Transient Plasmas Research Articles

Sensitivity Analysis of Excited-State Population in Plasma Based on Relative Entropy.

A highly versatile evaluation method is proposed for transient plasmas based on statistical physics. It would be beneficial in various industrial sectors, including semiconductors and automobiles. Our research focused on low-energy plasmas in laboratory settings, and they were assessed via our proposed method, which incorporates relative entropy and fractional Brownian motion, based on a revised collisional-radiative model. By introducing an indicator to evaluate how far a system is from its steady state, both the trend of entropy and the radiative process' contribution to the lifetime of excited states were considered. The high statistical weight of some excited states may act as a bottleneck in the plasma's energy relaxation throughout the system to a steady state. By deepening our understanding of how energy flows through plasmas, we anticipate potential contributions to resolving global environmental issues and fostering technological innovation in plasma-related industrial fields.

Dual-comb spectroscopy in the deep ultraviolet

This Letter reports dual-frequency-comb spectroscopy in the deep ultraviolet used to characterize transient laser-produced plasmas. Dual-comb spectroscopy at these wavelengths enables access to a large number of strong electronic transitions in neutral and ionized atoms and molecules. This broadband dual-frequency-comb system at 265 nm measures multiple transitions in neutral and singly ionized iron atoms in a laser-produced plasma. Analysis of absorption spectra yields time-resolved ion and neutral column densities, excitation temperatures, and electron densities, which are measured down to 1014cm−3 levels. The results provide insights into low-temperature plasma properties.

Data-driven electrical conductivities of dense plasmas

A wide range of theoretical and computational models have been developed to predict the electrical transport properties of dense plasmas, in part because dense plasma experiments explore order-of-magnitude excursions in temperature and density; in experiments with mixing, there may also be excursions in stoichiometry. In contrast, because high pressures create transient and heterogeneous plasmas, data from experiments that isolate transport are relatively rare. However, the aggregate of our datasets continues to increase in size and plays a key role in the validation of transport models. This trend suggests the possibility of using the data directly to make predictions, either alone or in combination with models, thereby creating a predictive capability with a controllable level of agreement with the data. Here, such a data-driven model is constructed by combining a theoretical model with extant data, using electrical conductivity as an example. Discrepancy learning is employed with a theoretical model appropriate for dense plasmas over wide ranges of conditions and a dataset of electrical conductivities in the solid to expanded warm dense matter regimes. The resulting discrepancy is learned via a radial basis function neural network. Regularization of the network is included through centers chosen with silhouette scores from k-means clustering. The covariance properties of each cluster are used with a scaled Mahalanobis distance metric to construct anisotropic basis functions for the network. The scale is used as a hyperparameter that is used to optimize prediction quality. The resulting predictions agree with the data and smoothly transition to the theoretical model away from the data. Detailed appendices describe the electrical conductivity model and compare various machine-learning methods. The electrical conductivity data and a library that yields the model are available at GitHub.

How transient effects can change the inferred plasma temperatures when using K-shell sulfur line ratios in a time dependent iron sulfide plasma

This paper describes how the steady state atomic kinetics approximation can underestimate the electron temperature determined from K-shell lines in ps-time-scale transient plasmas. In particular, we model the temperature determination of solid FeS targets used in opacity experiments at the Orion laser facility from the ratio of sulfur He-α to Ly-α lines. Such experiments use short-pulse lasers to heat a thin microdot of FeS buried in a plastic target to temperatures of more than 1 keV and densities of approximately 1–2 g/cm3. Using atomic kinetics calculations based on a temperature history from a radiation hydrodynamic simulation of the target evolution, the peak temperature inferred from the sulfur line ratios is 1.29 keV at 3.1 ps as compared with the input peak temperature of 1.41 keV at 2.0 ps. There is a time lag of 1.2 ps at the peak, and an overall 0.5 ps time lag in the temporal history of the temperature as the plasma cools over the next 10 ps.

