Negative Glow Region Research Articles

Simulation on the hollow cathode discharge in hydrogen

A rectangular hollow cathode discharge (HCD) in hydrogen with a pressure of 2 Torr is simulated using a 2-D fluid model. The potential, electric field, particle density, and average electron temperature are calculated. The discharge space consists of the cathode sheath region near the cathode electrode and the negative glow (NG) region in the central region of the discharge cell. A high electric field of thousands of V/cm and a low electric field of tens of V/cm appear in the cathode sheath region and NG region, respectively. The average electron temperature in the cathode sheath region is tens of eV, which is significantly higher than that in the NG region. Electrons and H3 + are the main negative particles and positive ions, whose peaks appear in the NG region, and the peak magnitude is on the order of 1010 cm−3. H atom is the highest-density neutral particle other than H2 with a peak density of 1013 cm−3. The reaction kinetics of the generation and consumption of different particles are explored. The results show that each reaction generates certain particles while consuming other particles, ultimately achieving a dynamic equilibrium in the density of various particles. The electrons mainly originate from the ground state ionization between electron and H2 (e+H2 → e+H2 ++e) and are consumed by the dissociative attachment (e+H2 → H−+H). The charge transfer collision reaction (H2 ++H2 → H3 ++H) is the only reaction that produces H3 + ions. Different reactions to the consumption of H3 + ions do not differ significantly. The generation and consumption of H mainly originate from the electron collision dissociation reaction (e+H2 → e+H+H) and the ionization reaction (e+H→H++2e).

Operating modes in a low-pressure glow discharge with hollow cathode

The paper deals with investigations of the discharge in hydrogen in conditions close to those for the trigger units of pseudospark switches. The following modes of discharge sustaining are classified: the Townsend discharge, the suppressed glow discharge with hollow cathode, the intermediate mode of the gradual transition from the suppressed discharge to the ordinary glow discharge, and the ordinary discharge in which the negative glow plasma occupies the whole depth of the cathode cavity. As applied to the ordinary discharge, the theoretical model that offers a possibility to estimate the negative glow parameters is developed. In the current range 1.6–10 mA the electron densities in the negative glow region are 1.7–8.7 108 cm−3. The interpretation for the mechanism of the current passage to the anode is also proposed based on the model. In particular, the explanation is given for the fact that the region of the positive column plasma near the flat anode appears at a low discharge current.

Application of $$\hbox {GK}^{{\mathbf {1}}} \Sigma ^{+}_{\mathbf {g}}$$, $$\nu ' = 0 \rightarrow \hbox {B}^{1} \Sigma ^{+}_{\mathbf {u}}$$, $$\nu '' = 0$$ hydrogen band for the axial temperature measurement in the cathode sheath region of an abnormal glow discharge

In this paper, optical emission spectroscopy technique was used to measure rotational and gas temperature distribution in the cathode-sheath region of an abnormal glow discharge operated in hydrogen and hydrogen–argon mixtures. The rotational temperature of excited electronic states of H $$_{\mathrm {2}}$$ was determined from the R-branch of the $$\hbox {GK}^{\mathrm {1}}\Sigma ^{\mathrm {+}}_{g}$$ , $$\nu '= 0 \rightarrow \hbox {B}^{\mathrm {1}}\Sigma ^{\mathrm {+}}_{u}$$ , $$\nu ''=0$$ band and compared with published results for the Q-branch of the Fulcher- $$\alpha $$ diagonal band $$\hbox {d}^{\mathrm {3}}\Pi ^{\mathrm {-}}_{\mathrm {u}}$$ , $$\nu ' \rightarrow $$ $$\hbox {a}^{\mathrm {3}}\Sigma ^{\mathrm {+}}_{\mathrm {g}}$$ , $$\nu '' (\nu '=\nu ''=0$$ ). The population of excited energy levels, determined from the relative line intensities, was used to derive the rotational temperature of the ground state of hydrogen molecule. The boundary between the cathode sheath and negative glow region is determined using Stark polarization spectroscopy of the hydrogen Balmer alpha line. The thickness of the cathode sheath is estimated by fitting the experimentally determined distribution of electric field strength to a suitable model function. Temperature distributions along the axis of the cathode sheath are also reported and discussed. The distributions of temperature show that both bands give consistent and reasonable values for the rotational temperature and they both mutually agree within the estimated experimental uncertainties of 15%. The reported results contribute to the development of the spectroscopic method for gas temperature measurement in pure molecular hydrogen and in the gas mixture with argon.

