Second Positive System Of Nitrogen Research Articles

Gas heating by nanosecond repetitively pulsed glow discharges applied to a methane–air flame

In this work, the thermal effect induced by non-equilibrium plasma produced by nanosecond repetitively pulsed (NRP) glow discharges applied across a lean premixed methane–air flame is investigated. The flame is laminar, stationary, and axis-symmetric. The discharges are applied on the symmetry axis of the flame, crossing the fresh reactants, flame front, and burned gases. The obtained plasma-assisted flame is stable and reproducible, allowing phase-locked averaged diagnostics. The thermal effect is investigated by acquiring spatially and temporally resolved temperature measurements in the plasma discharges using optical emission spectroscopy (OES) of the second positive system of nitrogen. The results show that for an applied voltage of 10 kV, NRP glow discharges increased the gas temperature by up to 130 K within 10 ns. However, this heating was location-dependent, i.e., high near the anode tip and not detectable 0.5 mm above the anode. It was also shown that reducing the applied voltage down to 9 kV caused this ultra-fast heating to disappear. On the other hand, the reactants near the anode tip had a constant temperature of 480 K, between discharges, while the reactants were injected at 293 K. The reason for this slow heating could be due to (1) heating by the flame which is pulled down to the anode when NRP discharges are applied, (2) slow gas heating induced by relaxation of vibrational excited states of nitrogen, or (3) post-discharge oxidation chemistry. Based on temperature measurements performed on NRP corona discharges, heating by the flame could be ruled out. Zero-dimensional numerical simulations’ results suggest that post-discharge oxidation chemistry can heat the gas by 110–130 K, indicating that this is an important heating mechanism. Further investigation will be necessary to assess the significance of the vibrational–translational relaxation of nitrogen.

Electron density and temperature in a diffuse nanosecond pulse discharge in air at atmospheric pressure

This work presents the first experimental results on the electron properties of a nanosecond diffuse fast ionisation wave generated in synthetic dry air at atmospheric pressure under very strong overvoltage. Both density and mean temperature of electrons are investigated by incoherent Thomson scattering. The electron density is also derived from the Stark broadening of oxygen lines resolved by optical emission spectroscopy. The extreme voltages applied question some common hypothesis of the diagnostics implemented. The solutions adopted and the remaining limitations are discussed in the paper. Each diagnostic covers a specific region of interest within the discharge and they show good agreement in conditions where they overlap. It is shown that most of the volume of the pin-to-plane discharge is quite representative of a quasi-steady state glow discharge dominated by the emission of the first and second positive systems of nitrogen. Once its propagation completed within the first two nanoseconds and until the end of the 10 ns pulse, it is characterized by rather homogeneous properties close to the axis. The electron density is of the order of 1015 cm−3 and the mean temperature is about 3 eV within the whole air gap. About 6 ns after the start of the discharge from the pin, a sub-millimetric region of strong ionization develops at the pin, which is consistent with the observation of a continuum of emission spreading from the UV to the near-IR spectral range. Within this part of the discharge, the electron density reaches values greater than 1017 cm−3 with an ionization degree higher than 1%. The radiative recombination of nitrogen ions N2 + and the three-body recombination of N+ with a large number of electrons could help to explain the continuum.

THz gas discharge in nitrogen as a source of ultraviolet radiation

This work presents the results of investigation of THz gas discharge in inhomogeneous flow of nitrogen. As a source of sub-millimetre radiation we used 40 kW@670 GHz gyrotron created in IAP RAS. It is shown that, despite the tendency of the discharge in electromagnetic wave beams to propagate towards the heating radiation from the breakdown region, in the case of a strongly inhomogeneous gas flow (nitrogen in our case), conditions can be created under which the discharge practically does not propagate and becomes point-like. The features of the glow of a THz discharge in a nitrogen flow in a wide range of background gas pressures are studied. The presence of a powerful afterglow in some bands of the second positive system of nitrogen at high pressures was demonstrated. The prospects of using such a discharge as a source of UV radiation are discussed.

