Plasma Emission Intensity Research Articles

Estimating the yield stress of soft materials via laser-induced breakdown spectroscopy

Taking three typical soft samples prepared respectively by loose packings of 77-, 95-, and 109-μm copper grains as examples, we perform an experiment to investigate the energy-dependent laser-induced breakdown spectroscopy (LIBS) of soft materials. We discovered a reversal phenomenon in the trend of energy dependence of plasma emission intensity: increasing initially and then decreasing separated by a well-defined critical energy. The trend reversal is attributed to the laser-induced recoil pressure at the critical energy just matching the sample’s yield strength. As a result, a one-to-one correspondence can be well established between the samples’ yield stress and the critical energy that is easily obtainable from LIBS measurements. This allows us to propose an innovative method for estimating the yield stress of soft materials via LIBS with attractive advantages including in-situ remote detection, real-time data collection, and minimal destructive to sample.

Investigation of thermodynamic properties in picosecond laser-produced plasmas on silicon

The validity of local thermodynamic equilibrium (LTE) in plasma is a long-term concern for laser-induced breakdown spectroscopy applications. In this paper, the spatial and temporal dependence of LTE deviation in picosecond laser-induced plasmas has been proved. A picosecond (∼20 ps) laser operated at 532 nm was used to ablate a silicon target to produce plasma at a pressure of 10−5 mbar. A general electron energy distribution function was used to provide access to the insight into population ratios of different energy levels for the spatially and temporally resolved optical emission intensity of laser-induced plasma. A precise temporal and spatial LTE boundary of plasma generated in picosecond laser ablation was obtained, and the results showed that only at delay times of 180–300 ns, the plasma away from the surface (>4 mm) is in LTE.

Laser ablation plasma expansion using microwaves

This study explores the potential of utilizing microwaves to sustain the expansion of transient laser ablation plasma of Zr target. By application of microwaves on the plasma, we observe a significant enhancement with a two to three order of magnitude increase in the plasma emission intensity, and 18 times increase in the plasma’s spatial volume. We investigate the temperature change of the plasma and observe that it decreases from 10,000 K to approximately 3000 K. Electron temperature decreased with volume expansion owing to increased surrounding air interaction, while the plasma can be sustained in air using microwaves. The increase in electron temperature during temperature drop is indicative of non-equilibrium plasma. Our results emphasize the contribution of microwaves in promoting enhanced emission and plasma formation at controlled, low temperature, thereby demonstrating the potential of microwaves to enhance the accuracy and performance of laser-induced breakdown spectroscopy. Importantly, our study suggests that microwaves could also mitigate the generation of toxic fumes and dust during ablation, a critical benefit when handling hazardous materials. The system we've developed is highly valuable for a range of applications, notably including the potential to reduce the possible emergence of toxic fumes during the decommissioning of nuclear debris.

Effect of Fe2O3 Nanoparticles Dust on the D.C Plasma Characteristics with Mirror Magnetron

In this paper, the effect of iron oxide nanoparticles dust (Fe2O3 NPs) on the parameters of DC electric discharge plasma under vacuum in argon gas was studied with the presence of a mirror magnetron behind the electrodes (cathode and anode) at constant pressure and with different amounts of Fe2O3 nanoparticles. Calculations presented a reduction of the plasma emission intensity with the NPs content. Both the plasma density (calculated by Stark's broadening method) and the mean electron temperature (calculated using Boltzmann's equation) decreased with increasing the Fe2O3 nanoparticles dust content, which indicates clearly the effect of dust density on restricting the movement of charge carriers, which in turn reduces inelastic plasma collisions.

A comprehensive study on the electron cyclotron resonance effect in a weakly magnetized capacitively coupled RF plasma: experiment, simulation and modeling

