Session DT22: Nonequilibrium Kinetics of Low-temperature Plasmas

Electron kinetics in low temperature plasmas

Low-temperature plasma (LTP) can be broadly defined as a quasi-neutral mixture of charged and neutral species with electron mean energy about the ionization potential of atoms and molecules. Its highly non-equilibrium nature is the source of numerous instabilities and collective effects observed in nature and the foundation for the widespread use of LTP in modern technologies. Collective behavior of magnetized space plasmas (associated with wave-particle interactions, turbulence, shock waves) and collisionless effects in plasma reactors (associated with stochastic electron heating and anomalous skin effect) call for a unified picture. This paper uncovers common features of electron kinetics at vastly different time- and space scales, from solar wind plasma to semiconductor manufacturing and nano-technologies. Nonlocal electron kinetics in low-pressure radio frequency (RF) discharges resemble collisionless effects in space science, fusion, terahertz technology, and plasmonics. Understanding scaling laws and closure rules for selecting kinetic and fluid models is critical for efficient plasma modeling and full utilization of the beneficial LTP properties. We describe common effects associated with the formation of electron groups in LTPs due to electron trapping by ambipolar electric fields, velocity filtration via magnetic focusing, and the nonlocal electro-dynamic related to violation of Ohm’s law. Using kinetic models of the solar wind and gas discharges as examples, we illustrate the formation of double layers and the associated appearance of different electron groups in glow discharges and in expanding plasmas of solar wind and magnetic nozzles. We also briefly discuss the runaway electrons responsible for a wide range of physical phenomena from nano- and picoscale breakdown of dielectrics to lightning initiation. Understanding the electron kinetics of LTPs could promote scientific advances in several topics of plasma physics and accelerate modern plasma technologies.   

Dependence of macro-kinetic paraments on the local electric field and mean energy in Nitrogen

The fluid models are frequently used to describe a non-thermal plasma such as a streamer discharge. The required electron transport data and rate coefficients for the fluid model are parametrized using the local-mean-energy approximation (LMEA) and the local field approximation (LFA). We performed Monte Carlo simulations in Nitrogen gas with step changes in the E/N (reduced electric field) to study the behavior of the transport properties in the transient phase. Our results show that rates equilibrate faster for step ups compared to step downs of the same amount. Also, the mean electron energy takes a considerably longer time to reach steady-state values compared to transport rates. During the transient phase of the simulation, we extract the mean energy and the corresponding transport parameters and rate coefficients. Our results show that for a given mean energy the electron drift has a significant spread and depends on the transient path. However, the high energy threshold rates such as ionization have a low spread. The use of electron mean energy to parametrize electron drift would lead to erroneous results.In either approach (LMEA or LFA), E/N would be a better parameter for electron drift.   

N2(A3Σu+,v) Energy Transfer Kinetics in Reacting N2-CO2-CH4 Plasmas

Energy transfer from metastable N₂(A³Σ_u⁺,v=0,1) molecules in plasmas sustained in mixtures of nitrogen with CO₂, CH₄, and H₂ is studied using Tunable Diode Laser Absorption Spectroscopy. The plasma is generated at 150 Torr using a repetitive ns pulse discharge sustained between two parallel copper electrodes external to a quartz cell. The pulse repetition rate is up to 100 kHz, with up to 50 pulses produced during each discharge burst, and the burst repetition rate of 20-40 Hz. The plasma remains diffuse and is confined between the electrodes, such that the absorption path is well defined. During the experiment, time-resolved, absolute populations of N₂(A³Σ_u⁺) are measured. The experimental data are compared with kinetic modeling to identify the dominant N₂(A³Σ_u⁺) generation and decay processes. Adding CO₂, CH₄, or H₂ to the mixture does not affect the N₂(A³Σ_u⁺) quenching in the beginning of the burst due to their slow quenching rates. However, dissociation of the added gases into H and O atoms results in a faster N₂(A³Σ_u⁺) quenching. Therefore, the increase of the N₂(A³Σ_u⁺) quenching rate can be used to indirectly indicate the enhanced reactivity of the mixture. The experimental results also show that methane results in a rapid vibrational relaxation of N₂(A³Σ_u⁺) molecules.   