Correlation between electric field, current and photon emission in subsequent barrier corona microdischarges

We investigate single microdischarges (MDs) in a sinusoidally operated barrier corona discharge in air. For the voltage amplitude being applied, two subsequent MDs appear in the anodic pin half-cycle. The developments of these subsequent MDs were studied and presented in detail in a previous contribution [Jahanbakhsh et al., Plasma Sources Sci. Technol. 27, 115011 (2018)]. In the present study, the reduced electric field strength (E/n) values of the MDs are determined. In addition, the current pulses are synchronized, with a subnanosecond time resolution, to the spatiotemporally resolved light emission and E/n development of the MDs. It is proposed that the current pulse derivative maximum corresponds to the streamer head arrival on the cathode surface. Therefore, the derivatives of the current pulses are used to synchronize the light emission and current measurements. Based on this synchronization, spatiotemporally resolved light emissions at different positions are compared to the averaged current pulses. Considering the observed correlations, it is proposed that after the arrival of the streamer head on the dielectric (cathode) surface and bulk plasma formation, the ionization processes near the dielectric surface are the dominant source of electron current production. The determination of the E/n is based on the analysis of the time-correlated single photon counting results for the molecular states of the first negative and the second positive systems of nitrogen. The E/n increases during the streamer propagation in the gap, reaching its maximum value at the impact of the streamer on the cathode. The E/n values for the second group MDs are lower only in the vicinity of the dielectric surface, which can be attributed to the positive residual surface charges from the first group MDs.

Optical Emission and Langmuir Probe Diagnostic Measurements in DC Electrode Pulse Discharge in Nitrogen

Optical emission of selected nitrogen bands is analyzed for different nitrogen fill pressure and input electrical power to find the changes in spectral intensities with changing discharge conditions. The electron temperature Te is inferred from the intensity ratio $$\left( {{{I_{{BX}}^{ + }} \mathord{\left/ {\vphantom {{I_{{BX}}^{ + }} {{{I}_{{CB}}}}}} \right. \kern-0em} {{{I}_{{CB}}}}}} \right)$$ of (0–0, 391.44 nm) and (0–2, 380.49 nm) band heads whereas electron number density ne from the intensity ratio and the corresponding rate coefficient X (cm3 s–1) for the given temperatures. Both band heads belonging to the first negative system and second positive system of nitrogen have a different threshold of excitation energies, and therefore the corresponding emission intensities provide a direct correlation between the group of electrons involved in optical emission (a part of electron energy distribution function above the excitation and ionization thresholds) and electron temperature. Measured intensity ratio $$\left( {{{I_{{BX}}^{ + }} \mathord{\left/ {\vphantom {{I_{{BX}}^{ + }} {{{I}_{{CB}}}}}} \right. \kern-0em} {{{I}_{{CB}}}}}} \right)$$ and resulting Te both increase with input power and decrease with gas fill pressure following almost the same trend. Besides, time-averaged triple probe measurements have been performed to determine Teff and ne under the same discharge conditions for the sake of comparison. The spectroscopic method provides the variation of Te and ne at various discharge power and gas pressure in comparison with probe measurements. This study will help to optimize the discharge conditions in terms of active species concentration, electron temperature and electron number density for technological applications.

On Pulsed Modes of the Glowing Corona Region

A corona discharge is experimentally studied in atmospheric air on a needle electrode with the curvature radius 40 μm. For both electrode voltage polarities, but different discharge burning modes, repetitive current pulses with the FWHM ~200 ns are registered. It is shown that in a wide range of negative-polarity voltages the area of the glow registered in the vicinity of the needle has a spherical shape, and in the case of a positive polarity of the needle tip, starting from a certain voltage value, cylindrical streamers are formed from the spherical glow area, whose length increases with the voltage. It is found out that the size of the glowing spherical formations near the needle tip, given the same voltage value, is larger in the case of negative polarity than in the positive one. It is validated that the radiation of the second positive system of nitrogen in the UVspectral region dominates in the corona discharge emission spectrum before its transition into a spark discharge.

High time-resolved spectroscopy of filament plasma in air

Abstract The results of spectral–temporal studies of the filament plasma emission in air with a time resolution of 2 ps were presented. The filament was created by laser radiation with pulse duration of 60 fs and wavelength of 940 nm. It was shown that a spectral composition of the filament plasma depended on the numerical aperture (NA). At NA > 0.035, the atomic and ion lines of oxygen and nitrogen dominate in the spectrum on the background of an intense continuous continuum in the visible region. Molecular lines of the second positive system of nitrogen and the first negative system of molecular ions appear in the spectrum for NA 0.03. In case of NA 0.01, the spectrum consists of only the molecular and ion lines of N 2 and N 2 + . The emission intensity maxima of these lines delay relative to the laser pulse. The atomic lines begin to glow per (80–100) ps after the laser pulse. The characteristic time of the luminescent lines decay at 1/e level for ions is 18 ps, for molecules is 27 ps, and for atomic lines is 20 ns. Rapid decrease in the intensity of molecular lines emission is due to the deactivation of the particles upper states by electrons. The formation of N 2 + ( B 2 Σ u + ) excited states is most likely through their N 2 + ( X 2 Σ g + ) ground state in the process of multiphoton absorption of pumping radiation by nitrogen molecules and subsequent their excitation by electron impact.

