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 following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[$^5S{^o}$]-3p[$^5P$] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics.
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, Rayleigh microwave scattering was utilized to measure the electron number density produced by nanosecond high voltage breakdown in air between two electrodes in a pin-to-pin configuration (peak voltage 26 kV and pulse duration 55 ns). The peak electron density decreased from 1*10^17 cm^-3 down to 7*10^14 cm^-3 when increasing the gap distance from 2 to 8 mm (total electron number decreased from 2*10^13 down to 5*10^11 respectively). Electron number density decayed on the timescale of about several microseconds due to dissociative recombination.
In this study we have studied the initial stage of the nanosecond-pulsed discharge development in liquid phase. Modeling predicts that in the case of fast rising strong nonhomogeneous electric fields in the vicinity of high voltage pin electrode a region saturated with nanoscale non-uniformities may be developed. This phenomenon is attributed to the electrostriction mechanisms and may be used to explain development of breakdown in liquid phase. In this work, schlieren method was used in order to demonstrate formation of negative pressure region in liquids with different dielectric permittivity…
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, 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…
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 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…
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…
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 …
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）
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…
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…
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…
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 symme…
We carry out simulations of laser plasmas generated during UV nanosecond pulsed laser ablation of the chalcogens selenium (Se) and tellurium (Te), and compare the results to experiments. We take advantage of a 2D-axisymmetric, adaptive Cartesian Mesh (ACM) framework that enables plume simulations out to centimeter distances over tens of microseconds. Our model and computational technique enable comparison to laser-plasma applications where the long-term behavior of the plume is of primary interest, such as pulsed laser synthesis and modification of materials. An effective plasma absorption ter…
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 …
A simplified and straightforward methodology that may be used to measure N(4 S) density by TALIF technique in strongly emissive plasmas is presented. The method makes use of the absorption peak intensities instead of the fully integrated absorption line intensity. The use of this method, along with a physics-based fitting/filtering procedure enables strongly reducing the experimental uncertainty and performing measurement even in strongly emitting discharges. After validation, the method was employed to investigate the transients of a strongly emissive pulsed microwave discharge. We especially…
Fast ignition is an alternative concept of laser fusion in which the task of compressing the fusion pellet to supersolid densities is accomplished by the conventional high energy nanosecond glass lasers and the task of igniting the compressed pellet is given to a high intensity, moderate energy pico second source which can set the pellet ablaze (with DT fusion reactions) by creating an adequate size hot spot in it. In this letter, we present a conceptual method by which energy from carbon dioxide ($ CO_2 $) laser could be coupled to heat up ions produced in situ in the plasma, which in turn pr…
Proof of operation of a micro-hollow cathode discharge driven by a true ns-pulsed high voltage: emission properties and atomic nitrogen production
Laboratoire des Sciences des Procédés et des Matériaux (LSPM), CNRS, Université Sorbonne Paris Nord（材料过程与科学实验室，法国国家科学研究中心，索邦巴黎北大学）；Laboratoire Energétique Moléculaire et Macroscopique, Combustion (EM2C), CNRS, Université Paris Saclay（分子与宏观能量、燃烧实验室，法国国家科学研究中心，巴黎萨克雷大学）
This paper introduces in full detail a methodology for the measurement of neutron yield and the necessary efficiency calibration, to be applied to the intensity measurement of neutron bursts where individual neutrons are not resolved in time, for any given moderated neutron proportional counter array. The method allows efficiency calibration employing the detection neutrons arising from an isotopic neutron source. Full statistical study of the procedure is descripted, taking into account contributions arising from counting statistics, piling-up statistics of real detector pulse-height spectra …
Pulsed plasmas in liquids exhibit complex interaction between three phases of matter (liquids, gas, plasmas) and are currently used in a wide range of applications across several fields, however significant knowledge gaps in our understanding of plasma initiation in liquids hinder additional application and control; this area of research currently lacks a comprehensive predictive model. To aid progress in this area experimentally, here we present the first-known ultrafast (50 ps) X-ray images of pulsed plasma initiation processes in water (+25 kV, 10 ns, 5 mJ), courtesy of the X-ray imaging te…
Coupling model of metallic target ablation-plasma evolution-radiation under nanosecond laser irradiation
State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University（西安交通大学电气绝缘与电力设备国家重点实验室）；Suzhou Nuclear Power Research Institute Co., Ltd.（苏州核动力研究所有限公司）；School of Aerospace Engineering, Xi’an Jiaotong University（西安交通大学航天航空学院）
