CO$_2$ dissociation stimulated by vibrational excitation in non-equilibrium discharges has drawn lots of attention. Ns-discharges are known for their highly non-equilibrium conditions. It is therefore of interest to investigate the CO$_2$ excitation in such discharges. In this paper, we demonstrate the ability for monitoring the time evolution of CO$_2$ ro-vibrational excitation with a well-selected wavelength window around 2289.0 cm$^{-1}$ and a single CW quantum cascade laser (QCL) with both high accuracy and temporal resolution. The rotational and vibrational temperatures for both the symmetric and the asymmetric modes of CO$_2$ in the afterglow of CO$_2$ + He ns-discharge were measured with a temporal resolution of 1.5 $μ$s. The non-thermal feature and the preferential excitation of the asymmetric stretch mode of CO$_2$ were experimentally observed, with a peak temperature of $T_{v3, max}$ = 966 $\pm$ 1.5 K, $T_{v12, max}$ = 438.4 $\pm$ 1.2 K and $T_{rot}$ = 334.6 $\pm$ 0.6 K reached at 3 $μ$s after the nanosecond pulse. In the following relaxation process, an exponential decay with a time constant of 69 $μ$s was observed for the asymmetric stretch (001) state, consistent with the dominant deexcitation mechanism due to VT transfer with He and deexcitation on the wall. Furthermore, a synchronous oscillation of the gas temperature and the total pressure was also observed and can be explained by a two-line thermometry and adiabatic process. The period of the oscillation and its dependence on the gas components is consistent with a standing acoustic wave excited by the ns-discharge.
In this contribution, nanosecond pulsed discharges in N$_{2}$ and N$_{2}$/0.9% H$_{2}$O at atmospheric pressure (at 300 K) are studied with time-resolved imaging, optical emission spectroscopy and Rayleigh scattering. A 170 ns high voltage pulse is applied across two pin-shaped electrodes at a frequency of 1 kHz. The discharge consists of three phases: an ignition phase, a spark phase and a recombination phase. During the ignition phase the emission is mainly caused by molecular nitrogen (N$_{2}$(C-B)). In the spark and recombination phase mainly atomic nitrogen emission is observed. The emiss…
In this work, evolution of parameters of nanosecond repetitively pulsed (NRP) discharges in pin-to-pin configuration in air was studied during transient stage of initial twenty discharge pulses. Gas and plasma parameters in the discharge gap were measured using coherent microwave scattering (CMS), optical emission spectroscopy (OES) and laser Rayleigh scattering (LRS) for NRP discharges at repetition frequencies of 1, 10 and 100 kHz. Memory effects (when perturbations induced by the previous discharge pulse would not decay fully till the subsequent pulse) were detected for the repetition frequ…
This work investigates the development of nanosecond pulsed discharges in water ignited with the application of both positive and negative polarity pulses to submerged pin to plane electrodes. Optical diagnostics are used to study two main aspects of these discharges: the initiation phase, and the development phase. Nanosecond pulses up to 24 kV with 4 ns rise time, 10 ns duration and 5 ns fall time are used to ignite discharges in a 1.5 mm gap between a copper plate and a tungsten needle with radius of curvature of 25 um. Fast ICCD imaging is used to trace the discharge development over varyi…
In this work, we present an experimental study of nanosecond high-voltage discharges in a pin-to-pin electrode configuration at atmospheric conditions operating in single-pulse mode (no memory effects). Various discharge parameters, including voltage, current, gas density, rotational/vibrational/gas temperature, and electron number density, were measured. Several different measurement techniques were used, including microwave Rayleigh scattering, laser Rayleigh scattering, optical emission spectroscopy enhanced with a nanosecond probing pulse, fast photography, and electrical parameter measure…
In this work, the temporal decay of electrons produced by an atmospheric pin-to-pin nanosecond discharge operating in the spark regime was measured via a combination of microwave Rayleigh scattering (MRS) and laser Rayleigh scattering (LRS). Due to the initial energy deposition of the nanosecond pulse, a variance in local gas density occurs on the timescale of electron decay. Thus, the assumption of a constant collisional frequency is no longer applicable when electron number data is extracted from the MRS measurements. To recalibrate the MRS measurements throughout the electron decay period, …
The formation of the electron energy distribution function in nanosecond atmospheric nitrogen discharges is investigated by means of self-consistent solution of the chemical kinetics and the Boltzmann equation for free electrons. The post-discharge phase is followed to few microseconds. The model is formulated in order to investigate the role of the cross section set, focusing on the vibrational-excitation by electron-impact through resonant channel. Four different cross section sets are considered, one based on internally consistent vibrational-excitation calculations which extend to the whol…