Spark-induced breakdown spectroscopy of methane/air and hydrogen-enriched methane/air mixtures at engine relevant conditions

An experimental study of temporally and spatially resolved spark-induced breakdown spectroscopy (SIBS) of methane and hydrogen-enriched methane mixtures is reported for premixed combustion in internal combustion engines. Experiments were conducted at quiescent conditions in a small constant volume chamber, varying pressure, stoichiometry and hydrogen admixture rates. Spectral emissions of hydroxyl (OH) at 306 nm, NH at 336 nm, the cyanogen (CN) band at 388 nm and the nitrogen second positive system (N2) were spatially resolved on a time-integrated setup. OH emissions were found to be strongest between the electrodes, whereas CN emissions were more pronounced at the center and ground electrode. Metal emission lines were observed, partially interfering with the radicals of interest. Energy dispersive X-ray spectroscopy showed nickel, copper and iron shares in the noble metal electrodes, confirming the origin of these metal emissions. Simultaneously to time-integrated spectra, temporally-resolved spectra at a frame rate of 100 kHz were recorded during the glow phase of the electrical discharge. Comparison of both spectra revealed a good agreement in terms of spectral characteristics and resolved species. Temporally-resolved spectra highlighted the strong dependence on the electric discharge characteristics of the inductive coil ignition system in the early phase of ignition. Ratios of CN/OH and CN/NH were found to correlate with the fuel-air equivalence ratio, but shot to shot repeatability was up to a factor 3 larger than the dependence on the mixture composition. Moreover, these ratios changed with increasing pressure at ignition timing, and with the higher pressure metal emission lines became more pronounced. Additionally, at pressures higher than 2 bar, the equilibrium position shifted to the disadvantage of the second positive system of nitrogen, suppressing this emission. Hydrogen admixture to methane of up to 50 vol% were found to not significantly alter the spectral signature between 300 and 400 nm, only marginally affecting the signal ratios of CN/OH and CN/NH.

Actuation of a lean-premixed flame by diffuse non-equilibrium nanosecond-pulsed plasma at atmospheric pressure

This study investigates the effect of diffuse non-equilibrium nanosecond-pulsed plasma at atmospheric pressure on a lean-premixed CH4-air flame (ϕ = 0.65, P ∼ 0.3 kW). The domain of diffuse plasma existence is explored for both the case of the cold flow (no flame) and the case where a flame is stabilized downstream. The dynamics of plasma propagation and the flame displacement, following a high-voltage pulse, were measured using intensified charge-coupled device imaging. The energy of the plasma was measured using electrical probes and measurements of the second positive system of nitrogen were used to determine the rotational temperature and vibrational populations in the plasma. The effect of plasma on a flame was investigated by varying the pulse repetition frequency gradually from 1 to 7 kHz. Time-resolved imaging of the plasma emission shows that the primary streamer travels at higher velocities with increased pulsing frequency and with the presence of a flame ignited downstream of the discharge. Time-resolved imaging of the flame, following a high-voltage pulse, shows that the flame moves upstream into the unburned methane-air mixture with increased pulsing frequency. As the flame is displaced upstream, the nature of the discharge also changes, whereby less energy is coupled to the gas volume. Spectroscopic results reveal that the region in which the flame stabilizes is that of highest vibrational excitation and lowest rotational temperature. This actuation method is evidence of low-temperature chemical flame enhancement and potential control of a lean-premixed laminar flame at atmospheric pressure.

