Positive System Of Nitrogen Research Articles

Vibrational distribution of nitrogen molecules in the C3Πu state in a near-surface microwave plasma in nitrogen at pressures of 1–5 Torr

The radiation of the second positive nitrogen system has been used to study the spatial dependence of the vibrational distribution of nitrogen molecules in the C3Πu state in the near-surface plasma layer of an electrode microwave discharge in nitrogen at pressures of 1–5 Torr. It has been shown that the vibrational distribution changes at a scale of 100 μm. It has been concluded that this state is populated owing to the electron impact from the ground state. The possibility of using the local approximation for the electron energy distribution function to explain the experimental results has been analyzed.

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.

Plasma diagnostics of barrier-torch discharge in argon flow in a capillary by cross-correlation spectroscopy

The radiation kinetics of the plasma of barrier-torch disrcharge in argon flow in a capillary has been studied by cross-correlation spectroscopy. It was established that the discharge emission spectrum consists of peaks of electronically excited states of argon, bands of hydroxyl radicals, and a second positive system of nitrogen. An analysis of the spatio-temporal distributions of emission intensity for the selected spectral indicators showed that the causes of the torch are ionization waves that extend through the capillary from the electrode system with a speed of 105 m/s and project up to 3–4 mm. It was established that the formation of electronically excited molecules of nitrogen N2(C3Πu) in the torch of discharge occurs mainly on the reaction between metastable electronically excited atoms of argon and molecules of nitrogen in the electronic ground state.

Similarity laws for cathode-directed streamers in gaps with an inhomogeneous field at elevated air pressures

Results are presented from experimental studies of cathode-directed streamers in the gap closure regime without a transition into spark breakdown. Spatiotemporal, electrodynamic, and spectroscopic characteristics of streamer discharges in air at different pressures were studied. Similarity laws for streamer discharges were formulated. These laws allow one to compare the discharge current characteristics and streamer propagation dynamics at different pressures. Substantial influence of gas photoionization on the deviations from the similarity laws was revealed. The existence of a pressure range in which the discharges develop in a similar way was demonstrated experimentally. In particular, for fixed values of the product pd and discharge voltage U, the average streamer velocity is also fixed. It is found that, although the similarity laws are violated in the interstreamer pause of the discharge, the average discharge current and the product of the pressure and the streamer repetition period remain the same at different pressures. The radiation spectra of the second positive system of nitrogen (the C3Π u -B3Π g transitions) in a wavelength range of 300–400 nm at air pressures of 1–3 atm were recorded. It is shown that, in the entire pressure range under study, the profiles of the observed radiation bands practically remain unchanged and the relative intensities of the spectral lines corresponding to the 3Π u -B3Π g transitions are preserved.

Radiation of nitrogen molecules in a dielectric barrier discharge with small additives of chlorine and bromine

Spectral and energy characteristics of nitrogen molecule radiation in dielectric barrier discharges in Ar-N2, Ar-N2-Cl2, and Ar-N2-Br2 mixtures were investigated experimentally. Small additives of molecular chlorine or bromine to an Ar-N2 mixture are found to increase the radiation intensity of the second positive system of nitrogen. The conditions at which the radiation spectrum predominantly consists of vibronic bands of this system are determined. Using a numerical model of plasmachemical processes, it is shown that, at electron temperatures typical of gas discharges (2–4 eV), a minor additive of molecular chlorine to an Ar-N2 mixture leads to an increase in the concentrations of electrons, positive ions, and metastable argon atoms. In turn, collisional energy transfer from metastable argon atoms to nitrogen molecules results in the excitation of the N2(C 3Π u ) state.

Radiative characteristics of nitrogen upon excitation by volume discharge initiated by runaway electron beam

The amplitude, time, spectral, and energetic characteristics of a volume (diffuse) discharge at an elevated pressure (up to 5 atm) in a gap with an inhomogeneous electric field without an additional preionization source are studied. The concentration and temperature of electrons in the discharge plasma are mea-sured by spectral methods. The effective lifetime of the C 3Π u state of the nitrogen molecule at the trailing edge of the radiation pulse is shown to be determined by radiative and collisional quenchings. For transitions of the second positive system of nitrogen, a plasma discharge radiation power into the complete solid angle is obtained to be ∼120 kW, with a specific radiation power of up to ∼50 kW/cm3.

