Helium Arc Research Articles

Variational and transport properties of high-pressure arc plasmas

One-dimensional steady-state transport equations are used to describe the equilibrium state of multiple species cylindrically symmetric arc plasmas. The effects of axial and azimuthal variations are introduced through the boundary conditions at the center. It is shown that the axisymmetric case of m=0 is actually the case of large m being smoothed out by thermal fluctuations where m is the azimuthal mode number. The conservation of mass density flux gives rise to mutual diffusion and a criterion is given to predict which neutral species should concentrate at the center and vice versa. By writing the energy balance equation in the form of action integral, it is clear that the power dissipation is always a minimum, thus proving the Steenbeck’s minimum principle. A local thermal equilibrium model is used to obtain the electron density. This model is tested against the experimental results of a helium arc with copper electrodes at atmospheric pressure. The steep positive density gradient of the copper is the consequence of pressure balance with the much higher density helium, and the steepness is directly related to the disparity of the two densities.

Experimental investigation of radially resolved plasma parameters in a low-pressure helium arc

The radial variation of plasma parameters in a pulsed low-pressure arc (100-150 Pa) in helium is studied by various methods. For an on-axis electron density of Ne=1.2 and 3.3*1021 m-3 the density profiles are measured by interferometric and spectroscopic methods. The gas temperature is derived from the Doppler broadening of the helium line He I at 388.9 nm. Relative line intensities of He II at 468.6 nm and He I at 471.3 nm yield the electron temperature. Collisional-radiative (CR) models put forward by Mewe and Drawin and Emard take deviations from local thermodynamic equilibrium into account. The necessity of introducing a 'channel model' for this discharge is supported by measurements of the plasma conductivity.

Compound-specific determination of arsenite at sub-nanogram concentrations in freshwater and seawater

Arsenite is quantitatively and selectively extracted into carbon tetrachloride in the presence of arsenate, methylarsonic acid, and dimethylarsinic acid by 0.5% (w/v) aqueous solutions of ammonium sec-butyldithiophosphate. The organic extract is evaporated to dryness and the residue mineralized with conc. nitric acid. The arsenic in this solution (5% oxalic acid) is reduced by sodium borohydride to arsine. The arsine is passed into a DC helium arc and the arsenic emission monitored at 228.8 nm. The detection limit of the extraction/hydride generation system is 0.6 ng As per liter (expressed as the concentration that must be present in a 500 ml sample subject to extraction), approximately 150-times better than hydride generation without extraction. Arsenic compounds other than arsenite do not interfere. The extraction/hydride generation method was successfully used to determine arsenite in seawater and freshwater samples with As(arsenite) contents in the range 3 to 137 ng/l.

Electron temperature determination in LTE and non-LTE plasmas

A method is presented for calculating electron temperatures ( T e ) in dense plasmas, which does not assume equivalence with the excited level distribution temperatures ( T ex). The method involves the upper-level Saha ionization equation at the ionization limit, the limiting weighted population density ( N I / g I ) obtained from measured population densities and the experimentally obtained electron density. Electron temperatures calculated for 0.1-bar hydrogen and 1-atm helium and argon arcs are found to be up to twice as large as excited level distribution temperatures. For subatmospheric argon arcs, the calculated T e are equivalent to the excitation temperature of the middle levels, but are two to three times smaller than the quoted T ex for the highest levels. Reasons are discussed for the apparent invisibility of true electron temperatures and for differences between them and the excitation temperatures.

Investigation of a Magnetically Stabilized Helium Arc

For verifying some theoretical predictions of light scattering from magnetized plasmas, a stable pulsed helium arc discharge in a magnetic field up to 5 teslas has been built. The arc has been investigated at filling pressures of 2 to 5 Torr and plasma currents between 1 and 5 kA. Plasma parameters were measured at a magnetic field of 4 teslas and a pressure of 3 Torr. At these values of the magnetic field and the filling pressure the arc is steady, approximately cylindrically symmetric up to a plasma current of 2.8 kA.Time dependence and radial distribution of the electron density and temperature have been determined from spectroscopic measurements, laser interferometry, and laser scattering. The latter two techniques were used successfully for the first time on a magnetically stabilized arc to measure the electron density and temperature in the hot core of the arc. The axial electron density is 1.2 × 1016cm−3 and is approximately constant over the arc cross section in the high temperature region as predicted by theory. The axial temperature is 145 000 K. Thus the arc design and the plasma properties are suitable for scattering experiments.

