Helium Arc Plasma Research Articles

Investigation of transient metal vapour transport processes in helium arc welding by imaging spectroscopy

In order to understand the transient transport mechanisms of metal vapour in an arc plasma, imaging spectroscopy was performed for helium arc plasmas burning on pure metals. The measured images were converted into the spectral intensity distributions at the central cross-section of the arc plasma by image processing and Abel inversion. The transient transport process of metal vapours inside the arc plasma are discussed in terms of the diffusion coefficients of metal vapour. The metal vapour generated from the weld pool surface was transported into the arc plasma and to the region near the tungsten electrode within a few seconds. As time passed, the metal vapour spectral intensities gradually increased near the weld pool surface, near the electrode and in the intermediate region. In contrast, the He I spectral intensity was weak and the He I distribution shrunk toward the centre of the arc. This is because the helium arc plasma was cooled by the strong radiation from metal vapour. For an arc burning on pure chromium, chromium ions reached to centre of the arc after about 40 s. In contrast, on pure iron, iron ions did not reach to the centre of the arc. This is because the helium arc plasma on pure chromium was cooled to a temperature at which the electric field and ordinary diffusion coefficients of chromium are largest, whereas the temperature of the plasma on pure iron remained high. Two factors contributed to this; the higher boiling point of iron, which reduces the iron vapour concentration, and the lower radiative emission coefficient of iron.

Highly Ionized Helium Arc Plasma. Determination of the Temperature in Nonequilibrium Conditions and the Effect of Plasma Microfields

Methods are proposed for the spectral measurement of electron temperature Те in a highly ionized nonequilibrium He plasma at atmospheric pressure based on a comparison of the intensities of atomic and ionic lines with a whole complex of experimental data (primarily, ne). The large energy gap between the emitting HeI and HeII excited levels (of more than 50 eV) and allowance for the nonequilibrium manifested in the overpopulation of the atomic levels with respect to the ionic levels make it possible to make reliable and simple Те estimates, especially if the ionization energies of the emitting levels of the atom and the ion are close. For practical use, three pairs of HeI and HeII lines observed in the spectral range of 200–1100 nm are recommended. The radial and temporal (upon the quasistationary pulsed heating of the electric arc plasma) dependences of Те are obtained in the range of axial values of 3.2–4.7 eV.

Specific Features of the Distribution of Atomic-Level Occupancies in Highly Ionized Helium Arc Plasma

The near-threshold spectral region of radiation of helium arc plasma under atmospheric pressure and at an electron temperature of 3 eV was investigated. Under these conditions, plasma is ionizationally nonequilibrial due to the overwhelming predominance of recombination on a wall over the bulk recombination. It is shown that the occupancies of atomic levels n ≥ 4 rapidly decrease; therefore, no spectral lines from levels n ≥ 7 are observed. This fact is related to the destruction of excited atomic states in the quasi-static (ion) plasma microfield (mainly, in the nearest-ion field).

Generation of a Large Diameter He Cascade Arc Plasma for a Plasma Window Application

We fabricated a cascade arc discharge source with a channel diameter of 8 mm for various applications to virtual vacuum interface (plasma window). Stationary helium arc plasma up to a discharge current of 100 A was generated, and the performance as plasma window was investigated. The pressure ratio of high-pressure discharge section to low-pressure vacuum side was above 70 at a 50-A He discharge with a gas flow rate of 0.2 L/min. To characterize the plasma at the anode electrode, emissions of vacuum UV and visible wavelengths were observed. Spectral analysis of He I continuum spectra and H- $\beta $ Stark broadening showed that the plasma temperature was around 0.9 eV and density was $3.4\times 10^{13}$ cm−3 for a 100-A He discharge with a gas flow rate of 0.05 L/min.

Mixing of multiple metal vapours into an arc plasma in gas tungsten arc welding of stainless steel

A computational model of the mixing of multiple metal vapours, formed by vaporization of the surface of an alloy workpiece, into the thermal arc plasma in gas tungsten arc welding (GTAW) is presented. The model incorporates the combined diffusion coefficient method extended to allow treatment of three gases, and is applied to treat the transport of both chromium and iron vapour in the helium arc plasma. In contrast to previous models of GTAW, which predict that metal vapours are swept away to the edge of the arc by the plasma flow, it is found that the metal vapours penetrate strongly into the arc plasma, reaching the cathode region. The predicted results are consistent with published measurements of the intensity of atomic line radiation from the metal vapours. The concentration of chromium vapour is predicted to be higher than that of iron vapour due to its larger vaporization rate. An accumulation of chromium vapour is predicted to occur on the cathode at about 1.5 mm from the cathode tip, in agreement with published measurements. The arc temperature is predicted to be strongly reduced due to the strong radiative emission from the metal vapours. The driving forces causing the diffusion of metal vapours into the helium arc are examined, and it is found that diffusion due to the applied electric field (cataphoresis) is dominant. This is explained in terms of large ionization energies and the small mass of helium compared to those of the metal vapours.

