Side-on Observation Research Articles

Electric field components within a micro-scaled DBD measured by Stark shifting and splitting of helium lines

Abstract Atmospheric pressure dielectric barrier discharges (DBDs), such as the micro cavity plasma array (MCPA), have emerged as promising technologies for the conversion of volatile gases. These conversion processes’ effectiveness can be enhanced by integrating catalytically active surfaces. To deepen the understanding of the plasma-catalyst interaction, it is crucial to study the transport dynamics of charged species to the catalytic surface, which, due to collisions with neutrals, also directly affects the transport of reactive species to the catalyst. Thereby the transport of the charged species is in particular influenced by the electric field perpendicular to the catalytic surface. However, experimental data on the component-wise electric field strength within SDBDs are rare. To address this issue, we performed polarized optical emission spectroscopy on the shifting and splitting of the allowed 492.19 nm (1D → 1P0) and forbidden 492.06 nm (1F0 → 1P0) helium line pair. This diagnostic approach requires a non-radially symmetric geometry, which leads to an adapted reactor design of the MCPA allowing the side-on observation of the discharge. The discharge operates in pure helium at atmospheric pressure, utilizing a triangular excitation voltage with a frequency of 15 kHz and an amplitude of 600 V. We performed phase-resolved measurements of the electric field components with a temporal resolution of 1 µs. Our results revealed an electric field strength of approximately 22 kV cm−1 for the component perpendicular to the dielectric surface, while the component parallel to the dielectric surface is about 5 kV cm−1 larger during the decreasing potential phase of the applied voltage.

Stark polarization spectroscopy of neon spectral lines for estimating cathode sheath parameters in Grimm-type glow discharge sources

We present the results of the optical emission spectroscopy (OES) study of the Ne I 503.775 nm and Ne I 508.038 nm atomic spectral lines observed in two DC glow discharge sources (GDS), standard Grimm analytical source and the modified plane cathode Grimm design. The standard Grimm GDS enables OES recording end-on only, i.e. when the radiation is collected from the entire discharge, in the direction perpendicular to the cathode surface. Our modification of Grimm GDS, on the other hand, allows for spectra recording of the complex line shapes from two optical directions, both end-on and side-on, the latter collecting radiation from a narrow slice of the discharge parallel to the cathode surface. Side-on observations enable spatially resolved Stark polarization spectroscopy at different positions along the whole cathode sheath (CS) region of the discharge. This allows the study of the Stark shifts of differently polarized (σ, π, or unpolarized) line components' dependence on the macroscopic CS electric field strength. Stark components of neon lines, encountered in side-on observations in the CS, are preserved in the end-on recordings as characteristic features – wavy wings with intensity maxima corresponding to the distinctly resolved Stark components in the CS. Following OES measurements in a broad range of discharge conditions (pressure, voltage, current), we report on a sustained correlation between the value of the maximum electric field strength Emax in the CS and the end-on recorded wing feature shifts Δλe. This correlation can be extrapolated to estimate the cathode sheath parameters Emax and the CS thickness dc in the standard Grimm-type GDS, with end-on optical access available only.

Further insights into prepeak emission in pulsed radiofrequency glow discharge

Side-on optical emission measurements, with spatial and temporal resolution, have been carried out on a modified Grimm type pulsed rf glow discharge, using a copper sample and argon as discharge gas. In particular, the early ignition phase of the pulsed discharge, prepeak, and the beginning of the plateau have been electrically characterized and spectroscopically evaluated along the plasma plume for several argon and copper emission lines. Different pulse repetition frequencies have been used while keeping a constant duty cycle of 25%, covering a range from the μs to the ms pulsed GD. In all the cases a spatially extended discharge has been observed during the first 10–20μs of the pulse, when the dc bias voltage has not been fully established yet. During this early phase of the pulse, the species present in the chamber are excited following the amplitude of the voltage symmetrical oscillations, and argon and copper emission can be detected at all the distances evaluated along the plasma axis at delays as short as 4μs. After the first 20μs approximately, the discharge has contracted close to the sample surface, the dc bias voltage has been almost established and the sputtering process is taking place. Emission registered through side-on observation, at positions relatively far from the sample, decreases and then progressively increases to reach a plateau if the voltage pulse lasts long enough. Moreover, the further the detection position is from the sample surface, the later the increase of the copper and argon emission is observed. Therefore transport phenomena involving both argon and copper species are now evidently leading to an estimated average speed of 100m/s.Additionally, end-on optical emission measurements have been carried out to evaluate the prepeak emission from the negative glow region close to the sample surface, and then to compare this emission with that occurring at farther positions (side-on detection experiments). The results have shown that the basis of both prepeaks might be different as the prepeak signal measured using end-on detection peaks around 10μs later than that observed using side-on detection.

