Spectral measurements of inductively coupled and helicon discharge modes of a laboratory argon plasma source

An experimental study was conducted to investigate the effects of several operational parameters in the emission spectra, in the 400-850 nm wavelength region, of a laboratory Argon plasma source. In particular, the emission spectra of the inductively coupled plasma and the Helicon plasma modes of operation were compared. Comparisons of spectra point to a significant increase in the ionization fraction of the plasma for the Helicon mode of operation. The spectral measurements allow one to determine the major trends in the plasma electron density for various parameters such as power delivered to the helical antenna, propellant mass flow rate, and applied external magnetic field intensity.Analysis of a prominent Argon single ion line, at 434.8 nm, points out that the plasma electron density increases linearly with the power delivered to the helical antenna, and that there is an optimum propellant mass flow rate for maximum ionization fraction. Additional analysis of the same line shows that above a minimum applied axial magnetic field intensity, the variation in the magnetic field strength has little effect on the plasma electron density.     

Some considerations on the excitation mechanism in the inductively coupled argon plasma

The excitation conditions in the analytical observation zone of the inductively coupled argon plasma cannot be described by the model of partial local thermal equilibrium. After some general remarks on excitation models the paper analyses in tutorial fashion four alternative models proposed in the literature, featuring metastable argon atoms, radiation trapping, reaction rates and ambipolar diffusion.It is concluded that none is completely satisfactory, but that they are complementary rather than contradictory. Finally, some elements of the four models are integrated into a new model that considers the ICAP as a plasma decaying from conditions of heterogeneous disequilibrium to a situation of homogeneous thermal equilibrium.     

Some considerations regarding temperature,electron density,and ionization in the argon inductively coupled plasma

The measured density of electrons in the ICP cannot be explained on the basis of a pure LTE calculation. A mechanism which involves radiation trapping and the transfer of excitation energy from the annular regions of the ICP to the aerosol channel is offered. This mechanism called “assisted ionization” leads to a more accurate prediction of electron density at a particular temperature. Assisted ionization is the result of the coupling of high energy resonance radiation from Ar(I) in the annular regions of the ICP into the analyte channel. The response of analyte atoms and ions to temperature and electron density in the channel can be estimated by inclusion of the analyte ionization equilibrium in an overall equilibrium which includes argon atoms and excited state argon species.     

Influence of water on conditions in the inductively coupled argon plasma

Aqueous media are used almost universally for sample introduction in both inductively coupled plasma atomic emission spectrometry (ICPAES) and in inductively coupled plasma/mass spectrometry (ICP/MS). In the process of aqueous sample introduction a substantial mass of water is introduced into the plasma as a combined aerosol/vapor mixture. In the present studies, the masses of water present as aerosol and vapor were controlled, in order to examine their separate influence on the key plasma properties of electron density n_e and ionization temperature T_ion. Water loading in the plasma was indeed found to have a major influence on n_e and T_ion, and plots of these parameters as a function of water loading are presented. Plasma viewing height and operating power were also found to be important variables in influencing the way in which water interacts with the plasma. The implications of water loading on background emission and noise level are also considered.     

Spatial profile measurement of ionization and excitation temperatures in an inductively coupled plasma

The developed instrument for spatial profile measurement [1] has been applied to the measurement of ionization and excitation temperatures in an inductively coupled plasma (ICP). The silicon intensified target (SIT) detector allowed it to measure a large number of emission spectra in a short period. The ease of acquisition enabled building up complete contour maps of ionization and excitation temperatures. The contour maps of various temperatures reveal that local thermal equilibrium does not exist in the whole ICP. The comparison between ionization temperature profiles for Ar and Ca indicates that in the normal analytical zone of the ICP, Ca is ionized as expected from the Ar ionization temperature. Excitation temperatures derived from low-level Fe I lines are lower than those derived from high-level Fe I lines over a large part of the plasma. The result confirms that for Fe I lines the ICP is characterized as an ionizing plasma in the whole ICP and the low atomic levels are overpopulated with respect to the high atomic levels.     

Analyte ionization in the inductively coupled plasma

Spatially resolved ion-atom emission intensity ratios for Sr, Ca, Mg, Cd and Zn have been measured at rf power settings of 1.00, 1.25, 1.50, 1.75 and 2.0 kW at a vertical height of 16 mm above the load coil. Measured values of electron density have been used to construct a theoretical local thermal equilibrium (LTE) framework, and ion-atom emission intensity ratios calculated from this framework have been compared to experimentally measured values. The measured ion-atom emission intensity ratios were found to be within an order of magnitude of these calculated LTE ratios.The experimental degree of ionization for these five elements was determined for the various rf input powers. These values have been compared to the analagous LTE values. Both degree of ionization and departure from LTE were found to be strongly correlated with the ionization potential of the element.The radial spatial dependence of the degree of ionization for Cd at an rf power of 1.25 kW has been measured for aerosol flow rates of 0.6, 0.8 and 1.21 m⁻¹ for vertical heights of 4, 8, 12, 16 and 20 mm above the load coil. The spatial distribution of electron number density was measured at an rf power of 1.25 kW and at aerosol flow rates of 0.6, 0.8 and 1.21 m⁻¹ and a correlation between degree of ionization and electron density identified. Finally the relative concentration of Cd ions has been calculated from ion spatial emission profiles and plasma operating conditions which produce a maximum in the ion density identified.     

