Helium Arc Research Articles

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.

Die Beeinflussung des Ionisationsgrades durch Diffusions­ und Massenströme im Plasma

Abstract A stationary, magnetically stabilized helium arc plasma is taken as an example to show that ionization degrees in such plasmas with large electron temperature and electron density gradients may not be calculated from static ionization relations valid for homogeneous plasmas (e. g. the Saha equation). Instead, ionization degrees in inhomogeneous plasmas must be determined from continuity equations of the form div (nΖυΖ)nΖ = JΖ-1 nΖ-1 -RΖnΖ nz where JΖ-1 and nΖ-1 are the ionization rate and the density of the Z -1 times charged ions, and RΖ , nΖ and υΖ are the recombination rate, the density and the centre-of-mass velocity of the Z times charged ions. Static ionization formulae may not be applied to inhomogeneous plasmas because the finite relaxation times for the attainment of static ionization equilibria result in these ionization equilibria being displaced by the motion of the ions parallel to the direction of electron temperature and density gradients. This is proved by means of two independent operations: firstly, the velocities of the helium ions, these being governed primarily by ambipolar diffusion, are calculated with the spectroscopically measured state variables from the momentum equations of the plasma; secondly, the velocities of the doubly charged ions are determined by integrating the continuity equation for these ions, use again being made of the measured state variables and of the ionization and recombination rates J1 and R2 of the helium ions that were calculated for this special plasma. The agreement between the independently obtained velocity values proves that the degree of ionization of the helium ions in this inhomogeneous plasma is described not by a static ionization formula, but by the continuity equations for these ions. Furthermore, it is shown that the ionization "equilibria" between singly charged and neutral helium particles and between three times (CIV) and four times (CY) charged carbon particles are not determined by means of static ionization formulae either. The influence of this hitherto disregarded effect on the spectroscopic determination of the electron temperature in the plasma discussed is illustrated in a diagram.

Electrical conductance decay of arc columns in some common gases

Experimental measurements are reported of the conductance decay following current interruption of wall stabilised arcs in air, nitrogen, argon, oxygen and helium within the current range 6–42 A. The results are compared with the predictions of several analytical models so that the influence of various gas properties and processes may be estimated. Measurements in the molecular gases show clearly how the arc structure affects the recovery. The results in the inert gases differ to some extent from the predictions of the simple models.

Calculation of the Raman shift in vacuum

Abstract A correction table, similar to that of Opler [1], is given to simplify conversion of the wavelengths of Raman lines observed as angstroms (in air) to wave numbers (in vacuum). A more elaborate table for direct conversion is described for some exciting lines of helium arc, helium-neon, argon+, and ruby lasers in addition to the common mercury lines. The computational program was based on the dispersion formula of Edlen [2].

Low-Frequency Instabilities of a Highly Ionized Magneto-Plasma

Measurements are made of the low-frequency oscillations occurring in a low-pressure helium arc plasma immersed in a magnetic field. A spectrum analyzer is employed in the frequency range below 200 kHz to study frequency spectra of signals from magnetic probes, a Rogowskii coil for the discharge current, and a monochromator selecting spectral lines of helium ions in the spatially definite regions of the plasma column. A drift mode instability arises from density inhomogeneity of the plasma in the boundary region and an ion acoustic mode instability occurs in the central region. The two modes are coupled, presenting the spectra similar to that of frequency modulation. The relevant collision effects are negligible and the pertinent instabilities develop due to an axial electron stream.

Doppler Broadening of Helium Ion Lines in a Low Pressure Helium Arc

Profiles of Spectral lines emitted from a plasma in a low pressure helium arc which has an electron temperature of about 105°K and an electron density of about 1014 cm-3 have been studied. The results show that the measured profile of HeII 4686 Å can be well explained by Doppler broadening if the fine structure is taken into account. The apparent half-widths of HeII 4686 Å and HeII 3203 Å are shown in a figure as a function of the Doppler half-width of a fine structure component. The profile of HeII 3203 Å is mainly formed by Doppler broadening but Stark effect contributes to it by a few percents.