Streamer-to-spark transition initiated by a nanosecond overvoltage pulsed discharge in air

This study is focused on the streamer-to-spark transition generated by an overvoltage nanosecond pulsed discharge under atmospheric pressure air in order to provide a quantitative insight into plasma-assisted ignition. The discharge is generated in atmospheric pressure air by the application of a positive high voltage pulse of 35 kV to pin-to-pin electrodes and a rise time of 5 ns. The generated discharge consists of a streamer phase with high voltage and high current followed by a spark phase characterized by a low voltage and a decreasing current in several hundreds of nanosecond. During the streamer phase, the gas temperature measured by optical emission spectroscopy related to the second positive system of nitrogen shows an ultra-fast gas heating up to 1200 K at 15 ns after the current rise. This ultra-fast gas heating, due to the quenching of electronically excited species by oxygen molecules, is followed by a quick dissociation of molecules and then the discharge transition to a spark. At this transition, the discharge contracts toward the channel axis and evolves into a highly conducting thin column. The spark phase is characterized by a high degree of ionization of nitrogen and oxygen atoms shown by the electron number density and temperature measured from optical emission spectroscopy measurements of N+ lines. Schlieren imaging and optical emission spectroscopy techniques provide the time evolution of the spark radius, from which the initial pressure in the spark is estimated. The expansion of the plasma is adiabatic in the early phase. The electronic temperature and density during this phase allows the determination of the isentropic coefficient. The value around 1.2–1.3 is coherent with the high ionization rate of the plasma in the early phase. The results obtained in this study provide a database and the initial conditions for the validation of numerical simulations of the ignition by plasma discharge.

Effect of air flow on the micro-discharge dynamics in an array of integrated coaxial microhollow dielectric barrier discharges

The micro-discharge properties and evolution in a 2D array of integrated coaxial microhollow dielectric barrier discharges are studied by using highly time-resolved electrical and optical diagnostics. The study is focused on the effect of the gas flow rate and gas residence time on discharge properties. The investigated integrated coaxial microhollow discharge geometry allows operating the discharge at exceptionally small residence times, which can be equal to or even smaller than the discharge period, at reasonable gas flow rates. The gas flow has an impact on gas heating, residual humidity, pre-ionization density and the densities of excited and reactive species produced by previous discharges. A unique voltage–charge plot is obtained with elongated periods without discharge activity. A very significant effect of flow on NO emission is observed that relates to the impact of flow on the NO production in these micro-discharges. Using the emission intensities of molecular bands of the second positive system of nitrogen and the first negative system of the nitrogen ion, effective reduced electric field strengths are obtained with a maximum equal to 870 Td. The reduced electric field decreases with increasing gas flow rate. This behavior is consistent with the reduction of the overall discharge intensity due to a reduced amount of charges present in the discharge gap. Both the flow rate and a reduction in water impurity changing the ion mobility can be responsible for the different effective electric field distributions at the highest and no flow conditions.

Electronic ground state OH(X) radical in a low-temperature atmospheric pressure plasma jet

The wide applicability of atmospheric pressure plasma jets in biomedicine stems from the presence of reactive nitrogen and oxygen species generated in these plasma jets. Knowing the absolute concentration of these reactive species is of utmost importance as it is critical, along with the particle flux obtained from the plasma feed gas flow rate to ensure that the correct dosage is applied during applications. In this study, we investigate and report the ground state OH(X) number density acquired using cavity ringdown spectroscopy, along the propagation axis (z-axis) of a cold atmospheric pressure helium plasma plume. The jet was generated by a repetitively pulsed mono-polar square wave of duration 1 μs running at a frequency of 9.9 kHz. The voltage supplied was 6.5 kV with the helium flow rate fixed at 3.6 standard liters per minute. The rotational and vibrational temperatures are simulated from the second positive system of nitrogen, N2(C3πu−B3πg), with the rotational temperature being spatially constant at 300 K along the propagation axis of the atmospheric pressure plasma jet while the vibrational temperature is 3620 K at the beginning of the plume and is observed to decrease downstream. The OH(A) emission intensity obtained via optical emission spectroscopy was observed to decrease downstream of the plasma jet. The OH(X) number density along the propagation axis was initially 2.2 × 1013 molecules cm−3 before increasing to a peak value of 2.4 × 1013 molecules cm−3, from which the number density was observed to decrease to 2.2 × 1013 molecules cm−3 downstream of the plasma jet. The total OH(A, X) in the plasma jet remained relatively constant along the propagation axis of the plasma jet before falling off at the tip of the jet. The increase in vibrational temperature downstream and the simultaneous measurements of both the excited state OH(A) and the ground state OH(X) reported in this study provide insights into the formation and consumption of this reactive oxygen species while also providing a reference for determining the radical number density dosage for future cold plasma irradiation studies.