Optical characteristics of radiation from N 2 * dimers in a barrier discharge

The spectral and power characteristics of radiation of the second positive system of nitrogen (C3Πu → B3Πg) in Ar-N2 and Ar-N2-Cl2 mixtures excited by barrier discharge have been studied experimentally. Addition of argon to N2 increased the radiation power by sixfold. In the triple mixture Ar-N2-Cl2 = 210/0.5/0.005, minor chlorine additions increased the intensity of the C3Πu → B3Πg transition by 26% compared to Ar-N2 mixtures. Radiation power density of 2.7 mW/cm2 has been achieved. In both binary and triple mixtures, the second positive system of nitrogen was the major contributor to radiation, while the contributions of the fourth positive system of N2* (D3Σu+ → B3Πg), the Vegard-Kaplan transition of N2* (A3Σu+ → X1Σg+), and the D′ → A′ band of Cl2* were negligibly small.

Specific features of radiation from a negative air corona operating in the Trichel-pulse mode

Experimental studies of spatiotemporal characteristics of radiation from a negative corona operating in the Trichel-pulse mode in the point-to-sphere electrode geometry have revealed two emission zones. In addition to the well-known glow near the point electrode, there is also an anode glow, whose intensity depends substantially on the shape of the anode. It is found that the anode glow is delayed with respect to the beginning of the Trichel pulse by a time that depends on the gap length and gap voltage. The emission spectrum of the anode glow in the wavelength range 300–400 nm is identified as the spectrum of the second positive system of nitrogen (the C 3Π u -B 3Π g transition).

Modeling of the electrode microwave discharge in nitrogen

Self-consistent two-dimensional modeling of a steady microwave discharge initiated at the end of the central electrode in nitrogen is presented. The discharge parameters are calculated at a gas pressure of 1 Torr and incident power of 30 W. The computational model includes the time-dependent Maxwell's equations, the balance equations describing the kinetics of charged and neutral plasma particles and the time-independent homogeneous Boltzmann equation for electrons. The processes involving vibrationally excited ground state molecular nitrogen are taken into account by the well-known analytic expression for the vibrational distribution of molecules in the diffusion approximation. It is shown that the spatially non-uniform microwave field causes the difference in plasma particles kinetics in different parts of the discharge. Results of numerical simulations for space distribution of electronically excited molecules have been found in good qualitative agreement with those taken from spectral measurements of first and second positive systems of nitrogen. Results confirm the concept according to which such a discharge comprises a self-sustained and a non-self-sustained discharge.

Wide-aperture electric-discharge nitrogen laser

The parameters of a wide-aperture nitrogen laser pumped by a generator with the inductive energy storage and a SOS diode opening switch or a generator with the capacitive energy storage are studied. The gas preionisation was performed by soft X-rays. The size of the active volume of the laser was 10×6×100 cm. The output energy and power obtained at the 337.1-nm C 3Πu — B 3Πg transition are maximal for electric-discharge nitrogen lasers. The output energy in the second positive system of nitrogen in the N2—SF6 mixture achieved 110 mJ for a peak power of 6 MW. Due to an increase in voltage across the laser gap in nitrogen mixtures with NF3, the generation of ∼35-mJ, 100-ns pulses was obtained in the quasi-stationary stage of the discharge.

Electrode microwave discharge in nitrogen: Structure and gas temperature

Electrode microwave discharges in nitrogen at pressures of 1–16 Torr and input microwave powers of 30–180 W have been studied by space-resolved emission spectroscopy. It is shown that the discharge is highly nonuniform. The relative intensities of the first and second positive nitrogen bands, as well as of the first negative band of nitrogen ions, are found to vary significantly throughout a discharge because, in different discharge regions, emitting particles are excited by different mechanisms. The gas temperature was determined by the method of the unresolved rotational structure of different sequences of the emission spectra of the second positive system of nitrogen.

