Nitrogen Arc Research Articles

Solutions of plasma arc equations by the orthogonal collocation method

The orthogonal collocation method is applied to the study of wall-stabilised plasma arcs. The numerical implementation of this method is simpler and faster than that of finite difference methods of comparable accuracy. The solutions obtained agree with experiments and with previous finite difference calculations. For the steady-state DC case, radial temperature results are obtained for arcs in nitrogen and sulphur hexafluoride at atmospheric pressure. It is shown that the inclusion of radiative self-absorption in sulphur hexafluoride arcs leads to a reduction in the centre temperature, a broadening of the temperature distributions and a small decrease in the axial electric field. Solutions are obtained also for the time-dependent behaviour of a nitrogen arc following an abrupt current interruption. In this transient case, self-induced radial flow is considered, leading to the additional process of convective loss.

Departures from CLTE composition in a nitrogen arc at atmospheric pressure. I. Experimental results

The positive column of a wall stabilised arc burning in pure nitrogen at atmospheric pressure is spectroscopically studied. The behavior of local population number densities of atomic and molecular excited levels shows deviations from CLTE composition. These departures are mainly related to diffusion phenomena and radiation escape.

Departures from CLTE composition in a nitrogen arc at atmospheric pressure. II. Interpretation of departures from CLTE

For pt.I see ibid., vol.16, 829-40 (1983). Departures from chemical equilibrium and from Boltzmann's distribution for excited level populations expected with the assumption of CLTE in a nitrogen plasma at atmospheric pressure are explained by means of a collisional-radiative model taking into account both monatomic and diatomic species, in which transport processes are treated by a parametric approach.

Experimental observations of cathode-spot surface phenomena in the transition from a vacuum metal-vapor arc to a nitrogen arc

This paper provides experimental data on the crater dimensions associated with low-current (50 A) arcs burning for periods of 100 ms on copper cathodes. The cathodes were arced in metal-vapor and nitrogen ambients, and the resulting cathode surface contamination was measured for each arcing condition. As the ambient pressure increased from 10−6 to 100 Torr, the crater diameters in the heavily arced areas decreased from 48–58 to 7–17 μm, the current carried per cathode spot decreased, and the surface contamination increased. We consider that the smaller crater dimensions are associated with the reduction in the maximum current per spot and that the maximum cathode-spot current is influenced by the thin layer of surface contamination. We compare the trend of pressure-related crater-size reduction with previous observations of crater sizes as a function of current for vacuum arcs. We conclude that an individual cathode spot operating at a given current level in a gaseous ambient has many of the same basic characteristics as an individual cathode spot operating at the same current level in vacuum. An unexpected feature though, confirmed by our measurements, is that the erosion rate at higher pressures can be significantly lower than the minimum vacuum erosion rate associated with the characteristic 8– 10% ion current. Redeposition may be a plausible explanation.

The influence of copper vapour on the transport coefficients in a nitrogen arc plasma

Using the method ofChapman and Enskog, the transport coefficients of a copper-nitrogen mixture at atmospheric pressure were calculated. With the most appropriate methods the authors determined the cross-sections of collisions between copper and the othe r particles. The results obtained show that at low temperatures (5000K<T<7000K) the electrical conductivity is clearly increased by the presence of copper. At higher temperatures (T approximately=15000K) the electrical conductivity of the mixture can be reduced by the presence of doubly ionised atoms of copper. The influence of the copper is also felt by the reactional thermal conductivity but has little influence on the translational thermal conductivity.

Modeling of the Anode Contraction Region of High Intensity Arcs

A self-consistent model for the anode contraction region of a high intensity dc arc is based on a wall-stabilized axisymmetric arc operated at atmospheric pressure with a plane cooled nonablating anode perpendicular to the arc axis. Arc constriction in front of the anode gives rise to an entrainment of cold gas from the vicinity of the anode leading to a more or less pronounced anode jet. The conservation equations for the anode region represent a set of highly nonlinear integro-differential equations which describe the temperature and the flow field in the arc. Numerical solutions of these equations are obtained by using an iterative finite-difference method. Results for a nitrogen arc at 250 A indicate that heat transfer close to the anode is dominated by the electron enthalpy transport. The cold gas approaches the arc fringes with velocities in the order of 1 m/s, and reaches velocities of up to 250 m/s in the hot core of the arc, indicating the existence of an anode jet which has been confirmed by experimental investigations.