Spectroscopic Investigations of the Helium Short Arc

A short (5 mm) helium arc between a ⅜-in.-diam thoriated tungsten cathode and a 1-in.-diam copper anode was examined spectroscopically. Electron densities were determined from the Inglis-Teller relation for a range of gas pressures and arc currents from 500 Torr, 50 A to 5000 Torr, 250 A. At 850 Torr, 100 A, the electron density was 1.2(+2.1/−0.2)×1016 per cm3. At 850 Torr, 100 A (2.4-kW power level) the atomic excitation temperature was measured using a line intensity ratio technique. Self-absorption corrections were determined experimentally using a tandem arc arrangement and were found to be significant. A numerical computational technique was solved for the spatial dependence of emission and absorption of the HeI 23S-N3P series. The state populations followed a Boltzmann distribution with an excitation temperature of 4800°K. A best straight line fit to the data was obtained with spontaneous decay coefficients computed by Bates and Damgaard and by Schiff and Perkeris. This excitation temperature is too low to account for the observed electron density assuming equilibrium so that local thermodynamic equilibrium (LTE) does not exist in the arc at this power level.

Spectroscopic measurement of electron temperature in helium arc plasma

The intensities of the neutral helium spectral, lines emitted from helium plasma produced by the energetic arc have been measured, and the population densities of the energy levels of neutral heliumhave been calculated from them. Electron temperature could be deduced from them, since the distribution of the measured population densities above the. fifth principal quantum number was Maxwellian. Discharge conditions of helium arc were as folows; helium pressure at 10-2 torr, currents of 150, and 240 amp., and magnetic field. strength of 3540 gauss. In these cases the electron temperaturess of 0.19, 0.34, and 0, 34 ev respectively were obtained. It was considered from the radial intensity distribution of neutral helium lines that the measured electron temperatures were the values around the arc column.

Microwave Reflection from Small-Diameter Plasma Columns

The scattering cross section of electromagnetic waves incident normally on a small-diameter plasma column exhibits resonances as a function of plasma density when the waves are polarized at right angles to the axis of the column. The electron density at resonance has been measured, using a microwave cavity technique, in pulsed helium arc discharges, as the frequency of the incident microwaves was varied from 2 to 4 kMc/sec. At resonance, the squared ratio of plasma frequency to incident frequency is found to be independent of frequency and neutral gas pressure, but somewhat dependent on pulse duration. Results are in qualitative agreement with recent analyses of the interaction between electromagnetic fields and thermal plasmas with finite dielectric boundaries.

Magnetic Annular Arc

The performance characteristics of an annular-arc configuration under the influence of a magnetic field have been experimentally analyzed, particularly with respect to the effects of tensor conductivity and j × B body forces. In this device the plasma conditions are such that the electrical conductivity is insufficient to prevent the diffusion of the plasma through the magnetic field, i.e., the magnetic Reynolds number was small. Experiments were conducted with this device using helium and argon at power levels up to 50 kW, magnetic field strengths to 10 000 G, and particle densities of the order of 1016/cm3 to 1017/cm3. The electron gyroradius was smaller than the electron mean free path, i.e., ωeτe > 1, where ωe is the gyrofrequency, and τe is the time between collisions for electrons. The impedance of the arc in helium was found to vary linearly with ωeτe. The existence of a circulating azimuthal Hall current was also demonstrated. A yaw probe was employed to measure the angle of the flow due to the j × B Lorentz forces. A good agreement was found between the measured rotation in argon and the predicted value obtained from momentum considerations.

