J. J. Lowke

Predicted electron transport coefficients and operating characteristics of CO2–N2–He laser mixtures

Calculations have been made of transport coefficients of electrons in gas mixtures for ratios CO2:N2:He of 1:1:8, 1:2:3, 1:7:30, and 1:0.25:3. New cross sections for CO2 derived from swarm experiments are used together with previously published cross sections for N2 and He. Curves are presented of the predicted electron drift velocity, transverse and longitudinal diffusion coefficients, and ionization and attachment coefficients for E/N values ranging from 10−18 to 1 × 10−15 V cm2; E is the electric field strength and N the gas number density. Examples are given of derived distribution functions and comparisons are made with a Maxwellian distribution function. The percentage of the input electrical power which excites vibrational processes coupled to the 001 upper laser level of CO2 is given as a function of E/N. The maximum efficiency from these calculations increases for increasing ratios of N2:CO2, because the proportion of energy used to excite the bending and stretching modes of CO2 is then reduced. ...

Characteristics of Radiation‐Dominated Electric Arcs

Temperature profiles of arcs when radiation is the principal energy transfer process have been derived from material functions. Arcs which are optically thin are approximately isothermal or constricted depending on whether eN/σ is an increasing or decreasing function of temperature; eN is the net emission coefficient and σ the electrical conductivity. Arcs which are optically thick have a temperature profile similar to conduction dominated arcs. Theoretically predicted examples of these three types of arcs are a 2000‐A arc at 30 atm in air, a low current arc in the vapor of a rare‐earth metal at 20 Torr, and a 4000‐A arc at 1000 atm in air. For arcs which are nearly isothermal, it is possible at any value of current and radius to make approximate calculations of (1) the temperature of the arc core, (2) the electric field strength, (3) the thickness of the outer arc sheath, and (4) the fraction of the input energy carried to the wall by conduction. Such calculations are made for arcs in air and are compare...

A simple model for high‐current arcs stabilized by forced convection

Using a one‐dimensional analysis investigations are made of properties of arcs stabilized by forced convection in a nozzle. For the current regime where arc diameters are large enough so that conduction and turbulence losses can be neglected at the arc center, yet small enough so that axial pressure and velocity distributions are determined by the nozzle shape, a simple model yields the following analytic properties: (i) Electric field E (z) and temperature distributions T (z) are independent of current, I; (ii) arc area is proportional to arc current and is given approximately by a simple formula; (iii) T (z) is approximately independent of p0, but E (z) ∝p01/2 and arc area ∝1/p01/2, where p0 is the tank pressure; and (iv) the Cassie equation for the time variation of arc conductance is valid. It is assumed that the effective radiation‐emission coefficient U (T) is independent of arc radius. We have made measurments of arc area as a function of current in sulfur hexafluoride. These results, together with...

Properties of electric discharges sustained by a uniform source of ionization

A theoretical examination is made of the class of discharges which are sustained by a uniform source of electron and ion pairs produced between plane‐parallel electrodes. The calculations account for drift, diffusion, and recombination of the charge carriers, ionization of the gas by electrons, and distortion of the applied field due to space charges. It is found that the number and current‐density profiles of electrons and positive ions in the gaseous discharge are of three different types, where (i) the electron density is several orders of magnitude lower than the positive‐ion density at all interelectrode positions, (ii) a plasma region exists, but the electric field in the plasma is negligible compared with that in both electrode sheaths, and (iii) significant ionization occurs in the cathode‐sheath region. Examples of such discharges for which numerical solutions are given are as follows: for type (i), a γ‐ray photoionization chamber with a current density j<1 μA/cm2; for type (ii), a fission fragme...