This work presents an experimental study of a nanosecond repetitively pulsed dielectric barrier discharge interacting with a transient laminar flame propagating in a channel of height near the quenching distance of the flame. The discharge and the flame are of comparable size, and the discharge is favoured at a location where it is coupled with the reaction zone and burned gas. The primary goal is to determine how the discharge evolves on the time scale of the flame passage, with the evolution driven by the changing gas state produced by the moving flame front. This work complements the large …
The characteristics of atmospheric-pressure microdischarges excited by nanosecond high-voltage pulses are investigated in helium-nitrogen mixtures, as a function of the parameters of the excitation voltage pulses. In particular, cases of single-pulse excitation, unipolar and bipolar double-pulse excitation are studied, at different pulse durations, voltage amplitudes, and delay times (for the case of double-pulse excitation). Our investigations are carried out with a particle-simulation code that also comprises the treatment of the VUV resonance radiation in the plasma. The simulations allow g…
An influence of a high-voltage (3-17 kV) 20 ns pulse on a weakly-ionized low-pressure (0.1-10 Pa) capacitively-coupled radiofrequency (RF) argon plasma is studied experimentally. The plasma evolution after pulse exhibits two characteristic regimes: a bright flash, occurring within 100 ns after the pulse (when the discharge emission increases by 2-3 orders of magnitude over the steady-state level), and a dark phase, lasting a few hundreds μs (when the intensity of the discharge emission drops significantly below the steady-state level). The electron density increases during the flash and remain…
The focusing of a secondary shock wave was experimentally recorded during the implementation of an extended nanosecond high-current discharge of a cylindrical configuration in air. Using high-speed shadowgraph recording (up to 300 000 frames/s) of the emerging unsteady flow, cylindrically symmetric gas-dynamic discontinuities were visualized: a diverging shock wave moving from the discharge plasma boundary, a diverging contact discontinuity separating the shock-heated gas from the nonequilibrium region excited by the discharge plasma, and also a converging compression wave (secondary shock wav…
The plasma dry reforming reaction of methane with carbon dioxide is investigated in a nanosecond repetitively pulsed discharge, a type of plasma that offers one of the highest non-equilibrium and performance characteristics. The experiment's purpose was to examine the effect of varying the sequence of high-voltage pulses for the same plasma energy. We find that when successive pulses are closer than 500 $\rm μ$s, a memory-dominated regime gradually develops, which influences subsequent breakdown events. As a result, both a higher reactant conversion and more efficient use of the energy injecte…
The process of electron multiplication through the bouncing-like accelerated motion of electrons inside nanovoids formed owing to external electric fields in bulk liquid water is investigated using Monte Carlo simulations in Geant4-DNA. Our results show that the initial charge developed at the metal/liquid interface can be multiplied and expanded along the direction of the external electric field on a picosecond timescale, owing to collision-free interiors of the nanoruptures. Characteristic features of two different electron multiplication mechanisms are revealed and characterized. We find th…
The efficiency of the conversion of CO$_2$ into CO with nanosecond repetitively pulsed discharges (NRP) is investigated in a high pressure batch reactor. Stable discharges are obtained at up to 12~bar. By-products of CO$_2$ splitting are measured with gas chromatography. The energy efficiency is determined for a range of processing times, pulse energy, and fill pressures. The energy efficiency is found to be approximately 20% and is only weakly sensitive to the plasma operating parameters, i.e., the extent of CO$_2$ conversion is almost linearly-dependent on the specific energy input. A conver…
We report first ns-resolved absorption measurements of the ro-vibrational excitation of CO$_2$. The high temporal resolution of 8 ns is made possible by a fast detector (rise-time 5 ns), sensitive in the mid-infrared region. The resolution is achieved by a slow temperature scan of a quantum cascade laser and a segmented data capturing scheme. A repetitively pulsed ns discharge in 10% CO$_2$ + 90% He at 150 mbar and a repetition rate of 2 kHz is investigated. The evolution of the population densities of the different vibration modes as well as the associated vibrational and rotational temperatu…
Spatiotemporal dynamics of nanosecond pulsed discharge in the form of a fast ionization wave: self-consistent two-dimensional modeling and comparison with experiments under negative and positive polarity