Spatial distributions of metastable argon,temperature and electron number density in an inductively coupled argon plasma

Spatially resolved radial distributions of excitation temperature and electron number density in an argon ICP were obtained. The argon excitation temperature and electron number density near the plasma center were found to 7000 K and 5 × 10¹⁵ cm^?3, respectively, at an RF power of 1.5 kW and a carrier argon flow rate 0.65 1 min^?1.Various distributions of the absorbance at the Ar I 811.5 nm line, which has one of the metastable levels as the lower level, were obtained with and without carrier argon flow, where an MIP was used as a light source. Introduction of a large amount of potassium did not influence the distribution of the absorbance. The emission intensities at Ar I 811.5 nm were also measured for comparison.     

Langmuir probe measurements of electron temperature in an inductively coupled plasma

The feasibility of using double Langmuir probes to measure electron temperature (T_e) in an Ar inductively coupled plasma (ICP) was evaluated. Experimental methods for probing the plasma and for reducing rf interference were devised. Despite these measures, the probe signal was noisy and erratic if the ICP had the normal analytical configuration with a hole through its center, so measurements were restricted to an ICP without an axial channel. Theoretical criteria indicated that Langmuir probe measurements in an atmospheric pressure ICP were in a borderline regime in which the measured T_e values may have been depressed somewhat (relative to the actual T_e values in the ICP) due to cooling of electrons as they approached the probe. The T_e values obtained from the center of the ICP were 7500 K at a forward power of 1.0 kW and 10 000 K at 1.25 kW for a measurement position 8 mm above the load coil. Electron density (n_e) measurements by the Langmuir probe method were comparable to or higher than n_e values calculated from the Saha equation at the measured T_es. The T_e and n_e values were high enough to indicate that, if electron cooling and ion-electron recombination occurred near the probes, these effects were not extreme and/or the use of two probes compensated for them in some fashion. The probe measurements also indicated that T_e increased with the potential difference between the probes. This latter observation provided tentative evidence that the electron kinetic energy distribution was non-Maxwellian with an excess of higher energy electrons relative to lower energy electrons.     

Spatial characterization of analyte emission and excitation temperature in an inductively coupled plasma

Vertical, lateral and radial profiles of analyte emission in an inductively coupled plasma have been measured using photodiode array spatial profiling spectrometers. These profiles have been measured for both neutral atom and ionic lines of several elements. Neutral atom lines can be sub-divided into two basic groups on the basis of their vertical spatial emission characteristics. One group in which the peak vertical position of emission correlates positively with normal temperature and a second group in which the peak vertical position of emission correlates negatively with normal temperature. Utilizing radially resolved emission intensities and vertical and radial profiles of neutral atom excitation temperature, the caracteristic emission patterns of both groups of neutral atom lines can be explained. Analyte ionic line spatial emission characteristics, both vertically and radially, are shown to be relatively species independent and radial emission intensity ratio maps of ionic and neutral atom lines of the same element are presented that indicate the potential importance of plasma boundary regions as regions of major non-LTE behavior.     

Electron temperature and radiative attachment continua in enclosed inductively coupled plasma in argon and chlorine

Inductively coupled plasma (ICP) discharges in chlorine, argon, and their mixtures sustained inside a spherical quartz container at atmospheric pressure have been investigated. Continua of radiative attachment of a free electron to a chlorine atom in ICP are elucidated and are utilized to derive a spatial profile of electron temperature. A qualitative picture of elementary processes in enclosed ICP is given. Differences between electron and gas temperatures are discussed. Electron temperature is maximum in a periphery layer near the induction coil that is chosen for impurity determination. Concentrations of carbon and metal impurities are estimated.     

Spatially resolved temperature measurements in a furnace atomization plasma excitation spectrometry source

Spatially resolved atomic emission intensities from helium, and molecular emission intensities from OH and N⁺₂ have been measured in a furnace atomization plasma excitation spectrometry (FAPES) source. He I emission at 388.86 nm was used to monitor the spatial structure of the plasma in the source while increasing the radio frequency (r.f.) power applied to its center electrode. At higher r.f. power the He I emission intensity increased significantly while its spatial structure remained relatively unchanged. The He I emission was found to be most intense adjacent to the center electrode. Some less intense emission was observed adjacent to the graphite cuvette wall and some very weak emission was seen throughout the volume of the source. These observations suggest that the FAPES source operates as an r.f. glow discharge.Emission intensities from the OH (0-0) rotational A ²Σ⁺ → X ²Π_i and N⁺₂ (0-0) rotational B ²Σ⁺_o → ²Σ⁼_g bands were used to monitor the effects of increasing the r.f. power applied to the center electrode of the source. From these measurements, rotational temperatures for these molecules were calculated. The intensity measurements showed that there is a significant thermal gradient in the source with OH rotational temperatures ranging between 680 and 1050 K and N⁺₂ rotational temperatures ranging between 580 and 1920 K with 60 W r.f. power applied to the center electrode. At higher r.f. powers there is an increase in rotational temperatures and an increase in the dissociation of molecular species in the FAPES source.Lead excitation temperatures were calculated using the line ratio method by measuring the emission of the Pb I 280.119 and 283.306 nm lines at different r.f. powers. The temperature was found to increase monotonically with r.f. power over the range of 35 to 75 W.     