The electron cyclotron resonance (ECR) effect in a weakly magnetized capacitively coupled radio frequency (RF) plasma was previously observed with optical emission spectroscopy (OES) in experiments and analyzed by particle-in-cell/Monte Carlo collision (PIC/MCC) simulations (Zhang et al 2022 Plasma Sources Sci. Technol. 31 07LT01). When the electron cyclotron frequency equals the RF driving frequency, the electron can gyrate in phase with the RF electric field inside the plasma bulk, being continuously accelerated like microwave ECR, leading to prominent increases in the electron temperature and the excitation or ionization rate in the bulk region. Here, we study further the basic features of the RF ECR and the effects of the driving frequency and the gas pressure on the RF ECR effect by OES and via PIC/MCC simulations. Additionally, a single electron model is employed to aid in understanding the ECR effect. It is found that the maximum of the measured plasma emission intensity caused by ECR is suppressed by either decreasing the driving frequency from 60 MHz to 13.56 MHz or increasing the gas pressure from 0.5 Pa to 5 Pa, which shows a qualitative agreement with the change of the excitation rate obtained in the simulations. Besides, the simulation results show that by decreasing the driving frequency the electron energy probability function (EEPF) changes from a convex to a concave shape, accompanied by a decreased electron temperature in the bulk region. By increasing the gas pressure, the EEPF and the electron temperature show a reduced dependence on the magnitude of the magnetic field. These results suggest that the ECR effect is more pronounced at a higher frequency and a lower gas pressure, primarily due to a stronger bulk electric field, together wih a shorter gyration radius and lower frequency of electron–neutral collisions.

Plasma emission intensity expansion of Zr metal and Zr oxide via microwave enhancement laser-induced breakdown spectroscopy

This study highlights the impact of microwaves in laser-induced breakdown spectroscopy, yielding vital implications for plasma analysis and spectroscopy.

Investigation of Pressure Dependence in Photoresist Ashing Process using Microwave Excited Water Vapor Plasma

Microwave-excited water plasma asher (WPA) is our laboratory's new technology for the photoresist (PR) ashing process in semiconductor manufacturing which uses water vapor as process gas. It has the potential to solve many problems of conventional PR ashing technologies including high temperature, oxidation of metal structure, low ashing rate for ion-implanted PR, and high costs of chemicals. In this study, the pressure dependence of various aspects of plasma in water plasma asher, including light emission area, emission intensity, and substrate temperature was investigated. The ashing rate of the novolak PR film on the silicon wafer at the various condition of process pressure was also investigated. Average emission intensity and emission area of plasma significantly increased as process pressure in the chamber were decreased, and they were the highest when the process pressure was 0.3 kPa. The average ashing rate of PR film was only 0.42 μm/min when process pressure was 2.14 kPa, and it gradually increased as process pressure was decreased. It is also found that the ashing rate was improved after reducing the temperature of the water. The ashing rate when using air-rich condition of plasma was observed to be higher than when only water vapor is used as process gas.

Quantification of plasma produced OH and electron fluxes at the liquid anode and their role in plasma driven solution electrochemistry

Plasma–liquid interactions enable various applications through the generation of a large range of reactive species in solution. In this work, we report on the interaction of a pulsed atmospheric pressure glow-like discharge with a liquid anode. Particularly, the flux of hydroxyl (OH) radicals and electrons in the plasma at the liquid anode are measured by laser induced fluorescence (LIF) spectroscopy and current measurements to investigate the role of OH and electrons in plasma-enabled redox chemistry in solution. The impact of the voltage pulse width, voltage amplitude, liquid temperature and conductivity on the OH density distribution was also investigated. We observed a significant OH density near the liquid surface, which showed a transition from a ring-shaped structure to a more uniform structure with increasing plasma power. This transition coincided with a similar transition in the plasma emission intensity and electron density profile. A Raman laser scattering study indicated that this transition can be attributed to an enhanced N2 mixing at larger plasma-dissipated powers. Besides, a time resolved measurement showed that the OH density segregates radially in the afterglow at velocities exceeding the gas velocity at room temperature due to enhanced gas convection resulting from the plasma-induced gas heating. While the OH flux was of the order of ∼1021 m−2 s−1, approximately two orders of magnitude lower than the electron flux, significant reduction in the solution occurs during the voltage pulse. Nonetheless, a slow oxidation was observed in the afterglow due to the much longer lifetime of OH radicals compared to electrons. The Faradaic efficiency of the liquid redox chemistry was evaluated with H cell measurements and showed a good agreement with a 1D liquid phase model with the measured electron and OH fluxes as the input. This result shows the capability to quantitatively describe the plasma-driven solution electrochemistry for a model redox couple based on OH and electron driven reactions.