Comparison of temperature and composition measurement by spectroscopic methods for argon–helium arc plasma

Abstract Three different spectroscopic methods were used to calculate the temperature and composition distribution of argon–helium arc plasma—the Fowler–Milne method, the two-line intensity correlation method and the Boltzmann plot method. Experimental errors, including random errors and systematic errors, were analyzed in detail to comparing the accuracy of different methods. Due to the large differences of physical characteristics between argon and helium, there were limited reports on the measurement of temperature and composition distribution in argon–helium arc plasma. To this end, The Fowler–Milne method and the Boltzmann plot method were modified in this paper. Three spectroscopic methods were compared with other׳s simulation result and showed good agreement with each other, except the Boltzmann plot method which had partly distinction. Through comparison and analysis of error bar in those methods, it was found that both the Fowler–Milne method and the two-line intensity correlation method had less error than the Boltzmann plot method, while the Fowler–Milne method, which is irrelevant to atomic transition probabilities and experimental apparatus calibration, had the minimum error.

A modified Fowler–Milne method for monochromatic image analysis in multi-element arc plasma welding

The monochromatic images of 794.8nm Ar I and 487.98nm Ar II spectrums are obtained by high speed camera, and these images are normalized with respect to their relative maximum emission coefficient in a pure argon (single element) arc. The relative maximum emission coefficient of 487.98nm Ar II is calculated based on Olsen-Richter diagram. Unlike other spectroscopic methods, the presented method does not require spectra scan or the maximum emission coefficient of non-argon elements. This technique is applied to the measurement of radial profile of temperature and gas composition of argon–helium arc plasma in gas tungsten arc welding (GTAW), significant demixing occurs in the argon–helium arc plasma. Helium concentration in arc plasma is higher than that in original gas mixture, and it decreases as the point moves away from the axis along radial direction and increases as the point gets closer to the cathode. The calculated arc temperature and gas composition are compared with the data obtained using conventional method where a spectrometer is used for spectral line intensity measurement and two-line relative intensity method for calculation. Very good agreements have been observed. That means, the developed technique measures arc temperature and gas composition as accurate as the conventional spectroscopic methods. With the usage of a high speed camera, the developed technique enables measurement of arc temperature and gas composition distribution in a dynamic arc such as arc ignition, pulsing arc.

Measurements of electron density and Stark width of neutral helium lines in a helium arc plasma

Electron densities in an atmospheric helium arc plasma have been measured with the Stark broadening parameters of helium spectral lines. The spatially distributed radiation intensities are converted to plasma emission coefficients at every wavelength by means of Abel inversion. From the inverted profiles of He I lines of 4713 A, 5016 A, and 6678 A electron density has been calculated, which ranges from 0.5 ×1016 to 4 ×1016 cm-3 for a helium arc with current 200 A. Stark widths of He I lines of 3889 A and 7065 A are determined based on the measurements and compared with existing data.

Adhesion of copper to poly(tetrafluoroethylene) surfaces modified with vacuum UV radiation from helium arc plasma

Poly(tetrafluoroethylene) (PTFE) film surfaces were exposed to vacuum UV (VUV) radiation from He dc arc plasmas that were made to rotate inside a graphite tube by the application of an auxiliary magnetic field. The films were covered with optical filters having different cutoff wavelengths to vary the VUV radiation that modified the fluoropolymer surface. Photo-etching was detected, as well as surface modification that showed the following: (1) water contact angles decreasing with wavelengths of 173 nm or shorter; (2) surface roughening; (3) defluorination of the surface and formation of cross-linking bonds in the top 10 nm of the surface as detected by XPS analysis; and (4) incorporation of oxygen upon exposure to air. An improvement in the adhesion of copper to these modified surfaces was observed.

Diagnostics and modeling of a hollow-cathode arc deposition plasma

Methane or acetylene added to an argon or helium arc plasma is dissociated and/or ionized effectively in order to deposit amorphous carbon (a-C:H) layers. The plasma can be confined or spread out by a homogeneous or cusp magnetic field. The issue of the present study is whether the magnetic confinement has an influence on the type of ions governing the deposition and their energy. To this end, an energy-resolved mass spectrometer (plasma monitor) was used, screened off by a μ-metal cage. It appears that, for strong magnetic fields of up to 10 mT, besides CH3+ and C2H2+ ions (from the methane and acetylene admixture, respectively), also relatively more dissociated species like CHx+ (x=2, 1, 0) and CyHz (y=2, 1, 0; z=1, 0) arise. Especially with the C2H2 admixture, atomic C+ ions appear to become more abundant with increasing magnetic field. The energy of the ions appears to decrease with increasing magnetic field. With Langmuir cylindrical electric-probe measurements it was confirmed that this is related to a decreasing plasma potential. By means of a numerical model, spatially resolved density profiles of the most important species were calculated. Depending on the reactor parameters, relevant ion densities are of the order of 109–1010 cm−3, resulting in deposition rates of up to some μm h−1.

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.

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

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.

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.

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.

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.