Laboratory of Plasma Physics (CNRS, Ecole Polytechnique, Univ. Paris-Sud, Observatoire de Paris, Sorbonne Université, l’Institut Polytechnique de Paris), Ecole Polytechnique（等离子体物理实验室（法国国家科学研究中心、巴黎综合理工学院、巴黎南大学、巴黎天文台、索邦大学、巴黎理工学院），巴黎综合理工学院）
We investigate the development of meter long negative discharges and focus on their X-ray emissions. We describe appearance, timing and spatial distribution of the X-rays. They appear in bursts of nanosecond duration mostly in the cathode area. The spectrum can be characterized by an exponential function with 200 keV characteristic photon energy. With nanosecond-fast photography we took detailed images of the pre-breakdown phenomena during the time when X-rays were registered. We found bipolar discharge structures, also called "pilot systems", in the vicinity of the cathode. As in our previous…
We present a novel microplasma flow reactor using a dielectric barrier discharge (DBD) driven by repetitively nanosecond high-voltage pulses. Our DBD-based geometry can generate a non-thermal plasma discharge at atmospheric pressure and below in a regular pattern of micro-channels. This reactor can work continuously up to about 100 minutes in air, depending on pulse repetition rate and operating pressure. We here present the geometry and the main characteristics of the reactor. Pulse energies of 1.9 $μ$J and 2.7 $μ$J per channel at atmospheric pressure and 50 mbar, respectively, have been dete…
The use of dielectric barrier discharge (DBD) plasmas has become a practical way to carry out surface treatment and one seeks to do it in a more efficient way, which requires to have control of the plasma parameters like rotational and vibrational temperatures (T_rot and T_vib). Since the T_vib of an atmospheric pressure plasma jet is an important parameter related to improvement of surface treatments, in this work it was analyzed two methods to increase the values of T_vib in the DBD plasma. One of the methods is to reduce the exit size of the DBD reactor, and the other is to change the gas f…
Non-equilibrium vibrational dissociation of CO$_2$ at low translational-rotational temperatures $T$ is investigated computationally for conditions of microwave induced plasmas. Semi-analytic treatment of vibrational relaxation in CO$_2$ in shock tube and acoustic experiments is summarized. A state-to-state vibrational kinetics model applied for the simulations is benchmarked and adjusted to the relaxation times obtained in gas dynamic experiments. The governing parameter $Q/n^2_0$ has been introduced, where $Q$ is the specific volumetric power coupled in plasma and $n_0$ is the initial number …
Nanosecond plasmas in liquids are being used for water treatment, electrolysis or biomedical applications. The exact nature of these very dynamic plasmas and most important their ignition physics are strongly debated. The ignition itself may be explained by two competing hypothesis: (i) ignition via field effects or (ii) via electron multiplication in nanovoids. Both hypothesis are supported by theory, but experimental data are very sparse due to the difficulty to monitor the very fast processes in space and time. In this paper, we are using fast camera measurements and fast emission spectrosc…
The non-equilibrium ro-vibrational distribution functions of molecules in a plasma can heavily influence the discharge operation and the plasma-chemistry. A convenient method for measuring the distribution function is by means of coherent anti-Stokes Raman scattering (CARS). CARS spectra are measured in a ns-pulsed plasma between two parallel, 1 mm spaced molybdenum electrodes in nitrogen at 200 mbar with pulse durations of 200 ns / 250 ns and a repetition rate of 1 kHz. The CARS spectra are analyzed by a fitting routine to extract information about the vibrational excitation of the nitrogen m…
The understanding of the ro-vibrational dynamics in molecular (near)-atmospheric pressure plasmas is essential to investigate the influence of vibrational excited molecules on the discharge properties. In a companion paper, results of ro-vibrational coherent anti-Stokes Raman scattering (CARS) measurements for a nanosecond pulsed plasma jet consisting of two conducting molybdenum electrodes with a gap of 1 mm in nitrogen at 200 mbar are presented. Here, those results are discussed and compared to theoretical predictions based on rate coefficients for the relevant processes found in the literat…
This study is designed to explore the two dimensional temperature maps of the atmospheric argon discharge consisting of pin-to-plane electrodes supplied by a high voltage DC source. After checking the stability of the micro discharge, the two dimensional image plane focused by a quartz lens was scanned by the fiber probe driven by a 3D Mobile Platform. The rotational and vibrational temperatures are calculated using nitrogen emissions collected by the high resolution spectrometer and high sensitive intensified charge coupled device (ICCD). The rotational temperature varies from 1558.15 K to 26…