Time-space distribution of laser-induced plasma parameters and its influence on emission spectra of the laser plumes

A physical model is developed accounting for dynamics and radiation of plasma plumes induced by nanosecond laser pulses on surface of solid samples. The model has been applied to simulate emission spectra of the laser erosion plasma at the elemental analysis of metals using single- and double-pulse excitation modes. Dynamics of the sample heating and expansion of the erosion products are accounted for by the thermal conductivity and gas dynamic equations, respectively, supposing axial symmetry. Using the resulting time-space distributions of the plasma parameters, emission spectra of the laser plumes are evaluated by solving the radiation transfer equation. Particle concentration in consecutive ionization stages is described by the Saha equation in the Debye approximation. The population of excited levels is determined according to Boltzmann distribution. Local characteristics determining spectral emission and absorption coefficients are obtained point-by-point along an observation line. Voigt spectral line profiles are considered with main broadening mechanisms taken into account. The plasma dynamics and plume emission spectra have been studied experimentally and by the model. A Q-switched Nd:YAG laser at 1064 nm wavelength has been used to irradiate Al sample with the pulses of 15 ns and 50 mJ duration and energy, respectively. It has resulted in maximum power density of 0.8 MW/cm 2 on the sample surface. The laser plume emission spectra have been recorded at a side-on observation. Problems of the spectra contrast and of the elemental analysis efficiency are considered relying on a comparative study of the measurement and simulation results at the both excitation modes.

Chemiluminescence Emission of C2, CH and OH Radicals from Opposed Jet Burner Flames.

Chemiluminescence emission from opposed jet burner flames was measured by a charge coupled device (CCD) camera with an image intensifier. Radial profiles of the emission intensity of OH, CH and C2 radicals were obtained by the Abel inversion of the side-on observations. The profiles of emission intensity of these radicals are similar, and each maximum is located at the center of the flame zone. The emission of C2 and CH radicals produced in the early stage of the combustion reaction was observed even on the burned-gas side of the flame zone where the mean temperature is significantly high, suggesting that unburned mixture exists on the burned-gas side of the flame zone. It was found that the profiles of OH emission intensity and NO concentration are similar. This fact suggests that NO originating from the prompt NO mechanism is predominant in the distributed reaction zone.

Easily ionizable element (EIE) interferences in inductively coupled plasma atomic emission spectrometry: II. Minimization of EIE effects by choice of observation volume

Measurements by inductively coupled plasma atomic emission spectrometry (ICP-AES) are expected to be immune to interferences caused by easily ionizable elements. This article demonstrates how observation width, viewing height and viewing volume can be chosen to minimize such interferences and to optimize emission-signal collection in either side-on or end-on configurations. At this time, results must be viewed as a confirmation of previous findings rather than a practical means of performing routine plasma optimizations. Volume data were computed from high-resolution two-dimensional laterally and radially resolved images of Ca I and Ca II emission in the presence and absence of a lithium concomitant. For side-on observation of calcium atomic emission, the customary arrangement, the optimal balance between signal-to-background ( S/B) ratio and interference effects arises from use of a 0.5 mm observation width. The optimal position for measurement and the ideal height of the observation volume were dependent on applied r.f. power. More extended contiguous regions of high S/B ratio became available with the proper spatial averaging, which is inherent in viewing an extended volume. Side-on observation of Ca II emission in the presence of a Li concomitant was best with a 5 mm viewing width at low r.f. power (500 and 1000 W) and a narrow width (< 1 mm) at higher power (1500 W). End-on observation of Ca atomic emission was optimal for a 1000 W plasma viewed within 1.5 mm of the central axis with an observation width of less than 6 mm. For end-on Ca II viewing, a 1500 W plasma minimized the lithium interference while maximizing the S/B ratio when the midpoint of the viewing position was within 1 mm of the central channel and the observation width was less than 2.5 mm.