Optical and mass-spectral characterization of mixed-gas flowing atmospheric-pressure afterglow sources

Plasma-based ambient desorption/ionization (ADI) sources for mass spectrometry (MS) often use helium as the primary discharge gas due to the large reaction cross section of excited helium species with atmospheric gases, which ultimately leads to efficient reagent-ion formation. However, some studies have shown that mixed-gas plasmas provide unique advantages. For instance, our group showed that a helium‑oxygen flowing atmospheric-pressure afterglow (He:O2-FAPA) source yielded enhanced ion signals for small polar analytes, but compounds with aromatic rings underwent chemical modification to produce pyrylium species. Here, it is shown that the addition of H2 to the helium FAPA gas significantly decreased the reagent-ion signals, but analyte-ion signal was not affected to the same degree. In addition, mass spectra obtained with He:H2-FAPA were chemically cleaner with fewer ions stemming from analyte species and produced less oxidation of analytes. Addition of nitrogen increased the abundance of negative reagent ions (e.g., NO2− and NO3−) which led to enhanced ion signal for RDX. To better understand these unique advantages of a mixed-gas FAPA, the impact of molecular-gas (O2, N2, or H2) addition on the optical-emission spectra from the discharge was also measured. Spatially resolved emission was obtained from the anode- and negative-glow regions. In general, addition of small fractions (~0.1%) of molecular gases decreased helium and OH emission intensities. Atomic oxygen and hydrogen emission increased with the addition of O2 and H2, respectively. Furthermore, emission characteristics of N2, N2+, and NO with respect to molecular gas composition on He-FAPA were measured. Fundamental plasma parameters, such as OH rotational temperature (Trot), were also calculated for these mixed-gas systems.

Q-branch of fulcher-α diagonal bands for determination of the axial temperature distribution in the cathode sheath region of hydrogen and hydrogen-argon abnormal glow discharge

In this paper, we present results of rotational, vibrational, and gas temperature distribution study in the cathode-sheath region of an abnormal glow discharge in hydrogen and hydrogen-argon mixture. For this purpose, we used Q-branch of the Fulcher-α diagonal bands d3Π−u, ν′=0, 1, 2, 3→ a3Σ+g, ν″=0, 1, 2, 3. The boundary between the cathode sheat and negative glow region is determined as the position where the electric field strength falls to zero. For the electric field measurement Stark polarization spectroscopy of hydrogen Balmer alpha line was employed and the thickness of cathode sheat is estimated by fitting the experimentally obtained distribution of electric field strength to a suitable model function.The temperature values, measured in the cathode sheath from different vibrational bands, decrease with vibrational quantum number v′ increase. The vibrational temperature is determined from the relative intensities of vibrational bands of the Fulcher-α system (d3Π−u→ a3Σ+g electronic transition; Q-branch v′= v″=2 and v′= v″=3). Temperature distributions along the axis of the cathode sheath are also reported and discussed. The distributions of rotational temperature show that all three temperatures (gas, rotational and vibrational) vary along the cathode sheath region of discharge and that all four bands give consistent values for the rotational temperature. The measurements performed with high spectral resolution provide further insight into understanding or modelling the DC glow discharge. The reported results represent the contribution to the development of the spectroscopic method for gas temperature measurement.