Optical emission spectroscopy applied to non-equilibrium plasmas in molecular gases can give important information on basic plasma parameters, including the rotational, vibrational temperatures and densities of the investigated radiative states. In order to precisely understand the non-equilibrium of rotational-vibrational state distribution from investigated spectra without limiting presumptions, a state-by- state temperature-independent fitting procedure is the ideal approach. In this paper we present a novel software tool developed for this purpose, freely available for scientific community…
Paper presents a review of modern works on the gasdynamic flow control using a heat release in highly nonequilibrium pulsed plasma. The major attention is paid to the effects based on ultrafast (on nanosecond time scale at atmospheric pressure) local heating of the gas, since at present the main successes in high-speed flow control using gas discharges are associated with namely this thermal effect. The physical mechanisms that control the interaction of the discharges with gas flows are considered in detail. The first part of the review briefly describes the most popular approaches in plasma …
In this work we studied processing of poly(dimethylsiloxane) (PDMS) surfaces using dielectric barrier discharge (DBD) plasma in two different assemblies, one using the primary plasma jet obtained from a conventional DBD and the other using a DBD plasma jet transfer. The evolution of water contact angle (WCA) in function of plasma processing time and in function of aging time as well as the changes in the surface roughness of PDMS samples for both plasma treatments have been studied. We also compared vibrational and rotational temperatures for both plasmas and for the first time the vibrational…
A Global Enhanced Vibrational Kinetic (GEVKM) model is presented for a new High Current Negative Hydrogen Ion Source (HCNHIS) developed by Busek Co. Inc. and Worcester Polytechnic Institute. The HCNHIS consists of a high-pressure radio-frequency discharge (RFD) chamber where high-lying vibrational states of the hydrogen molecules are produced, a bypass system, a nozzle, and a low-pressure negative hydrogen ion production region where $\text{H}^-$ are generated by the dissociative attachment of low energy electrons to rovibrationally excited hydrogen molecules. The GEVKM is developed from momen…
The Maxwell time is a typical time scale for the screening of an electric field in a medium with a given conductivity. We introduce a generalization of the Maxwell time that is valid for gas discharges: the \emph{ionization screening time}, that takes the growth of the conductivity due to impact ionization into account. We present an analytic estimate for this time scale, assuming a planar geometry, and evaluate its accuracy by comparing with numerical simulations in 1D and 3D. We investigate the minimum plasma density required to prevent the growth of streamers with local field enhancement, a…
Nanosecond Surface Dielectric Barrier Discharge (ns-SDBDs) are a class of plasma actuators that utilize a high-voltage pulse of nanosecond duration between two surface-mounted electrodes to create an electrical breakdown of air, along with rapid heating. These actuators usually produce multiple filaments when operated at high pulse frequencies, and the rapid heating leads to the formation of shock waves and complex flow fields. In this work we replicate a single filament of the ns-SDBDs and characterize the induced flow using velocity measurements from particle image velocimetry and density me…
We investigate structure and evolution of long positive spark breakdown; and we study at which stage pulses of hard X-rays are emitted. Positive high-voltage pulses of standardized lightning impulse wave form of about 1 MV were applied to about 1 meter of ambient air. The discharge evolution was imaged with a resolution of tens of nanoseconds with an intensified CCD camera. LaBr3(Ce+) scintillation detectors recorded the X-rays emitted during the process. The voltage and the currents on both electrodes were measured synchronously. All measurements indicate that first a large and dense corona o…
We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as $n_\text{e}=7\times10^{19}$ cm$^{-3}$, and decay to $1/e$ of their peak values over a period of about 50 ns followi…
The conventional ways of processing natural gas into more efficient and economical fuels usually either have low conversion rate or low energy efficiency. In this work, a new approach of methane liquefaction is proposed. Instead of direct treatment of only natural gas, plasma activated methane is reacting with liquid fuel. In this way, methane molecules are directly incorporated onto liquid fuel to achieve liquefaction. Nanosecond-pulsed dielectric barrier discharge and atmospheric pressure glow discharge are used here to ensure no local heating in gas bubbles. Effects of both discharges on me…
Stable DC atmospheric pressure normal glow discharges in ambient air were produced between the water surface and the metallic capillary coupled with influx of helium gas. Multiple independent repeated trials indicated that vibrational temperature of nitrogen rises from 3200 to 4622 K, and rotational temperature of nitrogen decreases from 1270 to 570 K as gas flux increasing from 20 to 80 sccm and discharge current decreasing from 11 to 3 mA. Furthermore, it was found that the vibrational degree of the nitrogen molecule has priority to gain energy than the rotational degree of nitrogen molecule…