The development of microdischarges of barrier discharges in N2/O2 mixtures—experimental investigations and modelling

The spatially resolved cross-correlation spectroscopy (CCS) was used for systematic investigations of the barrier discharge (BD) in N2/O2 mixtures at atmospheric pressure. The spatio-temporal distributions of the microdischarge (MD) radiation intensities were recorded for the spectral bands of the (0–0) transitions of the 2nd positive (λ = 337.1 nm) and 1st negative system of molecular nitrogen (λ = 391.4 nm). The velocities of the cathode-directed ionization waves were evaluated from the CCS data. In the middle of the gap, the MD channel diameter was found to be about 0.3 mm and to expand towards both electrodes. On the dielectrics, outward propagating discharges were observed. A computational model of the BD is proposed, to explain the MD formation in short (1–2 mm) air gaps by a Townsend mechanism. The two-dimensional dynamics of the MD development and the channel radiation for the second positive system of nitrogen are simulated. The proposed model explains satisfactorily the experimental results on the velocity of the cathode-directed ionization wave and the MD radiation of the 2nd positive system.

Generation of dielectric barrier discharge in high-pressure N 2 and CO 2 environments up to supercritical conditions

The generations of dielectric barrier discharge (DBD) in high-pressure nitrogen and carbon dioxide environments up to supercritical conditions with or without flow fields were performed. From the second positive system of nitrogen, the rotational temperature of N 2 was estimated to be approximately 400 K under a supercritical N 2 condition (4 MPa and 313 K). On the other hand, from emission spectra of DBD in a high-pressure CO 2 environment (1–8 MPa), C 2 and O spectra were remarkable in intensity instead of CO, CO +, CO 2 and CO 2 + spectra that were remarkable at atmospheric pressure. Furthermore, porous carbon film synthesis by DBD under a supercritical CO 2 condition (7.5 MPa and 305 K) was also identified by scanning electron microscopy observation.

Analysis of sterilization effect by pulsed dielectric barrier discharge

Atmospheric pressure (AP) plasmas can sterilize against almost all kinds of bacteria because many ions and reactive species, such as oxygen atoms and ozone, etc., are generated during AP plasmas. So AP plasmas are proper processes for application to air cleaners and sterilizers. The aim of this paper is to evaluate a germicidal effect caused by pulsed plasma system in air utilizing a dielectric barrier discharge (DBD) type reactor incorporating alumina, glass, etc. Escherichia coli, Bacillus subtilis and Pseudomonas aeruginosa bacteria were used for this sterilization experiment. For analysis of the relationship between sterilization results and chemical species generated in the discharge, we used optical emission spectroscopy and we checked emission spectra by atomic oxygen (394.2 and 436.8 nm) and second positive system of nitrogen (337.1 nm). Experimental results showed that DBD treatment during 70 s sterilized E. coli with 99.99% effectively and ozone molecules were the dominant germicidal species. From these results we concluded that the pulsed DBD system is very effective for sterilization.