Radiation transport in wall-stabilised nitrogen arcs

Radiation transport within wall-stabilised nitrogen arcs has been investigated for pressures of 1 and 10 atm. The computations allow for continuum and N I and N II line transport, and some molecular band absorption. The spectral data used gives reasonable agreement with experiment for the radiative balance on the arc axis at 1 atm. The calculated radiative arc loss accounts for a significant fraction of the electric power input, and it is found that losses at wavelengths less than 1000 AA are most important over a wide range of conditions. The dominant contribution to the loss is made by line radiation transport. Experimental temperature profiles were used for the 1 atm calculations, and for 10 atm the profiles were predicted using an approximate value for the local radiative balance determined from isothermal-arc calculations.

Spectroscopic measurements in nitrogen arcs with respect to local thermodynamic equilibrium

A wall-stabilized nitrogen arc at atmospheric pressure was studied spectroscopically in the current range from 20 to 70 A with the aim of detecting departures from LTE. Measurements of the relative intensity of several N(I) lines showed that the populations of the upper excited states are in equilibrium for currents greater than approx. 25 A. Comparisons of the excitation temperatures obtained from relative intensities with the electron temperature and with the temperature curves calculated for different departures from LTE show that deviations from equilibrium are small at the arc center for electron densities above approx. 4 × 10 16 cm -3. Deviations between the radial profiles of the electron and excitation temperatures were observed for currents between 35 and 55 A.

Transitions between thermionic and vapour arc modes

A theoretical comparison between the thermionic and vapour arcs is presented which illustrates the essential similarity of the two arcs modes. Using the carbon arc in nitrogen as an example, the arc spot parameters are derived for both arc modes and are in good agreement with experiment. An extension of these models provides a quantitative explanation for the observed transitions from one arc mode to the other. The transition from the thermionic arc to the vapour arc is initiated by excessive excited particle loss from the arc spot, whereas the reverse transition to the thermionic arc is caused by nitrogen gas quenching of excited carbon atoms. A value for the cross section of this quenching process is derived.

Prediction of properties of arcs stabilized by forced convection

A one-dimensional arc model has been used to calculate the central temperature, arc radius, and electric field as a function of axial position for steady-state arcs in forced convection in nozzles. Calculations indicate that the axial distributions of pressure, plasma velocity, and gas velocity are sensitive to the degree of gas heating by the arc of the surrounding gas, significant differences being obtained between assumptions of the gas being isothermal or expanding adiabatically. Despite the fact that turbulence effects are omitted in the model, good agreement is obtained with experimental results of arc temperature, radius, and electric field for a 2000-A arc in nitrogen. Calculations as a function of current indicate regimes of low, intermediate, and high current where arc voltage respectively decreases, is constant, and increases with increase of current. In the latter regime, the increase in voltage is caused by the arc ’’clogging’’ or restricting the gas flow in the nozzle.

Temperature Distribution in a dc Free Burning Arc in Nitrogen with and without Addition of Li2CO3

The investigation of the lithium effect on the temperature distribution of the arc is presented. Measurements were carried out by the Schlieren interference method to about 4000°K. The arc free burning in nitrogen was observed and the results of temperature measurements, carried out with and without lithium addition, were presented. Lithium was added to the arc by anode evaporation. Temperature measurements in outer areas were connected to previously presented spectroscopical measurements in the central part of the arc. The lithium addition affects the increase of the electroconductive central part of the arc.

A spectroscopic study of equilibrium in nitrogen arcs

Nitrogen arc measurements of the intensity of the 4915 Å-4935 Å NI doublet and of the 3995 Å NII line show that local thermodynamic equilibrium cannot be assumed in nitrogen arcs at electron densities below 1×10 17 cm -3. Below this point, the results suggest that gas and electron temperatures differ significantly and that ground states are overpopulated with respect to upper electronically excited states.

Measurement of thermal conductivity of argon-nitrogen gas mixtures at high temperatures†

The temperature profiles in wall-stabilized atmospheric pressure arcs in argon, nitrogen and argon-nitrogen mixtures have been obtained at currents of 26 A and 30 A using spectroscopic techniques. The temperature profiles were measured using both the absolute intensity of spectral lines and the absolute intensity of the continuum radiation. Good agreement between the two methods is found in argon but there is a considerable discrepancy in nitrogen. This is attributed to the existence of a N- ion. The temperature profiles and previously measured potential gradients are then used in conjunction with calculated values of the electrical conductivity to obtain the thermal conductivity of several argon-nitrogen mixtures. The results obtained are in good agreement with previously published data for the pure gases and show the gradual formation of an arc core and the increase in axis temperature as the gas mixture is varied from pure argon to pure nitrogen. Another result of these investigations is to show the im...