338. Concentration of charged particles in the plasma of an arc burning in an atmosphere of argon and helium: V. F. Kitaeva, V. V. Obukhov-Denisov and N. N. Sobolev, Optics & Spectrosc., 12 (2), Feb. 1962, 94–99

The contours of Balmer lines H/sub alpha /, H/sub beta /, H/sub gamma /, and H/sub s/- are measured in 1- to 200-amp area in argon and helium. The experimental contours are compared with theoretical contours. This comparison permits determination of the concentration of charged particles in arcs in argon (2 x 10/sup 14/ to 3 x 10: cm/sup -3/) and helium (4x 10/sup 13/ to 2 x 10/sup 16/ c m/sup -3/), at currents of 1 and 6 to 200 amp respectively. The low concentrations of charged particles existing in these arcs at low currents appear to be one of the basic causes of the nonequilibrium population of the levels. (auth)

Determination of Streaming Velocity and the Flow of Heat and Mass in High-Current Arcs

Arc plasma streaming velocities were calculated from previously unpublished experimental data obtained by H. N. Olsen on total pressure distributions at the anodes in 100- to 300-amp argon and helium arcs. The dynamic streaming component of pressure was calculated by subtracting the magnetic component (due to the self-magnetic field of the arc) from this total pressure. Arc temperatures also measured by Olsen were then used to calculate densities and heat contents of the plasma, and these were used with the streaming pressure to calculate velocities and mass and heat flow distributions. These in turn were integrated to give total mass and heat flows. The results compare reasonably with other measurements and estimates in the literature. Peak velocities range from 16 000 to 60 000 cm/sec, mass flows from 0.04 to 0.23 g/sec. The plasma heat flow distribution shows a close similarity in shape to values of heat transfer intensity at the anode measured by O. H. Nestor. A comparison of the total heat flow in the plasma to the total heat transfer due to streaming at a series of currents shows heat transfer proportional to (heat flow)0.62 which agrees with correlations used for heat transfer from gases and flames. A simplified model of arc pumping is presented which suggests that heat and mass flow should increase as (current)32. A simple analog experiment in mercury is described which illustrates graphically the streaming caused by a divergent current path in a conductor.

Some investigations on arc discharges in helium and hydrogen at high pressures

Synopsis In a high pressure discharge cell direct current arcs have been investigated in hydrogen and helium compressed up to 1000 atmospheres. For helium, the current, the voltage across the arc and its diameter have been measured as a function of pressure. From these data the temperature in the positive column was calculated with the assumption that the Saha equation is valid under the experimental conditions. For hydrogen, preliminary experiments also yielded an estimate of the temperature. Photographic analyses of the spectra of H2 and He have been made. As some H2 was present in the He used, it was also possible to observe the effect of high density He on the H2 spectrum. Finally, the total intensity of the helium arc in the range 4000–6500 was determined photoelectrically as a function of pressure.

Inert gas-shielded welding arc behavior and metal transfer characteristics

An investigation of inert gas-shielded consumable electrode arc behavior and metal transfer characteristics was undertaken on aluminum and steel in flat, vertical, and overhead welding positions with the use of argon, argon + 5-per-cent(%) oxygen, helium and helium + 5% oxygen shielding gases. Arc and metal transfer behavior were observed with high-speed motion pictures with the use of intense supplementary illumination to facilitate photography. Electrical behavior of the arc was examined during production of a considerable number of weld beads by recording oscilloscope traces of arc current or voltage on the high-speed pictures of the arc. Thus, relationships were observed between current transients and arc physical behavior. Arcs shielded with argon or a mixture of argon + 5% oxygen, known commercially as sigma grade argon, were more stable and operated with less spatter than similar arcs in helium or helium + 5% oxygen. Although addition of oxygen to argon was desirable, addition of oxygen to helium shielding gas did not improve arc behavior or metal transfer. Numerous large transients occurred in current and voltage of reverse polarity (RP) arcs in helium and helium + oxygen shielding gas. The transients or disturbances were frequently associated with drop severance from the filler wire and with formation of undesirable spatter. Electrical disturbances were not present in similar arcs in argon or argon + oxygen shielding gases.