1D PIC-DSMC analysis of a high-pressure nanosecond pulse discharge breakdown in helium

Kinetic simulations of plasma phenomena during and after formation of the conductive plasma channel of a nanosecond pulse discharge are analyzed and compared to existing experimental measurements. Particle-in-cell with direct simulation Monte Carlo collisions (PIC-DSMC) modeling is used to analyze a discharge in helium at 200 Torr and 300 K over a 1 cm gap. The analysis focuses on physics that would not be reproduced by fluid models commonly used at this high number density and collisionality, specifically non-local and stochastic phenomena. Similar analysis could be used to improve the predictive capability of lower fidelity or reduced order models. First, the modeling results compare favorably with experimental measurements of electron number density, temperature, and 1D electron energy distribution function at the same conditions. Second, it is shown that the ionization wave propagates in a stochastic, stepwise manner, dependent on rare, random ionization events ahead of the ionization wave when the ionization fraction in front of the ionization wave is very low, analagous to the stochastic branching of streamers in 3D. Third, analysis shows high-energy runaway electrons accelerated in the cathode layer produce electron densities in the negative glow region over an order of magnitude above those in the positive column. Future work to develop reduced order models of these two phenomena would improve the accuracy of fluid plasma models without the cost of PIC-DSMC simulations.

The smooth effect of fast electron detection in the positive column in DC glow discharge

Collision electron spectroscopy (CES) which is based on probe diagnosis and allows for the identification of the impurity gas in pure inert gas has provided a new method for gas analysis. Due to the higher electron temperature in the positive column, the negative glow region is frequently selected as the optimization for gas analysis in DC discharge operated at a relatively high gas pressure. In this paper, the effect of ambipolar diffusion of fast electrons, and the importance of ambipolar diffusion considered as a deep influence on the resolution of fast penning electrons in the positive column, have been verified by designed experiment. It was found that the increase in plasma density has an energy loss effect on fast electrons, resulting a smooth effect on fast electron detection at pressures of about few hundred Pascals.

Measurements of the Potential Profiles of Glow Discharges Using an Emissive Probe

Glow discharges are produced in air in a ~1-m-long glass vacuum tube under the pressure of 12-113 mtorr. We present the full potential profiles throughout the glow discharge regions characterized using a cold emissive probe with the current-bias method. Secondary electron emission from the probe likely determines the local plasma potential, which replaces the role of thermionic emission that fails in relatively high-pressure plasmas. The measured potential profiles show good agreement with the theoretical expectations. A large potential drop is measured in the cathode dark space followed by a potential plateau through the negative glow and Faraday dark space regions. Striations in the positive column are shown as stair-step-like multiple potential double layers with a potential step close to the ionization energy. This cold emissive probe method can be used for characterizing potentials in even higher pressure (>1 torr) glow discharges that are not yet well understood.

A novel gas sampling introduction interface for fast analysis of volatile organic compounds using radiofrequency pulsed glow discharge time of flight mass spectrometry

An improved gas sample introduction interface is developed and characterized for gas chromatography coupling and for direct injection of volatile organic compounds (VOCs), in a pulsed glow discharge (pulsed-GD) ion source coupled to a time of flight mass spectrometer (TOFMS) that is typically used for direct solid analysis. The novel interface allows the introduction of the analytes in the flowing afterglow region of the GD (a few mm away from the negative glow region) to reduce plasma quenching effects. Analyte ion signals are acquired in the temporal afterglow region, where low fragmentation of the molecular species is produced, providing useful qualitative and quantitative molecular information (e.g. molecular ion). Analytical capabilities of the pulsed-GD ion source with the novel gas sampling interface provides improved performance compared to previous designs. In particular, limits of detection for the analysis of VOCs in air were below (better) that legally established limits according to Directive 2008/50/EC of the European Parliament.

Comparative investigation of the emission characteristics between 6.78-MHz radio-frequency and direct-current glow discharge plasmas for atomic emission spectrometry

This paper focused on the characteristics of emission spectra excited by a Grimm-style glow discharge plasma source in a 6.78-MHz radio-frequency (RF) power mode, in comparison to a conventional direct-current (DC) power mode. For this purpose, Boltzmann plots for atomic emission lines of iron were analyzed in detail, when the RF forward power and argon pressure in the discharge chamber were selected as the experimental parameters. The Boltzmann plot for low-lying excited levels of iron atom (3.4–4.8 eV) was on a linear relationship to provide an excitation temperature, whereas emission lines having larger excitation energies (more than 5 eV) generally deviated upwards from the linear relation. Any systematic dependence on the spin multiplicity was not found among the high-lying excited levels in the Boltzmann plot. The reason for the phenomenon was considered as follows: (1) the excitation for the low-lying emission lines would be caused by a dominant thermal collision process and (2) any non-thermal process also contributed to excitations for the high-lying emission lines to cause their over-population as a whole. The resultant excitation temperature was obtained in a narrow range of 3400–3600 K, which was hardly affected by the discharge power and the argon pressure, and further, the excitation temperature was very similar to that of a DC plasma that we had already reported. This result suggested that the glow discharge plasma could be supported by a self-stabilized negative glow region, where the kinetic energy distribution of the particles would be changed to a lesser extent as far as a stable discharge could be maintained.