On the charge transfer in an inductively coupled argon plasma

Absolute population densities for several excited states of magnesium are obtained for several locations in an inductively coupled plasma (ICP). They were used to construct Boltzmann-Saha plots for these positions and show that magnesium is close-to-LTE. The deviations from LTE are mainly limited to the levels sensitive to charge exchange with argon ions. These measured deviations can be explained by a simple model which shows that, although charge transfer is a dominant excitation and ionization mechanism in an ICP, the associated LTE deviations are limited in magnitude.     

Evaluation of argon metastable number densities in the inductively coupled plasma by continuum source absorption spectrometry

Absolute number densities for the metastable and radiative 4s argon levels in an inductively coupled plasma have been determined for a variety of plasma conditions by the technique of continuum source absorption. As primary source, a 300 W xenon arc was used and much care was taken in screening the optical detection system from the intense background emission of the plasma. A 1.29-m focal length grating monochromator provided a variable bandwidth so that absorption measurements could be carried out, with varying degree of sensitivity, on 19 different argon lines. The number densities were derived from the corresponding curves of growth, calculated for each line. Concentrations ranging from 2.3 × 10¹⁰ to 7.4 × 10¹¹ cm⁻³ were obtained for the different levels, depending upon the presence or absence of nebulizing gas and water in the plasma. At observation heights greater than 20 mm above the coil, the number density approaches the value predicted by Boltzman equilibrium for a temperature of 6500 K. The detection sensitivity of the present apparatus is about 5 × 10⁹ cm⁻³. For seven lines, damping parameter values were also estimated and found to vary from 0.4 to 1.1.     

Influence of water on the spatial excitation behavior of selected elements in the inductively coupled plasma

A photodiode array spectrometer has been used to generate vertical spatial emission profiles of elements introduced into an inductively coupled plasma. Atomic (soft) and ionic (hard) lines have been studied in both the presence and absence of water. The data obtained suggest that the amount of water present has a marked influence on the vertical spatial profiles. Both atomic and ionic emission lines are similarly affected. The predominant role of water appears to be kinetic control of analyte desolvation which occurs prior to excitation.     

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density n_e and temperature T_e in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of n_e and T_e can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.     

Studies on lifetime measurements and collisional processes in an inductively coupled argon plasma using laser induced fluorescence

Lifetimes of excited atoms and ions in an inductively coupled argon plasma are obtained by the laser excited fluorescence technique, and quantum efficiencies are calculated for each species. Thermally assisted fluorescence is measured in order to explain the decrease in quantum efficiency with the existence of other excited levels of the same element near the laser excited level. Finally, a discussion is given on the possible collisional population processes.     

Entrainment of ambient air into a spectrochemical inductively coupled argon plasma

A spectrochemical inductively coupled argon plasma (ICP) is normally operated in the open air. Therefore, it is suggested in the literature that entrainment of air molecules into such an ICP may cause loss of electrons, especially so at the plasma's edge. The present study discusses the significance of this effect. The density and temperature of electrons and nitrogen molecules around the edge of the plasma were measured by Thomson and rotational Raman scattering. A region where both electrons and nitrogen were present in detectable amounts (10¹⁹ and 10²⁴ m⁻³, respectively) could not be observed. Above the torch inner wall the nitrogen concentration drops rapidly towards the plasma. Measurements suggest that the nitrogen concentration at 1 mm from the plasma is only a few percent, and in the active zones of the plasma (far) below 0.1%. This is not enough to affect the plasma significantly. Moreover, electron loss due to diffusion of nitrogen into the plasma is calculated to be much slower than the loss observed in earlier studies. Hence, air entrainment is unlikely to play a significant role in the ICP. A possible alternative is the formation and destruction of molecular rare gas ions.     

Novel instrument for high temperature thermogravimetric measurements in high water vapour contents

Summary A novel instrument for high temperature thermogravimetric measurements in atmospheres containing high water vapour contents was developed in a collaboration between Netzsch and Risø National Laboratory. The development of the instrument was initiated to facilitate the investigation of high temperature corrosion of steels in humidified atmospheres. The instrument consists of a standard thermal analyser unit, including a new water vapour furnace, balance and sample carrier. The design of the instrument is discussed and thermogravimetric measurements on a Fe₇₈Cr₂₂ steel are presented.