Multi-physics modeling of non-equilibrium phenomena in inductively coupled plasma discharges: Part II. Multi-temperature approach
Center for Hypersonics and Entry Systems Studies (CHESS)（高超音速与进入系统研究中心）；Department of Aerospace Engineering（航空航天工程系）；University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）
The effects of nanosecond pulsed non-equilibrium plasma discharges on ignition characteristics and low-temperature chemistry or LTC of DME/O2/Ar mixtures are numerically investigated in a plane-to-plane geometry at a reduced pressure of 76 Torr. Detailed numerical investigations of the two-stage ignition process in different temperature regimes of DME have been created through a high-fidelity one-dimensional, multi-timescale plasma-discharge model. Two different initial temperatures are set for comparison: 550K, the negative temperature coefficient regime, where LTC plays a dominate role; 800K…
Multi-physics modeling of non-equilibrium phenomena in inductively coupled plasma discharges: Part I. A state-to-state approach
Center for Hypersonics and Entry Systems Studies (CHESS)（高超音速与进入系统研究中心）；Department of Aerospace Engineering（航空航天工程系）；University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）
Electron dynamics in planar radio frequency magnetron plasmas: III. Comparison of experimental investigations of power absorption dynamics to simulation results
A 13.56 MHz RF discharge in hydrogen was studied within the pressure range of 1-17 10 Pa, and at power range of 400-1000 W. The electron energy distribution function and 18 electron density were measured by a Langmuir probe. The gas temperature was determined by 19 the Fulcher-$α$ system in pure H2, and by the second positive system of nitrogen using N2 as the 20 probing gas. The gas temperature was constant and equal to 450 $\pm$ 50 K in the Capacitively 21 Coupled Plasma mode (CCP), and it was increasing with pressure and power in the Inductively 22 Coupled Plasma mode (ICP). Also the vibrat…
We discovered a hysteresis in a microwave-driven low-pressure argon plasma during gas pressure change across the transition region between $α$ and $γ$ discharge modes. The hysteresis is manifested in that the critical pressure of mode transition depends on the direction of pressure change. As a corollary, the plasma would attain different discharge properties under the same operating parameters (pressure, power, and gas composition), suggesting a bi-stability or existence of memory effect. Analysis of the rotational and vibrational temperatures measured from the OH (A-X) line emissions shows t…
The electron and gas temperature of non-equilibrium argon plasma torch sustained by a focused 24 GHz microwave beam in the external atmosphere of air was measured by optical emission spectroscopy. The excitation temperature of argon atoms was found about 5000 K, while the vibrational and rotational temperatures of ambient nitrogen molecules were respectively estimated at 3000 K and 2000 K. The effective mixing of the external gas atmosphere into the plasma torch is experimentally demonstrated. The vibrational and rotational temperatures of nitrogen molecules slightly increased with the distanc…
Novel methods for diagnostics of molecular hydrogen plasma processes, such as ionization, production of high vibrational levels, dissociation of molecules via excitation to singlet and triplet states and production of metastable states, are presented for molecular hydrogen plasmas in corona equilibrium. The methods are based on comparison of rate coefficients of plasma processes and optical emission spectroscopy of lowest singlet and triplet transitions, i.e. Lyman-band ($B^1Σ^+_u \rightarrow X^1Σ^+_g$) and molecular continuum ($a^3Σ^+_g \rightarrow b^3Σ^+_u$), of the hydrogen molecule in VUV …
This paper studies a continuous atmospheric-pressure discharge maintained by focused subterahertz radiation of a 263 GHz gyrotron. A plasma torch propagating towards microwave radiation was sustained on an argon flow exiting a metal gas tube in the surrounding atmosphere of air. Using a high-speed camera, the spatial structure of the torch was studied and the temporal dynamics on timescales of the order of 20 ns was examined. The emission spectra of the optical wavelength range were used to estimate the excitation temperature of argon atoms and electron density. It was shown that a discharge o…
A selfconsistent 1D theoretical framework for plasma assisted ignition and combustion is reviewed. In this framework, a frozen electric field modeling approach is applied to take advantage of the quasiperiodic behaviors of the electrical characteristics to avoid the recalculation of electric field for each pulse. The correlated dynamic adaptive chemistry (CoDAC) method is employed to accelerate the calculation of large and stiff chemical mechanisms. The timestep is updated dynamically during the simulation through a three-stage multitimescale modeling strategy, which takes advantage of the lar…