Study of the Excitation Mechanism in the Low Current Carbon Arc in Argon and Helium

Arc temperatures have been measured, and relative electron densities have been estimated for dc arcs in Ar, Ar-CO, He, and He-CO. Two temperatures were measured: the excitation temperature, Tcxc, by means of the zinc two-line intensity ratio method, and the rotational temperature, Tr, by the C2band method. Relative electron densities were estimated by comparing Ha line widths in each of the four arc atmospheres. These measurements were performed to gain some understanding of the influence of small amounts of CO on the excitation characteristics of low amperage arcs in argon and helium. The resulting temperatures and electron density estimates indicate that only the c in argon with CO present approaches local thermodynamic equilibrium. Tables of the measured temperatures and plots of the Hα line widths for the estimation of electron densities are given. Implications with regard to the excitation of metal impurities vaporized into these arcs are discussed.

Scattering of laser light from a magnetically stabilized helium arc

The spectrum of laser light scattered from a magnetically stabilized helium arc has been measured. Aside from an enhancement near the plasma frequency the measured spectrum agrees with the predicted profile for a thermal plasma of kT e = 10 eV and n e = 10 16 cm −3.

Decay of a high temperature helium plasma: validity of local thermodynamic equilibrium and estimation of recombination coefficients for He++, He+and He2+

The current through a high temperature, high density helium arc plasma was removed in approximately 1 mu s and the subsequent plasma decay observed. Various spectroscopic methods of measuring electron temperature were investigated. The results were then used to deduce Collisional-Radiative recombination coefficients of He and He+ as functions of electron temperature. Dissociative recombination coefficients for He2+ were also obtained.

Parametric heating of a dense arc plasma with 0.337 mm laser radiation

The threshold intensity of 0.337 mm laser radiation for excitation of the parametric decay instability in a helium arc plasma at 4.0 eV is computed. The plasma density gradient reduces the intensity required from the laser by a factor of 16.

Spectroscopic Mapping of the Nonequilibrium between Electron and Excitation Temperatures in a 1 ATM Helium ARC

The thermostatic states of a 100 amp, 1.016 bar, free-burning helium short arc with a 10 mm electrode gap are mapped from spectroscopic measurements at eight cross sections. The theoretical model used is a multifluids model extended to consider nonequilibrium between electron and excitation temperatures, as well as simple nonequilibrium among excited electronic levels. Seven helium lines are used to determine population densities and upper level excitation temperatures. The electron density is calculated from continuum intensity measurements at C4690. Electron temperatures are found from an astrophysical method suggested by Athay and Menzel. The effective total excitation temperature is obtained by iteration using the multifluids model. The results indicate total excitation temperature values close to the usually calculated LTE temperatures, but electron temperatures up to three times larger than the total excitation temperature on the arc centerline near the electrodes. The ratio is approximately 1.5 in the middle of the arc. The heavy particle kinetic temperatures appear to follow the electron temperature, except near the anode, where they drop to values smaller than the total excitation temperatures.

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.

Spectroscopic intensity measurement of atmospheric-pressure helium arcs

Intensity distribution of six HeI spectral lines have been measured across wall-confined helium arcs at atmospheric-pressure and arc currents of 100, 160 and 210 A. Examination of Abel-inverted absolute intensities of these lines shows that the populations of the upper excited states of the neutral atoms are in local thermodynamic equilibrium (LTE) with the free electrons at a common electron temperature, but are not in equilibrium with the ground-state population. The temperatures calculated from the absolute intensities are different for different lines and different from the temperature derived from the relative intensity method.

Electric field intensity and wall heat transfer measurements for the heating region of an atmospheric cascade arc

Electric field intensity and wall heat transfer measurements have been made for the inlet and asymptotic regions of Ar, He and N 2 laminar, atmospheric arcs. Comparisons between the data and available equilibrium and nonequilibrium theories have been made in an attempt to determine the relative accuracy of single and multifluid arc models. Equilibrium predictions for the asymptotic arc region in argon agree with the experimental results to within the combined experimental and theoretical uncertainties. There exist, however, small but discernable departures at both the lower and higher arc currents in argon. In contrast, the nonequilibrium theory predictions are in excellent agreement with the data over the entire range of currents. Additional comparisons suggest that atmospheric helium arcs are characterized by radical departures from local thermodynamic equilibrium.