UV-radiation in nitrogen excited by an electrodeless discharge

The purpose of the present study is to investigate optical media emitting in the wavelength range from 320 to 400 nm. An optical medium emitting in the above-indicated wavelength range on the second positive system of nitrogen consists of a gas with low reactivity. Previously, nitrogen was not used for fabrication of unsealed electrodeless lamps because of its low emission efficiency. In the present study, discharges initiated in N2 and N2–Ar mixtures filling a cylindrical tube 15 cm long with an inner diameter of 3 cm are investigated. Ring electrodes 2 cm wide surrounded the external tube wall. An alternating voltage was applied to the electrodes. A generator of unipolar impulses with tunable frequency and variable voltage that allowed the average excitation power to be changed was used in our experiments. A ТDS-334 oscillograph with a bandwidth of 0.3 GHz and a sampling rate of 1.25 GS/s (5 points every 2 ns) was used to register signals from a capacitive divider and current shunt. The radiation intensity was measured with a calibrated FEC-22 SPU photodiode using the procedure described in [4]. The energy deposited into the medium was estimated by the method described in [5]. The lamp emission efficiency was determined from the ratio of the emission power to the power deposited into the medium. A digital camera was used to photograph the glow of a discharge. In our experiments, we obtained the following results. Lines of the CПu–BПg transition of the nitrogen molecule, including lines at λ = 316 nm of the (1–0) band, λ = 337.1 nm of the (0–0) band, λ = 358 nm of the (0–1) band, λ = 381 nm of the (0–2) band, and λ = 406 nm of the (0–3) band were the most intensive emission lines of excited nitrogen. The average emission power of excited nitrogen from the external lamp surface was 2 mW/cm in the wavelength range 310–410 nm, and the emission efficiency was 0.7% of deposited energy. The best result was obtained for nitrogen excited at a frequency of 30 kHz (at a pressure of ~18 Torr and an excitation power of ~1 W/cm). The lamp operation mode was optimized by varying the frequency of exciting radiation from 10 to 100 kHz, input power from 5 to 30 W, and pressure in range 1– 30 Torr. A decrease in the exciting pulse repetition rate relative to the optimum frequency at the given voltage hindered a breakdown and resulted in unstable lamp operation. The discharge was not ignited with further decrease of the exciting pulse repetition rate. The decreased efficiency of spontaneous emission on the CПu–BПg nitrogen transition with increase in the exciting pulse repetition rate was due to a decrease in the gap breakdown voltage. A decrease in the pressure relative to the optimum one decreased the emission power and efficiency. We also investigated an optical medium based on a nitrogen and argon mixture for a wide range of conditions. It is well known that the output laser energy generated on the second positive system of nitrogen increases when the argon and nitrogen mixture is excited with an electron beam [7]. However, an admixture of argon to nitrogen for a nitrogen laser pumped by a self-sustained discharge resulted in a decrease of the energy and lasing efficiency [8]. In the present investigation, the best results were obtained with N2: Ar = 1: 70 (1.5 Torr N2 and 105 Torr Ar). A double mixture with a comparatively small nitrogen content allowed the average output power and efficiency of the lamp to be increased several folds. The results of optimization of the frequency and excitation power are shown in Fig. 1. It illustrates the dependence of the specific emission power I and efficiency ν on the specific excitation power Р at frequencies of 30 and 50 kHz. From the dependence obtained it follows that for the same excitation power, the emission power and efficiency for a frequency of 30 kHz are twice greater than for 50 kHz. A decrease in the pulse repetition rate, as in the case of pure N2, hindered a breakdown, and a decrease in the pressure at the same excitation power decreased the emission power and efficiency. The typical emission spectrum of the argon and nitrogen mixture is shown in Fig. 2. The maximum intensity has the line with λ = 337.1 nm.

UV lasers on N2—SF6 and N2—NF3 mixtures pumped by transverse and longitudional discharges

Lasers on the second positive nitrogen system (the C3Πu—B3Πg electronic band, λ=337.1 and 357.7 nm) are studied. Mixtures of nitrogen with electronegative gases (SF6 and NF3) are pumped by transverse and longitudinal discharges. The amplitude—time and spectral characteristics of a laser pumped from capacitive and inductive energy storage units are measured. The output energies of the laser pumped by a transverse discharge from a capacitive and inductive storage units are 17 and 18.4 mJ, respectively. It is shown that the mean output energy in N2—NF3 mixtures continues to increase at higher pulse repetition rates than in nitrogen. Under the pumping by a transverse discharge, two-peak 357.7-nm lasing with a total pulse duration of >40 ns is achieved. It is established that adding SF6 to the mixture enhances the output power of a longitudinal-discharge-pumped nitrogen laser.

Study of the transition from oxidation to nitriding in a single N 2–H 2–O 2 post-discharge

The nitriding (understood here as the reduction of native oxide present on iron samples followed by the diffusion of nitrogen in the solid) or the oxidation of a 25-μm-thick iron foil is performed in a N 2–0.5 vol.% H 2–0.012 vol.% O 2 post-discharge by simply varying the temperature from 673 up to 773 K. The ɛ-Fe 2N 1− x phase is obtained at 773 K whereas the Fe 3O 4 phase is clearly identified at 673 K. At 723 K, either an oxide or a nitride phase is synthesized, depending on the leakage in the reactor. The measurement of the intensity of the first positive system of nitrogen indicates that the time evolution of this signal could be correlated with the growth of the oxide layer at low temperature and with the nitrogen uptake by solid diffusion at high temperature.

Measurement of intensity ratio of nitrogen bands as a function of field strength

The ratio of intensities of the first negative system and the second positive system of nitrogen as a function of the electric field strength was experimentally determined in atmospheric-pressure air. The non-self-sustained discharge was excited in a parallel-plane gap using the radiation of a pulsed laser. The field strength was varied in the range of 135–285 Td. The peculiarities of the measurement and data processing of signals of very low intensity are described. The results are compared with the theoretical predictions of other authors.