Determination of the rotational population of H2 and D2 including high-N states in low temperature plasmas via the Fulcher-α transition

Vibrational and rotational excitation of the hydrogen molecule can significantly affect molecular reaction rates in low pressure low temperature plasmas, for example for the creation of H−/D− ions via the dissociative attachment process. In general, the rotational population in these discharges is known to be non-thermal with an overpopulation of states with high rotational quantum number N. In contrast to a sophisticated direct measurement of the rotational distribution in the XΣg+1,v=0 state, it is demonstrated that the determination can also be carried out up to high-N levels rather easily via optical emission spectroscopy utilizing the Fulcher-α transition of H2 and D2. The measured rotational populations can be described with a two-temperature distribution where the cold part reflects the population according to the gas temperature of the discharge. This has been verified by using the emission of the second positive system of nitrogen as independent gas temperature diagnostic. The hot part where the rotational temperature reaches several thousand Kelvin arises most probably from recombinative desorption of hydrogen at the discharge vessel wall where parts of the binding energy are converted into rotational excitation. Neglecting the hot population – what is often done when using the Fulcher-α transition as gas temperature diagnostic – can lead to a strong overestimation of Tgas. No fundamental differences in the rotational distributions between hydrogen and deuterium have been found, only the hot rotational temperature is smaller for D2 indicating an isotope-dependency of the recombinative desorption process.

Diagnostics of low-pressure hydrogen discharge created in a 13.56 MHz RF plasma reactor

A 13.56 MHz RF discharge in hydrogen was studied within the pressure range of 1–10 Pa, and at a power range of 400–1000 W. The electron energy distribution function and electron density were measured by a Langmuir probe. The gas temperature was determined by the Fulcher-α system in pure H2, and by the second positive system of nitrogen using N2 as the probing gas. The gas temperature was constant and equal to 450 ± 50 K in the capacitively coupled plasma (CCP) mode, and it increased with pressure and power in the inductively coupled plasma (ICP) mode. Also, the vibrational temperature of the ground state of hydrogen molecules was determined to be around 3100 and 2000 ± 500 K in the ICP and CCP mode, respectively. The concentration of atomic hydrogen was determined by means of actinometry, either by using Ar (5%) as the probing gas, or by using H2 as the actinometer in pure hydrogen (Q1 rotational line of Fulcher-α system). The concentration of hydrogen density increased with pressure in both modes, but with a dissociation degree slightly higher in the ICP mode (a factor 2).

Spectral and amplitude–time characteristics of radiation of plasma of a repetitively pulsed discharge initiated by runaway electrons

Spectral and amplitude–time characteristics of radiation of plasma of a repetitively pulsed discharge initiated by runaway electrons were studied experimentally in nitrogen. Intense emission lines of copper atoms, nitrogen atoms, and ions, as well as the first and the second positive systems of nitrogen, NO, and CN, were observed in the regime of repetitively pulsed excitation.

Rotational temperature measurements in molecular plasmas using nonadditive Tsallis statistics

Tsallis’ nonadditive statistics was employed to determine rotational temperatures in low-temperature nonequilibrium molecular plasmas. The First Negative and Second Positive Systems of nitrogen were used as test cases with classical two-Boltzmann distributions that may be replaced successfully by one Tsallis distribution. In our experimental conditions for First Negative System, only one Boltzmann distribution fails to account the intensities of rotational levels. An alternative approach to resolve this problem is to consider the rotational levels as a linear combination of two-Boltzmann equilibrium distributions. However this method requires three adjustable parameters; two rotational temperatures and a free parameter which describes the relative contribution of the hot and cold temperatures. Tsallis’ temperature is close to the coldest one obtained by Boltzmann method and provides a consistent statistical interpretation. The nonadditive method could be, equally successfully, extended to other nitrogen systems and molecular plasmas.