Time-resolved CO2, CO, and N2 Vibrational Populations in In Ns Pulse Discharge Plasmas

Time-resolved CO₂, CO, and N₂ vibrational populations and translational-rotational temperature are measured in a ns pulse burst discharge plasma. The discharge is sustained in a CO₂-N₂ mixture slowly flowing through a rectangular cross section quartz channel, with two parallel plate electrodes external to the channel. CO₂ and CO vibrational populations are measured by mid-IR, tunable Quantum Cascade Laser (QCL) Absorption Spectroscopy, and N₂ vibrational populations are measured by the ns broadband, collinear CARS. The results indicate that a ns pulse discharge operated at a high pulse repetition rate, 100 kHz, results in a modest vibrational excitation of N₂ and CO₂. The number density of CO, a major product of CO₂ dissociation, is also inferred from the QCL measurements. The present diagnostics allow in situ monitoring of the time-resolved, state-specific CO₂ vibrational excitation and relaxation during the discharge and in the afterglow, along with the time-resolved CO vibrational populations. This makes possible isolating and quantifying two major mechanisms of CO₂ dissociation, (i) decay via excited electronic states populated by electron impact, and (ii) stepwise anharmonic vibration-vibration (V-V) pumping of the asymmetric stretch vibrational mode.   

Kinetics of Atomic and Metastable Species in N2 and H2-N2 Ns Pulse Discharges

Time-resolved, absolute number densities of metastable N₂(A³Σ_u⁺,v=0,1) molecules, ground state N and H atoms, and rotational-translational temperature are measured by TDLAS and TALIF in nitrogen and N₂-H₂ plasmas during and after a ns pulse discharge burst. Both N₂(A³Σ_u⁺,v) populations and the rate of N atom generation decrease significantly during the ns pulse discharge burst. Comparison of the measurement results and the modeling predictions indicates an additional major channel of N₂ dissociation in the plasma, by energy pooling in collisions of two N₂(A³Σ_u⁺) molecules. Additional measurements in a 1% H₂-N₂ mixture demonstrate a further significant reduction of N₂(A³Σ_u⁺,v) populations due to the rapid quenching by H atoms, which also diminishes the contribution of N₂(A³Σ_u⁺,v) dissociation, such that the N atom number density decreases significantly. On the other hand, the rate of H atom generation, produced predominantly by the dissociative quenching of the excited electronic states of nitrogen by H₂, remains about the same during the burst. The present quantitative results will be used for the further analysis of plasma-assisted catalysis generation of ammonia.   

The case for CO2 decomposition in plasma through vibrational activation: a closer look at the vibrational kinetics in a high excitation regime

Dissociation of CO₂ in plasmas has been shown to be theoretically and empirically promising. However, the bridge between throughput and underlying mechanisms is often brittle. Because at typical plasmas conditions vibrational excitation is often the dominant energy channel, it is natural to wonder how that excitation mechanism relates to the ultimate goal of dissociation. In this work we investigate the role of vibrationally activated CO₂ in its decomposition by modelling and studying CO₂ gas in a high vibrational excitation regime sustained by high power MIR lasers. We pinpoint and quantify three important aspects for maintaining vibrational non-equilibrium: energy deposited per molecule, time window of energy deposition and selectivity to the asymmetric stretch mode. Preliminary results show that, to achieve fully vibrational assisted dissociation, about 0.1 eV has to be deposited within 100 mean collisions of a CO₂ molecule (or about 25 ns at 1bar and 300K), after which the system starts to relax and efficiency of dissociation through vibrational ladder climbing starts plateauing. If the energy input is not fully selective to the asymmetric stretch mode, as would be the case in plasma activation of CO₂, more energy per molecule is needed in a shorter time window.   

Kinetic Mechanisms in CO2-N2 Plasmas

In this work we undertake a joint modelling and experimental investigation to study the impact of N₂ on the overall CO₂ plasma conversion. We perform our simulations solving a Boltzmann-chemistry 0D self-consistent kinetic model with the LoKI (LisbOn Kinetics) tool [1] and we compare our results with experimental data measured in low-pressure DC glow discharges.

The admixture of N₂ has a beneficial impact on CO₂ decomposition [2]. Several reasons can be assigned to it and will be discussed at the conference. Understanding the impact of the different processes on the overall kinetics, along with the validation against experimental data, will contribute to further develop the existing models [2,3] and to better control and enhance CO₂ conversion.

*This work was partially supported by the European Union's Horizon 2020 research and innovation programme under grant agreement MSCA ITN 813393, and by Portuguese FCT-Fundação para a Ciência e a Tecnologia, under projects UIDB/50010/2020 and UIDP/50010/2020.

[1] A. Tejero-del-Caz et al, Plasma Sources Sci. Technol. 28 (2019) 073001 [https://nprime.tecnico.ulisboa.pt/loki]

[2] L Terraz et al, J. Phys. D: Appl. Phys. 53 (2020) 094002

[3] A F Silva et al, Plasma Sources Sci. Technol. 29 125020 (2020)