Derivation of Plasma Propagation Velocity by Visual and Electrical Composite Analysis

For the plasma generated as a stationary wave in a vacuum vessel, the propagation velocity of the plasma was derived by a simple composite analysis consisting of a plasma emission image and a time fluctuation of the plasma current. The propagation wavelength of the plasma was derived from the shading interval length of the plasma emission intensity obtained by the digital camera in the visual analysis, and the propagation frequency of the plasma was derived from the time variation of the plasma current measured by a current probe in the electrical analysis. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Study of Some Plasma Characteristics in Dielectric Barrier Discharge (DBD) System

In this present paper, an experimental study of some plasma characteristics in dielectric barrier discharge (DBD) system using several variables, such as different frequencies and using two different electrodes metals(aluminium (Al) and copper (Cu)), is represented. The discharge plasma was produced by an AC power supply source of 6 and 7 kHz frequencies for the nitrogen gas spectrum and for two different electrodes metals(Al and Cu). Optical emission spectrometer was used to study plasma properties (such as electron temperature ( ), electron number density ( ), Debye length ( ), and plasma frequency ( )). In addition, images were analysed for the plasma emission intensity at atmospheric air pressure.

Feasibility of two-dimensional die-level plasma process monitoring using spatially resolvable optical emission spectrometers

We demonstrated the feasibility of die-level process monitoring using spatially resolvable optical emission spectrometers (SROESs) in a low-pressure plasma reactor. The spatially resolved Ar emission line intensities and their intensity ratios were obtained from uniformly generated and intentionally perturbed plasma by inserting conductors of known sizes at known locations. The sheath and perturbed plasma distributions were identified from the Ar emission line intensity and their intensity ratio in the lateral direction to the SROES installment orientation. However, the resolution in the axial direction was insufficient to identify the sheath and perturbed the plasma distribution. We propose the superimposition of measured plasma emission intensities from two perpendicularly aligned SROESs for die-level process monitoring. The insufficient axial resolution can be compensated for by the lateral resolution of another SROES, and die-level process monitoring using SROES can thus be realized.

Experimental study on the ignition process of a pulsed capacitively coupled RF discharge: Effects of gas pressure and voltage amplitude

The effects of gas pressure and voltage amplitude on the ignition process of a pulse capacitively coupled RF argon discharge are experimentally investigated. The electron density is measured by a hairpin probe, the spatiotemporal distribution of the electron impact excitation dynamics is determined by phase resolved optical emission spectroscopy, and the electrical parameters are obtained by analyzing the measured current and voltage waveforms. In this work, the pulse plasma is ignited with few initial electrons, so the ignition process behaves like gas breakdown. Based on the measured RF breakdown curve, the gas pressures and voltage amplitudes are selected, and then different characteristics of ignition processes are compared and discussed in detail. Particularly, the spatiotemporal pattern of the electron impact excitation rate obtained within the selected pressure range, as well as other results, aid the intuitive understanding of a typical “V-shaped” RF breakdown curve. At lower pressures, the excitation pattern exhibit shorter and tilted regions, ending at electrodes during the early ignition stage, implying a substantial electron energy loss, while at relatively high pressures, the excitation pattern becomes wider and less tilted, and the proportion of electron energy consumed by excitation processes increases. In addition, by increasing the voltage amplitude, the ignition is advanced and becomes more significant, manifesting a faster increase in discharge current and a stronger overshoot of RF power deposition. Meanwhile, at high voltage amplitude, the excitation pattern exhibits complex spatiotemporal distribution due to enhanced local electric field when the plasma emission intensity overshoots.

Determination of Magnesium and Sodium in Brine by Filter Paper Adsorption Laser-Induced Breakdown Spectroscopy

To make better use of brine, it is necessary to detect the element content before mining and processing. Laser-induced breakdown spectroscopy (LIBS) is a fast, simple, and flexible analytical method. However, the absorption of laser energy by liquid greatly reduces the plasma life and emission intensity. Although an improvement in the excitation source improves the characterization of the liquid, it makes the device more complicated and increases the cost. This article introduces a method for detecting Na and Mg in brine using LIBS with filter paper adsorption (FPA-LIBS). The limits of detection were 7.98 and 10.57 mg L−1 for sodium (Na) and magnesium (Mg), respectively. The results of six brine samples obtained using this method and inductively coupled plasma – optical emission spectrometry were compared. The discrepancy and recovery values were 1.34–8.94% and 91.64–107.35%, respectively. In addition, the results of the analysis after storage for five days were consistent with those measured initially. These results indicate that FPA-LIBS accurately determined Na and Mg in brine.