Studing of Some Characteristics and parameters of Argon Glow discharge plasma Using Hollow Anode Diameter

Hollow anode argon glow discharge plasma has been investigated experimentally at different argon gas pressure from constant discharge current. A sufficient high voltage has been applied among the electrodes to obtain breakdown. Firstly, we studied the influence of hollow anode diameter on the breakdown voltage and Paschens law. The inner diameters of hollow anodes used in our work were (10, 15, 20, 25, 30, 35, and 40) mm. Secondly under the same conditions we extended our study to measure some plasma parameters in the negative glow region using direct current argon glow discharge. The temperature and density of electrons in the negative glow were measured using double probes. From the (Ip-Vp) characteristics of double probes, we obtained plasma parameters by using computer MATLAB program. The results showed that the measured Pashence's curve closes to the well-known theoretical Pashence's law. The breakdown voltage and its minimum value decreased with increasing the hollow anode diameter. The Paschen’s curve became wide and shifted to lower pressure with increasing the diameter. The reduction area of hollow anode caused dens and luminous intensity of plasma to occur in the negative glow region. Increasing the diameter resulted in decreasing the temperature and density of electron.

DC glow microdischarge with a self-determined length in helium and argon at atmospheric pressure

The spatial characteristics of a stable DC glow microdischarge with currents of 1–20 mA ignited between a flat cathode and truncated cylindrical anode with a diameter of 100 μm in helium and argon at atmospheric pressure were studied. The focus of the present study is on the examination of the experimental conditions to obtain a stable and homogeneous state of the microdischarge at an electrode gap of 15–1500 μm. It is shown that such conditions are fulfilled when the parameter pL0 (p—gas pressure at 300 K; L0—electrode gap) corresponds to the values near the minimum of the Paschen curve, and the microdischarge choses its length, that is the microdischarge has a self-determined length. The main measured parameters are the voltage drops across the electrode gap and the discharge currents. In addition, the size and shape of the negative glow region of the discharge, cathode current density, and other characteristics were estimated.

Probe Diagnostics of Plasma Parameters in a Large-Volume Glow Discharge With Coaxial Gridded Hollow Electrodes

Results of the experimental investigation of glow discharge plasma created in a large-volume discharge chamber are presented. The discharge configuration consists of two coaxial gridded electrodes placed in a metallic vacuum chamber. The inner electrode and the chamber housing were grounded while time-varying power up to 2 kV with frequency 20 kHz was applied on the outer electrode. The discharge was created in air at a pressure of 50 Pa and in argon at pressures of 50 and 100 Pa. Measurements of plasma parameters were carried out with a single-probe technique using the commercial Impedans Langmuir Probe system. In order to perform time-resolved measurements, an additional synchronizing device was developed. Variation in time of such plasma parameters as electron temperature, charged particle densities, and electron energy distribution function was obtained during the experiments. It is shown that both electron temperature and density change slightly in time. Obtained results indicate that the plasma created in the considered chamber configuration is similar to the negative glow region of a direct-current glow discharge.

Emission characteristics of 6.78-MHz radio-frequency glow discharge plasma in a pulsed mode