Chemiluminescence Emission of C2, CH and OH Radicals from Opposed Jet Burner Flames.

Chemiluminescence emission from opposed jet burner flames was measured by a CCD (Charge Coupled Device) camera with an image intensifier. Radial profiles of the emission intensity were obtaine by the Abel transform of results of the side-on observation. The profiles of emission intensity of these radicals are similar and each peak is located at the center of the flame zone. The emission of C2 and CH radicals produced in the early stage of the combustion reaction was observed on the burned gas side of the flame zone where the mean temperature is significantly high, suggesting that unburned mixture exists even on the burned gas side of the flame zone. From the emission profiles of OH radical which can be used as a measure of the reaction zone, it can be seen that the outer diameter of the flame zone increases with the mixture supply velocity, while the inner diameter remains constant. It was found that the profiles of OH emission intensity and NO concentration are similar. This fact suggests that NO originating from prompt NO is predominant in the distributed reaction zone.

Structure of highly turbulent premixed flames

A highly turbulent flow was achieved by an opposed jet burner and an annular turbulent premixed flame was produced around the impinging mixture jets. In the present study, characteristic features of this flame were made clear experimentally. Schlieren pictures show that large scale wrinkles are produced on the unburned gas side of the turbulent flame zone. With increasing unburned gas velocity, the scale of these wrinkles becomes smaller. Concurrently, a stringy microstructure appears intermittently on the burned gas side. For the extremely high velocity case, these wrinkles disappear and the whole combustion flow field is filled with the bundles of stringy microstructure. For the moderate velocity case, the PDF's of temperature on the unburned gas side show an essentially bimodal profile with a high probability of intermediate temperature. On the burned gas side where the stringy microstructure is observed, the probability of the unburned gas temperature disappears, suggesting that the flame zone consists of the small eddies of completely burned products and preheated or partially reacted gas. This structure enhances the turbulent mixing within the flame zone and a uniform turbulent flow field is developed. The cross sectional radial distributions of the chemiluminescence emission of C 2 , CH and OH radicals were obtained by the Abel transform of side-on observation. The OH peak appears at the center of the flame zone and the OH profile indicates the thickness of the flame zone. The CH and C 2 peaks coincide with each other and fall on the burned gas side. From CH and C 2 emission profiles, it is found that the vortex tubes of unburned or partially reacted gas enter deeply into the burned gas region.

Transition Probabilities of Cull Lines

Abstract Relative transition probabilities of 7 CuII lines in the visible spectral region have been measured by side-on observation of a plasma jet emerging out of a capillary as well as by using an axial discharge-type of an exploding copper wire. The results are compared with those of two other experiments.

End-on observation of a horizontal low-flow inductively coupled plasma

End-on observation of a horizontal water-cooled low-flow plasma has been compared to side-on observation of a conventional high flow plasma. The analytical performance is similar to the conventional plasma. The detection limits are close to the detection limits measured with a Perkin-Elmer Plasma II conventional ICP. The linearity of calibration curves is also similar to the conventional ICP. However, a remarkable reduction of interferences has been observed for end-on observation of the low-flow plasma.

Experimental Stark parameters for some spectral lines in SF6 plasma

Stark widths of some sulphur and fluorine spectral lines emitted by a wall-stabilised arc have been measured in order to provide simple relations for practical diagnostics. Different mixtures of SF6, H2 and Ar were used and end-on or side-on observations were performed in order to yield information about the electronic concentration and temperature. An alternative method of electronic density diagnostics is proposed by measuring the relative dip between two close lines of the 4s 5S0-5p 5P multiplet of SI.

