Spatially-resolved Optical Emission Spectroscopy Research Articles

Application of emission spectroscopy in plasma-assisted NH3/air combustion using nanosecond pulsed discharge

This work investigates the effects of atmospheric-pressure nanosecond (ns) pulsed plasma discharges on NH3/air flames using optical emission spectroscopy (OES). Firstly, spatially-resolved OES in the 200–800 nm range of NH3/air flames with and without discharges are presented to identify the plasma-induced excited species (such as N2* and H*). Then, time-resolved OES with ns timescale resolution are achieved to illustrate the discharge dynamics using a statistical imaging strategy, which is specifically designed to overcome the timing jitter of the discharge. From the time-resolved spectra of excited N2, the first and second emission events in NH3/air flames with a time interval of 80 ns are revealed, and each exhibits a profile with two exponential decays. Furthermore, spatial- and time-resolved rotational and vibrational temperatures are derived from the spectral fitting of N2(C→B) to quantify the thermal effects of ns plasmas, thus revealing the ultrafast and slow gas heating processes. In addition, the electron number density along the flame is obtained from the intense Hα line, which locally reaches up to 1 × 1017 cm−3. Besides, the spatial-resolved Hα line intensity suggests that the flame chemistry does not directly affect, but the discharge dominates the production of the atomic hydrogen. It provides insight into the kinetic effects of ns pulsed discharge on NH3 combustion enhancement.

Investigation on EDM Plasmas Using Time and Spatially-Resolved Optical Emission Spectroscopy

Electrical discharge machining (EDM) is a well-established non-conventional manufacturing process. The underlying working principles of EDM relay on interactions of electric discharge plasmas with the electrodes material and dielectric in micrometer gaps. Key aspects of the process, such as material removal mechanisms, are strongly related to this complex interaction. Therefore, observation of small gaps plasma-material interactions can help to better understand this particular EDM process, supply new boundary conditions for existing models, and lead to more sophisticated processing. Different methods of analysis have been used to study and explain the physics of EDM discharges. In the present work, spatially-resolved high-speed optical emission spectroscopy is used to analyze emission spectra from small gap discharges under different conditions. Results on the plasma expansion in function of time as well as time resolved profiles of plasma components and of different plasma parameters are obtained and interpreted. Optical emission spectra supported by emission spectra simulations suggest that the center of the micro discharge is dominated by highly ionized species, whereas single ionized and atomic species are more uniformly distributed over the whole plasma. Furthermore, the spatially-resolved measurements shows that the center of the plasma is not in thermodynamic equilibrium, probably field emission leads to an ion-enhanced FE driven Townsend discharge in the small gaps.

Laboratory Hydrogen-Beta Emission Spectroscopy for Analysis of Astrophysical White Dwarf Spectra

This work communicates a review on Balmer series hydrogen beta line measurements and applications for analysis of white dwarf stars. Laser-induced plasma investigations explore electron density and temperature ranges comparable to white dwarf star signatures such as Sirius B, the companion to the brightest star observable from the earth. Spectral line shape characteristics of the hydrogen beta line include width, peak separation, and central dip-shift, thereby providing three indicators for electron density measurements. The hydrogen alpha line shows two primary line-profile parameters for electron density determination, namely, width and shift. Both Boltzmann plot and line-to-continuum ratios yield temperature. The line-shifts recorded with temporally- and spatially-resolved optical emission spectroscopy of hydrogen plasma in laboratory settings can be larger than gravitational redshifts that occur in absorption spectra from radiating white dwarfs. Published astrophysical spectra display significantly diminished Stark or pressure broadening contributions to red-shifted atomic lines. Gravitational redshifts allow one to assess the ratio of mass and radius of these stars, and, subsequently, the mass from cooling models.

Spatial characterization of H 2 :CH 4 dissociation level in microwave ECR plasma source by fibre-optic OES

Spatially resolved optical emission spectroscopy (SR-OES) was used to investigate microwave activated H2/Ar/CH4 plasma under conditions of the electron cyclotron resonance (ECR). The chemistry and composition of the gas phase were studied using self-designed fibre-optic system with echelle type spectrometer during CVD deposition of polycrystalline diamond. One-dimensional intensity profiles of the main species were collected along the vertical axis of chamber. The dominant species in the flux, originating from excited hydrogen and hydrocarbons, were identified as H, H+, CH and CH+; they are crucial for the diamond deposition process. The effect of ECR on the spatial distribution of H2 and CH4 dissociation profiles was studied in depth. The influence of processing parameters (gas flow rates, input power, pressure and magnetic field level) on species excitation as a function of the distance above substrate was asessed. The obtained data can be used for the ECR system optimization.