Spot formation at the anode of high intensity arcs

A simple analytical model is proposed which describes the region between a plane, cooled nonablating anode and an arc column normal to the anode The model takes an initial arc contraction into account due to the low temperature of the anode (“thermal pinch”). The resulting magnetic pinch effect leads to an entrainment of cold gas which determines via an energy balance the final shape of the arc in the anode region. The conservation equations are solved numerically for the anode region with a relaxation method, and results are presented based on atmospheric arcs in nitrogen. Finally, limitations of this model and necessary refinements are briefly discussed; possible extensions of the arc parameter range for future work are proposed.

Potential gradient measurements in argon-nitrogen mixtures using a wall-stabilized arc†

Abstract The potential gradient in wall-stabilized atmospheric pressure arcs in argon, nitrogen and argon-nitrogen mixtures has been measured over the current range 5 to 40 A. The potential gradient was measured by four methods which were found to agree to within 3%. The values obtained were found to be a function of the state of the surface of the discs forming the arc channel and independent of the gas flow rate in the range l·-9·0 1/min. The results demonstrate that the effect of a small trace of nitrogen in argon is much greater than that of a small trace of argon in nitrogen. For example, at 26 A 5% nitrogen in argon increases the potential gradient by ∼30%, while 5% argon in nitrogen reduces it by ∼ 3% from the pure argon and pure nitrogen values, respectively. A simple two zone arc model is used to explain the results. This model indicates that the arc in argon entirely fills the channel for currents greater than 5 A. However, when nitrogen is added a central core is formed whose diameter decreases as the nitrogen concentration is increased, the current being kept constant.

The Effective Periphery of a High Pressure Arc

A brief account is given of ideas which are aimed at providing a common reference point for theoretical treatments of arc problems through the equations of continuum magne-to-fluid-dynamics. By unifying theoretical solutions in the manner described, their usefulness in analyzing the results of experimental investigations may be greatly enhanced. The ideas center on the concept of an isothermal arc periphery, the temperature of which is regarded as an artificial gas property to be derived from the measured voltage gradient-current characteristic of a wall-stabilized arc of low power gradient. Evidence in support of this concept is given for various types of arc in nitrogen at one atmosphere pressure. Some of the implications of the use of an effective periphery of a high pressure arc are examined, and a suggestion is made for the complete set of boundary conditions to be applied at the periphery.

Measurements of the current-zero behaviour of constant-pressure axial-flow electric arcs in nitrogen

It is shown that the equations for the arc in constant-pressure axial flow can be described in terms of nondimensional variables that account for the effects of changes in the gas-flow velocity, pressure, arc length and circuit-element values. Experiments in nitrogen confirm the predicted scaling laws for arcs with an impressed linear current wave, as well as arcs for which an added parallel capacitance causes a strong arccircuit interaction. Sinusoidal current waves up to 150 A peak were used at flow velocities up to 300 m/s and pressures up to 2.5 atm.

The power balance in electrode-dominated arcs with a tungsten anode and a cadmium or zinc cathode in nitrogen

The power transfer to the tungsten anode and the cadmium or zinc cathode of an arc in nitrogen is measured, for currents of 2, 3 and 4 A and voltages of 9 to 20 V. The power balance equations are solved to calculate the positive-ion current to the cathode, the electric field and the electron energy in the plasma column, and the anode fall voltage.

Spectroscopic measurements for atmospheric nitrogen and helium arcs

Thermal conditions in the asymptotic region of atmospheric nitrogen and helium arcs are determined spectroscopically and calculated using an equilibrium flow model. The equilibrium model is shown to be ill-suited for both gases. The helium arc is characterized by a high degree of thermochemical nonequilibrium, and significant thermochemical and excitation nonequilibrium effects characterize the nitrogen arc.

Radiative energy balance in cylindrical nitrogen arcs

A systematic investigation of the radiative energy balance in cylindrical arcs is described. It is shown that consideration of the overlapping of lines within multiplets themselves, or even of different multiplets at higher pressures, is of particular importance. Methods of simplifying the frequency and spatial integration of the equation of radiative transfer are developed. These are tested by comparison with exact calculations. Considering the contributions of the continuum radiation, as well as of all important N I-, N II- and N III-lines, the radiative energy balance in nitrogen is computed for a wall-stabilized arc (1 atm, 5 mm dia., axial temperatures up to 26,000°K) and for a convection-stabilized arc (pressure between 12 and 24 atm, assumption of rectangular temperature profiles). Comparison with experimental results gives good agreement.