Characterization of plasma discharge channel in EDM using photography and spectroscopy

The plasma discharge channel of electrical discharge machining (EDM) brings a temperature of several thousands of degrees Celsius into the gap between the anode and cathode, resulting in the material removal by means of melting and evaporating. To investigate the characteristics of the plasma discharge channel, high-speed imaging technology is adopted to observe the dimension of the plasma. A spectrometer is used to calibrate the temperature of the plasma as well. Then the particle-in-cell with Monte Carlo collision (PIC-MCC) simulation is conducted to analyze the variation and distribution of the plasma and heat flux. The experimental results demonstrate that the discharge channels’ morphology obtained is stable after a few microseconds of expansion after breakdown, and the diameter of discharge channel and the current density increases with the increase of discharge current. Besides, the emission intensity of plasma increases with the increase of electrical parameters. However, the temperature of plasma decreases after high current, which means debris particles make machining condition deteriorated. Finally, the simulation results demonstrate that despite the current density increases, the maximum heat flux at the center of electrode reduces rapidly with the expansion of the discharge channel. The distribution of heat flux upon electrode evolves towards "wider and flatter", leading to larger debris particles than exceptional size.

Influence of the Distance between Focusing Lens and Target Surface on the Characteristics of Laser-excited Lead Plasma

The present work investigated the effect of distance from target surface on the parameters of lead plasma excited by 1064nm Q-switched Nd:YAG laser. The excitation was conducted in air, at atmospheric pressure, with pulse length of 5 ns, and at different pulse laser energies. Electron temperature was calculated by Boltzmann plot method based on the PbI emission spectral lines (369.03 nm, 416.98 nm, 523.48, and 561.94 nm). The PbI lines were recorded at different distances from the target surface at laser pulse energies of 260 and 280 mJ. The emission intensity of plasma increased with increasing the lens-to-target distance. The results also detected an increase in electron temperature with increasing the distance between the focal lens and the surface of the target in all laser energies under study. In addition, the electron number density was determined by using the Stark broadening method. The data illustrated that the electron number density was increased with increasing the distance from target surface, reaching the maximum at a distance of 11 cm for all pulse laser energy levels under study.

Enhancement of the ion flux to the substrate through high-voltage biasing in an electron cyclotron resonance plasma and its application to high-speed deposition of conductive carbon film

A method for enhancing ion flux to the substrate via high-voltage pulse biasing is investigated in an electron cyclotron resonance plasma. When high-voltage pulse biases above 500 V are applied to the stage, an increase in the stage current is observed, especially in the case of diverging magnetic field configurations in front of the bias stage. The growth and decay time constants of the plasma density and emission intensity are evaluated using a time-resolved Langmuir probe and emission spectroscopy while the pulse is on, and the enhancement of the ionization rate during the bias application is estimated using the zero-dimensional global model. The estimated density enhancement from the model is in good agreement with the measured one. From the numerical simulation of secondary electron trajectory, it is concluded that the electron confinement from the magnetic field is the key factor in plasma density enhancement during stage biasing. Using the high-density plasma produced by the bias voltage, conductive carbon is deposited at a high deposition rate of ∼4 nm s−1.

Plasma Discharge across Metal Nanoparticle Surfaces and Plasma-Driven CO2 Reduction to CO and Higher Order Hydrocarbons