Time- and spatially-resolved characterization of electrical discharge machining plasma

Plasma created during electrical discharge machining (EDM) is investigated with fast imaging and with time- and spatially-resolved optical emission spectroscopy. After the breakdown, the plasma develops very fast (<50 ns) and then remains stable. The plasma excites a broad volume around the electrode gap. Light is still emitted after the discharge by incandescent liquid metallic particles coming from the erosion of the workpiece. Typical plasma spectra show a strong Hα and continuum radiation, with many lines emitted by impurities coming from electrodes materials. The plasma is thus composed of species coming from the cracking of the dielectric molecules, mainly hydrogen, with contamination from the electrodes. This contamination is mostly concentrated in the vicinity of the electrodes. The electron density reaches 2 × 1018 cm−3 at the beginning of the discharge. This extreme density causes merging of lines, strong Stark broadening and shift of the Hα line. The density decreases afterwards rapidly with time, remaining nevertheless above 1016 cm−3 after 50 µs. The electron temperature remains roughly constant around 0.7 eV in the whole plasma. The low temperature, the high density measured and other spectroscopic evidence prove that the EDM plasma is weakly non-ideal (Γ ≃ 0.33).

High rate growth of high-quality microcrystalline silicon films from plasma by interconnected multi-hollow cathode

We present guiding principles for obtaining high-quality microcrystalline silicon (μc-Si:H) films with regard to film-precursors and to their reactions on the film-growing surface, in which the contribution of short lifetime reactive species to film-growth determines the defect density of the resulting films except for temperature dependence. The importance of annihilation reactions of the short lifetime species with SiH 4 as well as that of their low generation rate is emphasized. The consideration of co-existing annihilation reactions of atomic hydrogen, important species for μc-Si:H formation, is also included. Based on the guiding principles, we propose a high rate deposition technique using high density very-high-frequency plasma produced with a newly designed interconnected multi-hollow cathode. Spatially-resolved optical emission spectroscopy indicated a radical separation effect introduced by the cathode. Device-grade μc-Si:H films with low defect densities (∼ 5 × 10 15 cm − 3 ) have been successfully prepared at deposition rates approaching 8 nm/s by means of this technique.

Study of the plasmas produced during the deposition of TiC/SiC thin films in a hybrid magnetron-laser system

Time- and spatially-resolved optical emission spectroscopy was performed to characterize the plasma produced in a hybrid magnetron-sputtering-laser deposition system, which is used for TiC or SiC thin films preparation. A graphite target was ablated by a KrF excimer laser (λ=248 nm,τ=20 ns) and either Ti or Si targets were used for DC magnetron sputtering in argon ambient. Spectra were measured in the range 250–850 nm. The evolution of the spectra with varying magnetron powers (0–100 W) and argon pressures (0.3–10 Pa) was studied. Spectra of the plasmas produced by a) the magnetron alone, b) the ablation laser alone, and c) the magnetron and the ablation laser together, were recorded. Spectra (a) were dominated by Ar atoms and Ar+ ions. Emission lines of Ti and Si were detected, when Ti target and Si target was used, respectively. Spectra (b) revealed emission of C, C+, C2, Ar, Ar+. Spectra (c) showed presence of all previously mentioned species and further of Ti+ ions emission was detected.

Time-resolved imaging and spatially-resolved spectroscopy of electrical discharge machining plasma

The plasma created during electrical discharge machining is investigated with imaging and spatially-resolved optical emission spectroscopy. Analysis of the pre-breakdown duration shows that the breakdown is of stochastic nature. Due to the presence of gas bubbles created by electrolysis, the breakdown mechanism in water could be different from the one in oil. After the breakdown, the plasma develops very fast (<50 ns) and then remains stable. The plasma excites a broad volume around the electrode gap, between 50 and 400 µm in diameter depending on the discharge current. The Hα line is emitted from the whole plasma, since the hydrogen atoms originate from the dielectric. On the other hand, spatially-resolved spectroscopy shows that plasma contamination from the electrodes is concentrated in their vicinity. After the discharge, light is still emitted by incandescent metallic particles coming from erosion of the workpiece. Profiles between the electrodes of electron density and of electron temperature have been estimated from spatially-resolved emission spectra. The density is found to slightly increase towards the plasma centre, whereas the temperature is quite constant across the plasma (∼0.7 eV).

Plasma emission diagnostics for the transition from microcrystalline to amorphous silicon solar cells

Abstract The possibility to employ spatially resolved optical emission spectroscopy (SROES) as a diagnostic tool for the prediction of the transition from microcrystalline to amorphous silicon growth was investigated. The transition was achieved by increasing the silane fraction in the mixture and was identified through the solar cell performance. A drastic change of the shape of the emission profiles, characterized by an enhancement of the production of species closer to the substrate, was observed in the transition region when increasing the silane fraction. Calculations of the probability of various species to reach the surface have shown that the change of the shape of the radical generation distribution in space finally leads to an increase of the contribution of highly reactive, highly sticking radicals like SiH2 to the film growth. On the other hand less reactive species like H atoms are less affected by the shape of their generation profiles. Their probability to reach the surface drops because of the increase of the collision frequency. Both these factors can explain the transition to amorphous silicon growth and the relation between emission profiles and the transition indicating a clear potential for using SROES in thin film solar cell performance optimization.

In situ sensor of spatially resolved optical emission

An experimental study of the uniformity of the plasma emission from the gaseous electronics conference reference cell was conducted using a diagnostic technique refined at the University of Michigan known as spatially resolved optical emission spectroscopy (SR-OES). This diagnostic will provide immediate spatially resolved feedback regarding the plasma species emissivity.