Low-temperature plasma simulation based on physics-informed neural networks: Frameworks and preliminary applications

Plasma simulation is an important, and sometimes the only, approach to investigating plasma behavior. In this work, we propose two general artificial-intelligence-driven frameworks for low-temperature plasma simulation: Coefficient-Subnet Physics-Informed Neural Network (CS-PINN) and Runge–Kutta Physics-Informed Neural Network (RK-PINN). CS-PINN uses either a neural network or an interpolation function (e.g., spline function) as the subnet to approximate solution-dependent coefficients (e.g., electron-impact cross sections, thermodynamic properties, transport coefficients, etc.) in plasma equations. Based on this, RK-PINN incorporates the implicit Runge–Kutta formalism in neural networks to achieve a large-time step prediction of transient plasmas. Both CS-PINN and RK-PINN learn the complex non-linear relationship mapping from spatiotemporal space to the equation's solution. Based on these two frameworks, we demonstrate preliminary applications in four cases covering plasma kinetic and fluid modeling. The results verify that both CS-PINN and RK-PINN have good performance in solving plasma equations. Moreover, RK-PINN has the ability to yield a good solution for transient plasma simulation with not only large time steps but also limited noisy sensing data.

On the Dynamics of Transient Plasmas Generated by Nanosecond Laser Ablation of Several Metals.

The dynamics of transient plasma generated by UV ns-laser ablation of selected metals (Co, Cu, Ag, Bi) were investigated by the Langmuir Probe method in angle- and time-resolved modes. Multiple ionic and electronic structures were seen for all plasmas with some corresponding to anions or nanoparticle-dominated structures. The addition of an Ar atmosphere energetically confined the plasma and increased the charge density by several orders of magnitude. For pressure ranges exceeding 0.5 Pa fast ions were generated in the plasma as a result of Ar ionization and acceleration in the double layer defining the front of the plasma plume. Several correlations between the target nature plasma properties were attempted. The individual plasma structure expansion velocity increases with the melting point and decreases with the atomic mass while the corresponding charged particle densities decrease with the melting point, evidencing the relationship between the volatility of the sample and the overall abated mass.

Insight into the plasma oxidation process during pulsed laser deposition

AbstractThe dynamics of transient plasmas generated by ns laser ablation of Ag targets under various O2gas pressures (10−5–10 Pa) were investigated by implementing angle‐ and time‐resolved Langmuir probe (LP) analysis and space‐ and time‐resolved optical emission spectroscopy (OES). The effects of an O2atmosphere on silver oxide formation and the ionic kinetics and the overall plasma energy were systematically investigated. Measurements performed in the LP floating regime revealed the presence of three‐ionic structures with kinetic energies varying from 0.8 to 350 eV, which can be tailored by the addition of O2. The time‐modulated regime of electron temperature in the 0.5–3 eV observed for pressures higher than 2 Pa of O2could be explained by the formation and dissociation of silver oxide molecules. The energetic balance sustains molecular oxide formation, as confirmed by OES measurements, which revealed emission signatures of AgO (B 2Π3/2and X 2Π3/2) in the 350 nm (B 2Π3/2) and 460 nm (X 2Π3/2) spectral regions. Finally, thin films were deposited to understand the range of possibilities offered by pulsed laser deposition of Ag in O2. The morphology and atomic structure of the films were analysed for large‐scale deposition. Silver oxides were detected in the films starting from 2 Pa, while at higher values, we found mixtures of different AgxOyphases. A cauliflower‐like morphology was dominant above 5 Pa of O2atmosphere pressures, which signals the formation of the AgO phase in the deposited film. Our results demonstrate that the properties of deposited thin films are influenced by plasma properties.

In situ optical and electrical analysis of transient plasmas generated by ns-laser ablation for Ag nanostructured film production

The continuous effort for transitioning pulsed laser deposition (PLD) technique from an experimental tool into an industrial one can be sustained by the use of complex tools for in situ real-time monitoring of the deposition process. Langmuir Probe (LP) and optical emission spectroscopy (OES) measurements were used for plasma monitoring during PLD of silver films under various Ar pressure conditions. The LP measurements revealed a multi-structured distribution of the ions, which was strongly influenced by the increase of Ar pressure. The generation of highly energetic ions with the addition of Ar and the degree of Ar gas ionization was confirmed by OES measurements. A perturbative effect was seen under pressures higher than 10 Pa, where the presence of high electronic charges leads to an unstable behavior of plasma in the vicinity of the LP. The effect of these complex dynamics on the structure and quality of the deposited thin film was investigated revealing a strong correlation between the electronic distribution in plasma and thin film thickness profile.