: By discharging nanosecond high voltage pulses (up to 25kV) across an insulating substrate coated with metal nanoparticles (Au or Pt), significantly enhanced plasma generation is observed through local field enhancement on the surface of the nanoparticles. The structural integrity of metal nanoparticles can be maintained due to the low-temperature nature of this transient plasma. Electrostatic simulations based on high-resolution transmission electron microscope (HETEM) image of the nanoparticles demonstrate a 3-fold enhancement of the electric field near the nanoparticle surfaces. Since field emission of electrons, which initiates plasma generation, depends exponentially on electric field, this 3-fold increase in the local electric field can significantly enhance plasma generation at a given applied external voltage. Similar enhancement was also observed by discharging in a methane environment, revealing a 50X enhancement in methane upconversion to C2 and higher order hydrocarbons, presenting the exciting possibility of converting a notorious greenhouse gas into an energy dense hydrocarbon fuel. Under laser irradiation, we observe a 200-fold enhancement in the plasma emission intensity, indicating the important role that hot electrons play in the electron emission process. In addition to gas phase plasma discharge, we also demonstrate CO2 reduction to CO and higher order hydrocarbons by discharging nanosecond high votlage pulses in CO2-saturated water, as revealed by C2 Swan band emission, cryogenic NMR spectroscopy and liquid ion chromatography, which indicate clear peaks corresponding to formic acid, methanol, and acetic acid.

Spatial characterization of ns-laser induced Tungsten plasma in air using laser induced breakdown spectroscopy

The laser induced plasma (LIP) of tungsten is studied using space-resolved laser induced breakdown spectroscopy (SR-LIBS). In the present work a Q-switched Nd:YAG laser working at wavelength of 532 nm having maximum energy of 600 mJ and pulse duration of 7 ns with repetition rate of 1 Hz is used to generate the plasma onto the target sample surface. The LIP is generated at four different incident laser energy of 25, 50, 75 and 100 mJ. The spectra recorded from the different location of the LIP is analyzed to get the spatial distribution of plasma emission intensity of atomic and ionic transitions, plasma temperature and electron density. The plasma temperature is determined using the Boltzmann plot from the WI lines and that of the electron density estimated using Stark broadened profile of WI transition at 430.2 nm. The intensity of atomic lines as well as plasma parameters attain the maximum value at a distance ranging from 1.6 mm to 2.0 mm away from the target surface for the incident laser energy of 25 mJ and 50–100 mJ, respectively. The thermal conduction towards the solid target and radiative cooling of the plasma and conversion of thermal energy into kinetic energy is responsible for the decrease in plasma temperature towards the plasma edge away from the target surface whereas the direct ejection of low velocity neutrals around the target surface is responsible for lower temperature. Both these plasma parameters are also studied at incident laser energy of 25, 50, 75 and 100 mJ. The validity of the local thermodynamic equilibrium (LTE) is studied by Mc-Whirter criteria and diffusion length is estimated at different location of the plasma to understand the effect of inhomogeneous nature of LIP on the validity of LTE. The branching ratio method is employed to study optical thin condition of the LIP at different axial location as function of energy. From this an optimized spatial window is identified for the LIBS studies.

Splitting of laser-induced neutral and plasma plumes: hydrodynamic origin of bimodal distributions of vapor density and plasma emission intensity

The splitting of laser-induced plumes into the fast and slow components is usually explained by acceleration of ions by the ambipolar electric field. The kinetic simulations show that the splitting is observed even in neutral plumes due to the snow-plow effect. In plasma plumes, the simulations predict bimodal distributions of plasma emission intensity, where the slow maximum appears due to radiation absorption during the laser pulse, while the fast maximum emerges after the pulse at the plume edge. The snow-plow effect explains why the fast component is observed in a limited range of ambient pressure, exists during a limited time, and is characterized by larger degree of ionization.

Investigation on the electron density and temperature in a nanosecond pulsed helium plasma jet with Thomson scattering

AbstractDiagnostic of spatiotemporal distribution of the electron density and temperature of atmospheric pressure plasma jets is a difficult task, as Thomson scattering, which is the most used measurement, suffers from the stray light induced by Rayleigh scattering. In this paper, a new strategy is reported to restrain the stray light in Thomson scattering. With a physical mask placed at the output of a single spectrometer, the scattering signal is obtained with a relatively high ratio of signal to noise. The electron density in a pulsed kHz‐driven atmospheric plasma jet interacting with a dielectric target is measured by using this method. A donut‐shaped distribution of electron density is observed at an axial position 1 mm away from the nozzle. The radius of the ring structure decreases with the reduction of the gas flow rate. The electron density reaches a maximum of about 4.5 × 1020 m−3 at a delay time of 150 ns from the onset of the pulse. A similar time relationship between the electron density and plasma emission intensity is also identified. Higher applied voltage leads to a faster increasing and a higher peak value of the electron density. Pulse width shows little impact on the generation and decay of electrons.