dissociative recombination for this gas, together with the higher io...

Scalar-tensor theories and asymmetric resonant cavities

The recently published experimental results indicate the appearance of unusual forces on asymmetric, electromagnetic resonant cavities. It is argued here that a particular class of scalar-tensor theories of gravity could account for this effect.

Experimental and theoretical study of an atmospheric air plasma-jet

In this work, we present an experimental and theoretical study of a low frequency, atmospheric plasma-jet discharge in air. Voltage-current characteristics and spectroscopic data were experimentally obtained, and a theoretical model developed to gain information of different aspects of the discharge. The discharge is modeled as a cathode layer with different mechanisms of electron emission and a main discharge channel that includes the most important kinetic reactions and species. From the electric measurements, it is determined that high electric field magnitudes are attained in the main channel, depending on the gas flow rate. Using the voltage-current characteristics as an input, the model allows to determine the plasma state in the discharge, including electron, gas, and molecular nitrogen vibrational temperatures. The model also allows to infer the mechanisms of secondary electron emission that sustain the discharge.

Hydrodynamic model for the plasma-gas flow in a cutting torch nozzle

We present a simple hydrodynamic model to obtain the profiles of the relevant physical quantities along a nozzle of arbitrary cross-section in a cutting torch. The model uses a two-zone approximation (a hot central plasma carrying the discharge current wrapped by a relatively cold gas which thermally isolates the nozzle wall from the plasma). Seeking for a solution with sonic conditions at the nozzle exit, the model allows expressing all the profiles in terms of the externally controlled parameters of the torch (geometry of the torch, discharge current, mass flow of the gas and plenum pressure) and the values of the arc and gas temperatures at the nozzle entrance. These last two values can be estimated simply appealing to energy conservation in the cathode-nozzle region. The model contains additional features compared with previous reported models, while retaining simplicity. The detailed consideration of an arc region coupled to the surrounding gas dynamics allows determining voltage drops and consequent delivered power with less assumptions than those found in other published works, and at the same time reduces the set of parameters needed to determine the solution.

Experimental characterization of a low-current cutting torch

An experimental characterization of a low-current (30-40 A) cutting torch is presented. To avoid contamination of the plasma arc by removed anode material, a rotating steel cylinder was used as the anode and the arc was anchored onto the cylinder lateral surface. The cathode-anode and cathode-nozzle voltage drops, together with the gas pressure in the plenum chamber were registered for different values of the mass flow rate injected into the plenum chamber. By employing an optical system with a large magnifi cation (» 15 X), the arc radius the nozzle exit was also determined with a digital optical camera. The obtained experimental quantities were used to evaluate several flow properties at the nozzle exit (hot arc plasma and cold gas temperatures, arc and gaz velocities, etc.) by employing a simplifi ed theoretical model for the plasma flow in the nozzle. The obtained results are in reasonable agreement with the data reported in the literature by other authors. Explanations of the origin of the clogging effect and the nozzle voltage are also presented.

Numerical investigation of the double-arcing phenomenon in a cutting arc torch

A numerical investigation of the double-arcing phenomenon in a cutting arc torch is reported. The dynamics of the double-arcing were simulated by using a two-dimensional model of the gas breakdown development in the space-charge layer contiguous to the nozzle of a cutting arc torch operated with oxygen. The kinetic scheme includes ionization of heavy particles by electron impact, electron attachment, electron detachment, electron–ion recombination, and ion–ion recombination. Complementary measurements during double-arcing phenomena were also conducted. A marked rise of the nozzle voltage was found. The numerical results showed that the dynamics of a cathode spot at the exit of the nozzle inner surface play a key role in the raising of the nozzle voltage, which in turn allows more electrons to return to the wall at the nozzle inlet. The return flow of electrons thus closes the current loop of the double-arcing. The increase in the (floating) nozzle voltage is due to the fact that the increased electron emi...

Effects on light propagating in an electromagnetized vacuum, as predicted by a particular class of scalar–tensor theory of gravitation

The effect of static electromagnetic fields on the propagation of light is analyzed in the context of a particular class of scalar-tensor gravitational theories. It is found that for appropriate field configurations and light polarization, anomalous amplitude variations of the light as it propagates in either a magnetized or electrified vacuum are strong enough to be detectable in relatively simple laboratory experiments. rtheo@dat.demokritos.gr minotti@df.uba.arThe effect of static electromagnetic fields on the propagation of light is analyzed in the context of a particular class of scalar–tensor gravitational theories. It is found that for appropriate field configurations and light polarization, anomalous amplitude variations of the light as it propagates in either a magnetized or an electrified vacuum are strong enough to be detectable in relatively simple laboratory experiments.

Spectroscopic Measurements in a Titanium Vacuum Arc with Different Ambient Gases

gas pressure up to pressure values of the order of 0:2i0:4 mbar, while they decrease for higher pressure values. With Ar gas, a different behavior of the Ti + intensity was found; it presents an increasing general trend. The behavior of the lines was qualitatively analyzed in terms of the most relevant atomic processes that take place in the metallic plasma - gas structure (charge-exchange, electron impact excitation and ionization, etc.). It is found that the behavior of the observed spectral lines can be satisfactorily explained in terms of the relevance of these processes as functions of the neutral gas density and electron temperature.