Langmuir Probe Technique for Plasma Characterization during Pulsed Laser Deposition Process

The history of pulsed laser deposition (PLD) and transient plasmas generated by laser ablation is intertwined with the development of various techniques for its fundamental understanding. Some diagnostic tools have been developed to better suit the rapid transient nature of the plasma (space and time dependence of all parameters, fast decay and complex chemistry inside the plasma), whereas others have been adapted from basic plasma physics studies. Langmuir probe method has been used as a real-time in situ diagnostic tool for laser ablation and later for PLD. It remains a useful tool for the PLD community arsenal, which can easily be adapted to the development of new lasers and ablation regimes and new deposition configuration, being one of the most versatile techniques for plasma diagnostics. It is the cornerstone on which charge particles are analyzed and has led to several important discoveries, such as multiple peak distribution, selective acceleration during expansion, plume splitting, plasma turbulences and fluctuations. However, because the Langmuir probe theory adaptation from classical plasma physics is not straightforward, it might lead to misinterpretation and often incorrect analysis of data. This review analyzes the limits and understanding of the technique as a foundation for attaining its full potential, which can impact the way PLD is used. This is especially useful for the pressing need of real-time, in-situ diagnostics and feedback loops for systematic semi-industrial implementation of the PLD technique.

Effects of pulse rise time and repetition frequency on nanosecond pulsed plasma ignition for combustion

Transient plasma ignition (TPI) employs highly non-equilibrium plasmas driven by nanosecond high-voltage pulses for combustion ignition. The effects of rise time and pulse repetition frequency (PRF) on combustion ignition are evaluated when transient plasmas, powered by 10-ns, kilovolt pulses, are employed to initiate combustion of lean, stoichiometric, or rich methane-dry air mixtures at atmospheric pressure. It was found that the plasmas driven by high voltage pulses with a faster rise time and PRF between 6–10 kHz enhanced combustion with higher peak pressure and/or shorter ignition delay. A mode transition in the plasma, corresponding to the combustion initiation, was observed based on voltage and current waveforms of four consecutive pulsed plasmas driven by a pulse burst at kilohertz PRFs. Although benefits in combustion ignition from a shorter rise time and optimal PRF were observed for all three equivalence mixture ratios, the highest peak pressure and shortest ignition delay were associated with the stoichiometric mixtures. In addition, the gas temperature of the transient plasma was measured by determining the rotational temperature of the 2nd positive system of nitrogen to be ∼1000 K for the first pulse and increased up to 2000 K after the application of the 4th pulse in a four-pulse burst TPI.

Long-range oscillations of a laser-produced carbon plasma recorded by an electrostatic energy analyzer

Understanding peculiar dynamics in transient laser-produced plasmas is of great importance for a number of applications such as energetic ion production, ion implantation, laser micro-machining, etc. Continuing a series of papers on the electrical and optical characterization of laser ablation plasma plumes, we report here on the observation of oscillations in the ionic and electronic currents at a long distance (17.5 cm) from the ablation target. An electrostatic energy analyzer was used to finely probe the dynamics of charge carriers in a carbon plasma generated in vacuum by KrF excimer laser ablation of a graphite target. A short-time Fourier transform data processing approach was applied to extract valuable information from the recorded currents. This method not only reveals different frequencies for two distinct expanding plasma structures, but also unveils different trends in their temporal evolution. These new experimental results corroborate previous studies supporting a scenario based on the plasma structuring in multiple double layers. A discussion with respect to an alternative scenario based on low-frequency sheath instabilities is also presented.