Effect of Alkali and Alkaline Earth Chlorides on the Anode Temperature of the dc Arc*

Temperatures of graphite anodes have been measured for the dc arc in helium and argon by means of a recording thermistor pyrometer when pure chlorides of the alkali and alkaline earth metals are present as samples. Anode temperatures and gap voltages were depressed by all of the samples. The data for helium indicate a correlation of anode temperature to the boiling point of the sample salt and the lowest excitation potential of the cation. Correlation was also observed between the boiling point of the salt and its rate of volatilization. Temperatures in argon are lower and correlation between anode temperatures and either boiling points or excitation potentials was not observed. However, rates of volatilization were related to the boiling points and excitation potentials of the samples. These effects are large enough to account for the poor precision of the dc arc as a spectrochemical source and show the physical-chemical basis for the use of “spectroscopic buffers.”

Effects of Inert Atmospheres on Emission Spectra in the Interrupted Arc

A comparison of the spectra of several elements produced in the interrupted arc in helium, helium-oxygen mixtures, argon, and air has been made. It has been found that the differences observed in the spectra are not uniform, but vary according to the ionization states of the spectral lines; those lines requiring large energies for excitation being more intense in a rare gas atmosphere than in air, and those lines requiring small energies for excitation being more intense in air than in a rare gas. It is shown that with the interrupted arc the use of rare gas atmospheres cannot lead, except in isolated cases, to an improvement in the sensitivity of emission spectrographic analysis.

The electric arc in argon and helium

THE EXACT NATURE OF THE MECHANISM of the high-current electric arc is not yet understood clearly even though the arc has been used and studied for the past century and a half. Much of the difficulty in the interpretation of arc data has been associated with the complexity of the test conditions. An example of this is found in much of the data reported on welding arcs∗∗∗ in air. Such arcs are drawn in an atmosphere composed of a mixture of several gases (some of which are very active chemically) with electrodes which are usually of complex metallic and chemical structure.

The electric arc in argon and helium

Measurements of the electrical characteristics of arcs, such as the variation of voltage with current and with arc length, have been carried out by Ayrton,1 Nottingham,2 and Suits,3 and others for currents below 20 amperes and by Finkelnburg4 for carbon arcs for currents above 20 amperes. Most of the work has been done at atmospheric pressure. The great increase in the use of inert gases for arc welding has resulted in the publication of much valuable data on the behavior of arcs in these atmospheres under typical welding conditions. It appears, however, that there is a need for more quantitative information on the characteristics of the inert gas arc under controlled laboratory test conditions. In particular, this information is needed for arc currents in excess of 20 amperes and for gas pressures in the neighborhood of atmospheric such as pertain when the inert gas arc is used for welding.

Stability of Short Arcs in Oxygen, Hydrogen, and Helium

An arc drawn between separating contacts is known to be very unstable, in the absence of oxygen, in the usual power circuit. To study this instability experiments have been made in which a four-ampere arc in a 125-volt dc resistive circuit was repeatedly restruck between separating contacts. Various metals were used as cathodes, with spectroscopically pure carbons as anodes, in various gases.Data have been obtained showing the dependence of average arc duration after ignition on the cathode material and on the pressure and kind of surrounding atmosphere for a number of metals in oxygen, hydrogen, and helium. Oxygen exhibits an extraordinary stabilizing influence for most metals, whereas in hydrogen the mean life of the arc is generally less than ten milliseconds even at atmospheric pressure.

Radiative Processes in Thermionically Controlled Discharges in Helium

Recent measurements have been made of the intensity of radiation from the low voltage arc in helium as a function of gas density, tube current and tube potential. The experimental results indicate that the radiation is the result of a primary electron process. This process has been generally assumed to be direct excitation. Such an explanation is not fully in accord with the phenomena observed. The possibility of an unrecognized process is suggested.