Microwave Radiation from a Steady Helium Arc Plasma Upon Superposition of High Voltage Pulse

Microwave Radiation from a Steady Helium Arc Plasma Upon Superposition of High Voltage Pulse

Experimental study of the plasma parameters and the populations of excited atomic states in a low current helium arc

Experimental study of the plasma parameters and the populations of excited atomic states in a low current helium arc

Parameters Involved in the Spectroscopic Monitoring of Atmospheric Contaminants in the Tungsten—Inert Gas Arc Process

The parameters involved in the spectral monitoring of atmospheric contaminants in the tungsten–helium arc process were studied under welding conditions. An internal standard was necessary because small variations in arc length, current, condition of metal surface, alloy, the portion of the arc in the aperture, etc. resulted in relatively large variations in spectral response, proportionally much greater than for those same parameters in conventional low current–graphite electrode spectroscopy. Only the argon line internal standard was satisfactory. Warping of the work metal caused the arc to change in space, so a side arm mount for the torch was designed to carry the entrance optics. The high and varying background that resulted from electrode reflections, glowing oxide particulate, focus on hot metal, and possible radiation from the electrode tip required the use of a rotating circular wedge to oscillate the emission lines across the exit slits. Alternating current electronics amplified the difference in signal levels.

Investigation of Helium Arcs at 10 atm Pressure

The measured “EI-ER” characteristics of wall-confined helium arcs at 10 atm pressure, when interpreted with the Elenbaas-Heller equation, show that the arcs are close to local thermodynamic equilibrium. A spectroscopic measurement is also made to investigate possible deviations from local thermodynamic equilibrium. The results show that, despite over-all indications of a closer approach to local thermodynamic equilibrium at 10 atm than at 1 atm, the 10 atm arcs are still not in local thermodynamic equilibrium in the region near the walls. Theoretical criteria of local thermodynamic equilibrium tend to support the above results that local thermodynamic equilibrium is approached when the arc pressure is raised, but the complete local thermodynamic equilibrium is not attained at 10 atm. It is believed that the deduced transport properties from the arc measurement with the aid of the Elenbaas-Heller equation are the first approximations. The deduced electrical conductivity and thermal conductivity agree with the theoretical prediction of Lick and Emmons within a factor of 2.

Theoretical model for a nonequilibrium helium arc plasma

A semiempirical model for a nonequilibrium helium arc plasma is used to interpret systematic deviations in existing spectroscopic data and to aid in computing nonequilibrium plasma transport properties. Two groups of bound electronic states are assumed to exist which follow Boltzmann distributions at T e (upper states) and T R ≠ T e (lower states). Under certain conditions, T R can be interpreted as a radiation temperature characterizing the optically thick part of the spectrum and can be calculated using a restricted Rosseland approximation. The nonequilibrium plasma is characterized by electron densities as much as several orders of magnitude greater than the local thermodynamic equilibrium value. The model reproduces the experimental data in detail once the electron temperature profile is given.

A radiation standard for the vacuum ultraviolet

A constricted dc argon arc to which nitrogen or oxygen has been added has been calibrated as a radiation standard for the vacuum ultraviolet. The calibration data are given for the strongest atomic lines in the wavelength range of 1800-1000 Å. Operation for calibrated output of continuum or blackbody radiation down to 600 Å is given for a helium or neon arc using hydrogen or argon as the radiating gas. The source has been found to be stable and reproducible, thereby allowing use of the comparison method to measure the number of photons incident on the entrance slit of a spectrometer.

Bestimmung der elektrischen Leitfähigkeit von Helium aus integralen Lichtbogenmessungen

Abstract In Ohmic heated plasmas the temperature of the electrons is always higher than the gas temperature. In this case the electrical conductivity can be assumed as a function of the gas temperature and the electric field strength. With this assumption comparing the characteristics of two arcs with different radii the electrical conductivity of an helium arc plasma (1 atm) is determined and reduced to the value for a field free plasma.