Characteristics of plasma stream evolution in a pulsed plasma accelerator

The time-integrated spectroscopic measurements of argon plasma are carried out to characterize the transient plasma stream produced in a coaxial pulsed plasma accelerator. Transient plasmas produced in these types of devices have a temporal change in density and temperature. The temporal evolution of the plasma stream and its characteristic variation are studied from a time of 50–300 μs in steps of 50 μs. Analysis of different spectral lines shows the existence of some metastable states of Ar+ having a long lifetime. The evolution of different Ar+ transitions to metastable and non-metastable lower levels is observed for different time frames. From the Stark broadening and Boltzmann plot method assuming the partial local thermodynamic equilibrium model, the plasma densities and excitation temperatures are estimated. The temporal evolution study shows a decrease in plasma densities from 1.96 × 1021 m−3 at 50 μs to 1.23 × 1020 m−3 at 300 μs after the initiation of plasma formation. A reduction in excitation temperature is observed from 0.86 to 0.77 eV. Based on the density profile, the estimated energy density of the plasma stream shows a time-dependent decreasing trend from 0.06 to 0.004 MJ/m2 during the observed period. The results may be useful for optimization of the plasma stream in pulsed plasma accelerators concerning its different parameters.

Formation of transient high-β plasmas in a magnetized, weakly collisional regime

We present experimental data providing evidence for the formation of transient ( ${\sim }20\ \mathrm {\mu }\textrm {s}$ ) plasmas that are simultaneously weakly magnetized (i.e. Hall magnetization parameter $\omega \tau > 1$ ) and dominated by thermal pressure (i.e. ratio of thermal-to-magnetic pressure $\beta > 1$ ). Particle collisional mean free paths are an appreciable fraction of the overall system size. These plasmas are formed via the head-on merging of two plasmas launched by magnetized coaxial guns. The ratio $\lambda _{\textrm {gun}}=\mu _0 I_{\textrm {gun}}/\psi _{\textrm {gun}}$ of gun current $I_{\textrm {gun}}$ to applied magnetic flux $\psi _{\textrm {gun}}$ is an experimental knob for exploring the parameter space of $\beta$ and $\omega \tau$ . These experiments were conducted on the Big Red Ball at the Wisconsin Plasma Physics Laboratory. The transient formation of such plasmas can potentially open up new regimes for the laboratory study of weakly collisional, magnetized, high- $\beta$ plasma physics; processes relevant to astrophysical objects and phenomena; and novel magnetized plasma targets for magneto-inertial fusion.

Non-Linear Behaviors of Transient Periodic Plasma Dynamics in a Multifractal Paradigm

In a multifractal paradigm of motion, nonlinear behavior of transient periodic plasmas, such as Schrodinger and hydrodynamic-type regimes, at various scale resolutions are represented. In a stationary case of Schrodinger-type regimes, the functionality of “hidden symmetry” of the group SL (2R) is implied though Riccati–Gauge different “synchronization modes” among period plasmas’ structural units. These modes, expressed in the form of period doubling, damped oscillations, quasi-periodicity, intermittences, etc., mimic the various non-linear behaviors of the transient plasma dynamics similar to chaos transitions scenarios. In the hydrodynamic regime, the non-Newtonian behavior of the transient plasma dynamics can be corelated with the viscous tension tensor of the multifractal type. The predictions given by our theoretical model are confronted with experimental data depicting electronic and ionic oscillatory dynamics seen by implementing the Langmuir probe technique on transient plasmas generated by ns-laser ablation of nickel and manganese targets.

Space-and time-resolved optical investigations on ns-laser produced plasmas on various geological samples

Experimental and theoretical investigations were performed on transient plasmas generated by ns-laser ablation on ten mineral samples. Space-and time-resolved optical emission spectroscopy and ICCD fast camera imaging were implemented to understand the kinetics of the ablated particles and how the samples physical properties can be reflected by those of the plasma plume. Strong correlations were found between the global and individual plasma velocity and the melting temperature of the geomaterials. The thermal energy of the plasma expressed through the excitation temperature of the ejected particle is correlated with the same target physical parameter, results corelated with the presence of strong thermal ejection mechanisms. To better understand the global empirical laws, we implemented the fractal model to interpret the results. The model manages to emulate the target-plasma correlations using a multifractal hydrodynamic approach and adequately calibrating the theoretical model on the empirical tools used to describe the laser produced plasmas.