Measurements of Stark broadening of He(II) 4686Å

Side-on observations were performed on a Stark-broadened He(II) 4686Å line emitted from a pulsed, high-current arc plasma preionized by CW microwave radiation at atmospheric pressure. A two-dimensional, multichannel detection method was used to obtain the radial distribution of the line profile in a single test. The line was emitted from the central hot core of the plasma, which had a homogeneous radial electron-density distribution. Comparisons with theoretical predictions show that the inclusion of ion-dynamic effects leads to better agreement between experiment and theory.

Stark Shifts of Ionized Helium Lines

The shifts of the ionized helium lines λ4686A ( n =4→3) and λ3203A ( n =5→3) were measured relative to the centers of gravity of the fine structures of these lines. A capillary tube, through which condensed discharge was passed, was used as a plasma light source. Side-on observation enabled one to obtain optically thin plasma. In each case the center of gravity of the fine structure lines excited by a liquid-nitrogen-cooled hollow cathode discharge was assumed to be the unperturbed position. The measured red shifts of the He II lines λ4686A and λ3203A are -0.041±0.023 cm -1 and -1.3×0.3 cm -1 respectively, under the plasma condition of N e =(1.6×0.4)×10 16 cm -3 , T e =(2.2±0.3)×10 4 K, and N e =(1.2±0.3)×10 16 cm -3 , T e =(1.3±0.3)×10 4 K respectively. The present results are consistent with those of Murakawa but not with those of Pittman et al.

Stark broadening of visible neutral helium lines in a plasma

Side-on observations of the visible spectrum emitted by a helium plasma generated in a wall-stabilized arc are reported. Electron densities range from 0.2 to 1.3 × 10 22m -3 and electron temperatures vary from 10,000 to 20,000 K. Most of the seventeen measured lines are “isolated” at the conditions of the experiment. Here, “isolated” refers to quantum levels which are non-degenerate even in the presence of perturbing fields. Electron densities derived from the Stark-widths of the isolated lines agree to within 15% of the values determined from Hβ and from the quasi-degenerate He(I) lines at 4471 and 4921 Å. Similar agreement is observed for the static-ion Stark broadening parameters. However, at the conditions of this experiment, the static-ion approximation is not valid near the center of some of the isolated lines. The observation of ion-dynamic effects in neutral isolated lines is reported here for the first time. In the isolated lines, the measured ion-dynamic effects are reasonably consistent with calculations based on an adiabatic “unified” theory for the ion perturbers. A simple parametric expression closely approximates the ion-dynamic contribution to the half-width of isolated lines. Hydrogenic (non-isolated) neutral helium lines exhibit a large discrepancy with theoretical profiles near the line center, where ion-dynamic effects are important.

Effect of optical aperture on the derivation of volume emission coefficient from side-on intensity measurements of cycling cylindrically symmetrical sources

An integral equation, analogous to the Abel integral equation for the derivation of volume emission coefficient from side-on observations, is developed which takes account of finite optical aperture. This integral equation is partially inverted and the solution is used to examine the effect of finite optical aperture for two simple distributions of volume emission coefficient. It appears that the error arising from the effect of finite optical aperture is negligible in most practical cases. One important and interesting result that emerges from this treatment is that the volume emission coefficient derived on the axis ( r=0) is independent of optical aperture and is therefore correct whatever the aperture.

Spektrochemische analysen mit einer glimmentladungslampe als lichtquelle—I: Elektrische eigenschaften, probenabbau und spektraler charakter

The Grimm glow discharge lamp is a new type of spectrochemical light source that is distinguished from other types of light sources in several ways. Therefore the more important physical properties of the source were initially investigated. The electrical behaviour of the discharge, characterized by the voltage-current parameters, depends on the type of discharge gas and its pressure as well as on the composition of the sample. The rate of erosion by cathode sputtering differs with samples of different composition and structure, and also depends upon the electrical parameters of the discharge gas and its pressure. The spectral nature of the discharge is due to the fact that no thermal equilibrium exists between atoms, ions, and electrons in the source. The higher the ionization energy of the discharge gas is, the stronger are the ion line intensities of the sample. For the discharge in a pure noble gas alone, the background is a continuum, but the spectral distribution differs for the various gases. From these observations, and from further investigations on the dependence of atom lines, ion lines, and background on the discharge parameters, conclusions could be drawn of the suitability of the source for qualitative and quantitative analysis. Side-on observations of the source indicate that the radiating layer of the source is very thin, and at low power densities is only about 1–2 mm. When such a discharge is observed normal to the sample (i.e. end-on) then inhomogeneities in the sample can be observed by the suitable illumination of a spectrograph.