Ultrafast heating and oxygen dissociation in atmospheric pressure air by nanosecond repetitively pulsed discharges

A detailed experimental investigation of the mechanism of ultrafast heating and oxygen dissociation produced by nanosecond repetitively pulsed discharges in atmospheric pressure air preheated at 1000 K is presented. The ultrafast mechanism creates excited electronic states of nitrogen, which then dissociate molecular oxygen through quenching reactions, with the remaining energy dissipated as heat. Optical and electrical diagnostic techniques have been applied to provide a self-consistent set of experimental data for a reference test-case with well-defined discharge and gas conditions. The pulses have a duration of 10 ns, an amplitude of 5.7 kV, a repetition frequency of 10 kHz and the pin-to-pin electrodes are separated by 4 mm. We present measurements of the gas temperature during and after the discharge using optical emission spectroscopy of the second positive system of nitrogen, spatially resolved profiles of the absolute densities of excited electronic states, determined using Abel-inverted spectra of the first and second positive systems of nitrogen, as well as the temporal evolution of the absolute densities of N2(A), N2(B), N2(C), electrons and atomic oxygen. These measurements are synchronized with electrical measurements of pulse current, voltage, and energy. The discharge is found to dissociate about 50% of molecular oxygen and to produce a temperature increase of about 900 K within 20 ns, corresponding to an ultrafast heating rate of about 5 × 1010 K s−1. Comparisons with numerical simulations show good agreement with the measurements and validate the ultrafast mechanism. About 35% of the electric energy deposited into the gas goes into O2 dissociation, and about 21% into gas heating. Finally, the dissociative quenching rates of N2(B) and N2(C) with O2 at 2200 K were measured and found to be 2.8(±0.6) × 10−10 cm3 s−1 and 5.8(±0.9) × 10−10 cm3 s−1, respectively. Combining these measurements with the literature values at 300 K, we propose a functional temperature dependence in the range 300–2200 K of 3.0 × 10−10(T/300)0.3 cm3 s−1 for the C state, and a constant value of 3.0 × 10−10 cm3 s−1 for the B state.

Experimental–Modeling Study of an Atmospheric-Pressure Helium Discharge Propagating in a Thin Dielectric Tube

This paper presents an experimental and numerical study of the dynamics and structure of an atmospheric-pressure helium discharge with nitrogen admixture propagating through a thin dielectric tube surrounded by two ring electrodes. The experimental unfiltered optical emission of the discharge has been compared to the computed emission of the second positive system of nitrogen and shows very good agreement. In all studied cases, the discharge ignition occurs inside the tube at the outer edges of the high-voltage electrode ring, and the discharge propagates outward of both sides of the high-voltage ring with a rather homogeneous discharge front structure. When the discharge approaches the grounded ring, the emission is enhanced and moves from the symmetry axis to the tube surface. In both simulation and experiment, an initial front propagation velocity of about 3 ×106 cm/s is observed as the plasma traverses the first part of the electrode gap. This velocity increases in the later part of the electrode gap as the plasma approaches the grounded ring, and the observed velocity in the experiment is slightly higher than that in the simulation. In experiments, when the admixture is decreased, the discharge constricts close to the tube axis during its propagation between the electrode rings before moving to the tube surface close to the grounded ring. In simulations, as a first step, we have considered that, in reducing the amount of nitrogen admixture, the efficiency of photoionization decreases significantly. This allows the formation of an electron depleted volume close to the grounded electrode ring, which then constrains the discharge to propagate close to the tube axis, in agreement with experiments.

Effect of the dc field on the near-surface plasma of a highly nonuniform microwave discharge

The effect of the dc electric field on the near-surface plasma of an electrode microwave discharge at pressures of 1–5 Torr was studied by the emission spectroscopy method. It is shown that the dc field weakly affects the vibrational distribution of nitrogen molecules in the C3Πu state, but changes the structure of the near-surface plasma (shifting the intensity maxima of the emission bands) and the strength of the microwave field near the electrode surface. It is also found that the ratio between the intensities of bands of different sequences of the second positive system of nitrogen radiated from the same state depends on the position along the discharge axis.

Optical and electrical diagnostics of a spark-plug discharge in air

Optical and electrical diagnostics were used to investigate a spark discharge using a commercial spark-plug operating in the glow phase regime. Voltage and current were measured in order to characterize the discharge. The gas temperature was estimated as a function of time and duty cycle using ro-vibrational spectra of the second positive system of nitrogen by comparison between experimental and simulated spectra. It was found that 1600 K ⩽ Tg ⩽ 2800 K. The reduced electric field varied between 1 and 1000 Td after the end of the current pulse. Using the line intensity method between one line from the ionic argon and the other from the neutral atom, the electronic temperature was measured and found to be between 17 000 and 20 000 K. The electron density was determined from the broadening of Hα line and was found to be 4.0 × 1014 cm−3 ⩽ ne ⩽ 1.7 × 1015 cm−3.