Addendum: Mapping electron dynamics in highly transient EUV photon-induced plasmas: a novel diagnostic approach using multi-mode microwave cavity resonance spectroscopy (2018 J. Phys: D. Appl. Phys. 52 034004)

A new approach for an in-line beam monitor for ionizing radiation was introduced in a recent publication (Beckers J et al 2018 J. Phys. D: Appl. Phys. 52 034004). Due to the recent detection and investigation of an additional third decay regime of the afterglow of an extreme ultraviolet photon-induced plasma described in a later article (Platier B et al 2020 Appl. Phys. Lett. 116 103703) there is an additional reason for a minimum number of photons for this approach to work. Near or below this threshold, we explain that the response time of the diagnostic method is a limiting factor. Further, a second limit for the number of photons within a pulse is formalized related to the trapping of highly energetic free electrons.

Charged Particle Oscillations in Transient Plasmas Generated by Nanosecond Laser Ablation on Mg Target

The dynamics of a transient plasma generated by laser ablation on a Mg target was investigated by means of the Langmuir probe method and fractal analysis. The empirical data showcased the presence of an oscillatory behavior at short expansion times (<1 μs) characterized by two oscillation frequencies and a classical behavior for longer evolution times. Space- and time-resolved analysis was implemented in order to determine main plasma parameters like the electron temperature, plasma potential, or charged particle density. In the motion fractal paradigm, a theoretical model was built for the description of laser-produced plasma dynamics expressed through fractal-type equations. The calibration of such dynamics was performed through a fractal-type tunneling effect for physical systems with spontaneous symmetry breaking. This allows both the self-structuring of laser-produced plasma in two structures based on its separation on different oscillation modes and the determination of some characteristics involved in the self-structuring process. The mutual conditionings between the two structures are given as joint invariant functions on the action of two isomorph groups of SL(2R) type through the Stoler-type transformation, explicitly given through amplitude self-modulation.

Multiple structure formation and molecule dynamics in transient plasmas generated by laser ablation of graphite

Transient plasmas generated by ns - laser ablation of graphite in air have been investigated by complementary optical diagnosis techniques to study complex multiple structures generated in carbon laser produced plasmas (C-LPP) during the expansion phase. The overall emission was investigated by ICCD fast camera imaging and revealed the structuring of C-LPP into four distinct structures with different evolutions. The dynamic of the C-LPP was analyzed on various expansion directions in order to understand the formation of the main CN dominant structure and of lateral “flaps” with dominant C2 emission. The kinetic and thermal energies of the resulted atoms, ions and molecules were investigated by means of space-and time-resolved optical emission spectroscopy and using the Boltzmann plot and Stark broadening approaches. The effect of the background gas pressure on the formation of molecular carbon structures is discussed and an optimum range of working pressure which maximizes the generation of C2 is found. Based on the acquired data a scenario for the complex carbon structures generation in C-PLL as function of the background gas is proposed.

Nanosecond second harmonic generation for electric field measurements with temporal resolution shorter than laser pulse duration

Electric field induced second harmonic (EFISH) generation using a ns pulse duration laser has been employed for time-resolved measurements of the axial and transverse electric field in a pin-to-pin ns pulse discharge in ambient air, at atmospheric pressure and at 2 bar. The results demonstrate that the time-varying electric field can be measured accurately over the duration of the laser pulse 15–20 ns long, with the temporal resolution (potentially sub-ns) limited by the response time of the detector. Each data set provides time-accurate electric field over a period of up to 10 ns. Time-resolved electric fields over longer time periods are measured by adjusting the laser Q-switch delay, and using the composite data obtained by overlapping the individual data sets. Absolute calibration of the electric field is obtained from the comparison of the experimental data with the Laplacian field distribution in the discharge gap before breakdown, taking into account the residual charge accumulation on the surface of the dielectric encapsulating the grounded pin electrode. The results demonstrate the feasibility of ns EFISH measurements in high-pressure transient plasmas, using widely available ns pulse duration Nd:YAG lasers.

Spectroscopic investigations of YAG-laser-driven microdroplet-tin plasma sources of extreme ultraviolet radiation for nanolithography

SynopsisHighly charged tin ions are the sources of extreme ultraviolet (EUV) light at 13.5-nm wavelength in laser-produced transient plasmas for next-generation nanolithography. Generating this EUV light at the required power, reliability, and stability however presents a formidable task that combines industrial innovations with challenging scientific questions. We will present work on the spectroscopy of tin ions in and out of YAG-laser-driven plasma and present a surprising answer to the key question: what makes that light?