Präzisionsmessungen am Spektrum des Wasserstoffplasmas

Detailed and precise investigations on the Balmer spectrum of a hydrogen arc have been made in order to compare the measured absolute values of the spectral emission coefficients with the existing theories, the results being of special interest with respect to line broadening calculations and to the lowering of the ionization energy. The plasma is generated in a stationary high power cascade arc at atmospheric pressure. The experiment covers the temperature range between 12000 and 20000 K, where all emission coefficients show maxima in their temperature dependence. To keep self-absorption within the plasma as low as possible, the investigations have been carried out in side-on observation. The extended gas layer of almost constant intensity surrounding the maxima ensures very accurate results of the side-on measurements. For the necessary precise calibration of the optical system both a standard carbon arc and tungsten strip lamps have been used. As the intensity of these different radiation standards only shows a few percent deviation, a very high accuracy can also be established for the measured hydrogen emission coefficients.

Experimental Determination of Transition Probabilities and Stark Widths of S I and S II Lines

Absolute transition probabilities and Stark widths for some prominent multiplets of S I and S II have been determined experimentally. A wall-stabilized arc was operated in S${\mathrm{O}}_{2}$ with a small hydrogen admixture. The spectroscopic measurements were performed photoelectrically. Side-on observations were transformed via the Abel inversion to give the radial dependence of the measured quantities. Measurements of the absolute intensity of O I lines and the width of ${\mathrm{H}}_{\ensuremath{\beta}}$ served for the plasma analysis; the results of these measurements together with the application of equilibrium and conservation relations for arc plasmas enabled the determination of the temperature and various particle densities. Mass-separation effects are taken into account. The measured Stark widths generally agree well with values calculated from Stark-broadening theory; in the case of the S II lines the agreement with recently calculated values is substantially better than that with earlier theoretical data.

Application of the Abel Integral Equation to Spectrographic Data

An improved method is proposed for applying the Abel integral equation to radiance data gathered from rotationally symmetric sources by side-on observation. The method involves dividing the data into a number of segments going out from the center. A least-squares polynomial is then fitted to each segment so that the inverted Abel integral equation can be integrated exactly to yield the emission coefficient. It is shown, with test curves approximating spectral line shapes from nonhomogeneous sources, that the method becomes superior to the other more common numerical methods of reducing such data as the number of data points becomes large. This superiority increases as the data scatter increases.

Shifts and Widths of Some Stark-Broadened Oxygen Lines in an Arc Plasma

A thermal oxygen plasma was generated in a wall-stabilized arc and its temperature and electron density were determined spectroscopically from the intensity measurement of an oxygen line of known transition probability. Observations were made over the range of electron densities from (1 to 6)\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and for temperatures around 10 000\ifmmode^\circ\else\textdegree\fi{}K. The profiles of several oxygen lines were scanned end on and side on, and the shifts and half-widths were compared with the results of a recently developed Stark-broadening theory for isolated lines of heavy elements. The agreement in the half-widths is very good, with deviations usually smaller than 10%. The ratio of measured to calculated width averaged over the six investigated multiplets is 1.00 with a standard deviation of \ifmmode\pm\else\textpm\fi{}0.06. However, the agreement in the shifts is good only in those cases where the shifts are large. For the two strongly broadened multiplets at 6046 and 6455 \AA{}, detailed side-on observations were made at various radial distances. They show the theoretically expected linear dependence of half-widths and shifts on the electron density over the measured range. For the strong multiplet at 6455 \AA{} the intensity distribution in the distant line wings was measured, too, and could be represented by power laws with exponents of -1.96 for the blue wing and -1.91 for the red wing, in close agreement with theoretical asymptotic formulas.