Calculated properties of vertical arcs stabilized by natural convection

The equations of energy, continuity, and momentum are solved to give axial and radial values of temperature, pressure, and axial and radial velocity for vertical arcs stabilized by natural convection. It is assumed that the electric field is purely axial and magnetic forces are neglected. For a free‐burning arc of 10 A in air, calculated temperature and velocity profiles are in good agreement with published experimental results. For an arc in an enclosed vertical tube of mercury vapor, results of central velocities as a function of axial position for a 2.9‐A arc are in good agreement with the experimental results of Kenty. Comparisons are made of the differing properties predicted for free‐burning arcs of 10 A in air, sulfur hexafluoride, and hydrogen. Removal of the maxima in the thermal conductivity has a negligible effect on the degree of predicted arc constriction in SF6. Convective flow in all calculations is assumed to be laminar. The influence of radial pressure variations is found to be almost neg...

Decay of electrical conductance and temperature of arc plasmas

Calculations assuming thermal equilibrium have been made of properties of cylindrical arc plasmas which decay after the removal of the electric field. Account is taken of energy losses due to thermal conduction and radiation, self‐absorption effects being included where possible in the treatment of radiation. The maximum possible influence of radial convection is calculated by assuming that convective flow maintains a uniform pressure in space and time. Comparisons are made of the calculated decay of electrical conductance with experimental measurements, where available, for A, N2, SF6, H2, and air; curves are given showing the contributions of thermal conduction, convection, and radiation to the decay. The theoretical results underestimate the rapid decays of electrical conductance that have been observed for plasma temperatures of less than 10 000 °K in air, and less than 7000 °K in SF6, but good agreement between theory and experiment is obtained for argon. Information from rate coefficients supports t...

Arc constriction in lamps containing mercury and iodine

Quantitative calculations indicate that the mechanism by which iodine causes constriction of arcs in mercury vapor lamps is the emission of molecular radiation from HgI. Calculated arc temperature profiles from the energy balance equation for various mixtures of mercury and iodine are found to agree with measured profiles when allowance is made for the emission and self‐absorption of radiation from all atomic and molecular species. Approximate emission coefficients for HgI molecular radiation and atomic iodine radiation have been determined as a function of temperature. Molecular radiation emitted from the plasma at temperatures between 2500 and 4500 °K is found to be the major cause of arc constriction for plasma pressures of the order of a few atmospheres.

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.

Theoretical description of ac arcs in mercury and argon

The energy balance and circuit equations, together with Ohm’s law, are solved numerically to obtain temperature profiles, current, and voltage as functions of time for wall−stabilized ac arcs. Experimental measurements of current and voltage oscillograms and also temperature profiles have been made for 1.5− and 7.5−A rms arcs in mercury vapor. Derived net emission coefficients of radiation as a function of temperature for the 1.5− and 7.5−A arcs are consistent with one another; furthermore, the experimental temperature profiles for both arcs are in good agreement with our theoretical predictions. Theoretical current and voltage waveforms and central temperatures for a 20−A rms arc in argon are in good agreement with the experimental results of Detloff and Uhlenbusch. Radial convection has little influence on these results. An approximate criterion for arc extinction or reignition at current zero is given in terms of the steady−state V−I characteristic and the arc time constant. By relating the present ana...

Predicted Arc Properties in Sulfur Hexafluoride

Calculations of spectral absorptivities of sulfur hexafluoride for continuum radiation ranging from the far ir to 300 A have been made as a function of temperature and pressure. These values are used together with theoretical values of thermal and electrical conductivity to derive temperature profiles of arcs in sulfur hexafluoride, taking into account effects due to self‐absorption of radiation, but assuming local thermal equilibrium. The temperature profile for a 100‐A arc is in good agreement with the experimental measurements of Motschmann. The sensitivity of the derived temperature profile to the input material functions is examined. It is predicted that radiation losses at the arc center are dominated by radiation of wavelengths in the region of 1000 A. Using the theoretical material functions, an estimate is made of central arc temperatures and electric field strengths for currents of up to 60 000 A and arc radii of up to 2 cm for gas pressures of 1 and 8 atm.