This paper investigated Boltzmann plots for both atomic and ionic emission lines of iron in an argon glow discharge plasma driven by 6.78-MHz radio-frequency (RF) voltage in a pulsed operation, in order to discuss how the excitation/ionization process was affected by the pulsation. For this purpose, a pulse frequency as well as a duty ratio of the pulsed RF voltage was selected as the experimenter parameters. A Grimm-style radiation source was employed at a forward RF power of 70W and at an argon pressures of 670Pa. The Boltzmann plot for low-lying excited levels of iron atom was on a linear relationship, which was probably attributed to thermal collisions with ultimate electrons in the negative glow region; in this case, the excitation temperature was obtained in a narrow range of 3300–3400K, which was hardly affected by the duty ratio as well as the pulse frequency of the pulsed RF glow discharge plasma. This observation suggested that the RF plasma could be supported by a self-stabilized negative glow region, where the kinetic energy distribution of the electrons would be changed to a lesser extent. Additional non-thermal excitation processes, such as a Penning-type collision and a charge-transfer collision, led to deviations (overpopulation) of particular energy levels of iron atom or iron ion from the normal Boltzmann distribution. However, their contributions to the overall excitation/ionization were not altered so greatly, when the pulse frequency or the duty ratio was varied in the pulsed RF glow discharge plasma.

Formation and energy relaxation of a fast electron beam in cathode regions of a glow discharge in helium

Results of a three-dimensional direct statistical simulation of formation and energy relaxation of a group of high-energy electrons in cathode regions of the low pressure (p < 1 Torr) glow discharge in helium are given. It is demonstrated that the electron distribution function at the exit from the cathode sheath contains a group (beam) of high-energy electrons which did not suffer inelastic collisions and had energy close to the cathode fall potential. The beam is shown to be a main source of charged particle production in the negative glow region. The calculated electron energy distribution function is compared with experimental data.

Characterization of the DC glow discharge in longitudinal flowing helium at atmospheric pressure

A one-dimensional self-consistent fluid model was developed to study the direct-current (DC) glow discharge in longitudinal flowing helium (He) at atmospheric pressure. Distinct dynamical features of charged particles are observed at different He flow velocities, which is accompanied by a dramatic change of spatial discharge structure. The positive He flow leads to the reduction of charged particle densities in the positive column and negative glow regions, and even makes the negative glow region disappear. The negative He flow allows for the enhancement of charged particle densities in the negative glow regions. These observations above mainly lie in the fact that the convection effects due to the He gas flow influence the transport properties of charged particles through different ways by comparing the He flow velocity with the ion drift one. These findings are of guiding significance for the improvement of the chemical activity and work efficiency of plasma source by controlling the gas flow in practical industrial fields.

Influence of longitudinal argon flow on DC glow discharge at atmospheric pressure

A one-dimensional self-consistent fluid model was employed to investigate the influence of longitudinal argon flow on the DC glow discharge at atmospheric pressure. It is found that the charges exhibit distinct dynamic behaviors at different argon flow velocities, accompanied by a considerable change in the discharge structure. The positive argon flow allows for the reduction of charge densities in the positive column and negative glow regions, and even leads to the disappearance of negative glow. The negative argon flow gives rise to the enhancement of charge densities in the positive column and negative glow regions. These observations are attributed to the fact that the gas flow convection influences the transport of charges through different manners by comparing the argon flow velocity with the ion drift velocity. The findings are important for improving the chemical activity and work efficiency of the plasma source by controlling the gas flow in practical applications.

Electric field gradient in microplasmas

&#61472;Linear Stark splitting of the H&#946; Balmer line components and spatially resolved optical emission spectroscopy (OES) measurements were used to estimate the electric &#64257;eld gradient in the cathode sheath region (&#8764;70 &#956;m long) of an atmospheric pressure direct current argon &#64258;ow-stabilized microplasma jet. Also, plasma parameters in the negative glow region were investigated by both OES and electrical diagnostics. OES was used to diagnose near-cathode region (negative glow and cathode sheath) of the microplasma jet in order to measure both, the basic discharge parameters, in the plasma bulk, and electric &#64257;eld distribution along the cathode sheath. In addition, the main mechanisms responsible for H&#946; spectral line broadenings were investigated.

PIC/MCC Simulation of Radio Frequency Hollow Cathode Discharge in Nitrogen**supported by Natural Science Foundation of Hebei Province, China (No. A2012205072)

A two-dimensional PIC/MCC model is developed to simulate the nitrogen radio frequency hollow cathode discharge (rf-HCD). It is found that both the sheath oscillation heating and the secondary electron heating together play a role to maintain the rf-HCD under the simulated conditions. The mean energy of ions (N2+, N+) in the negative glow region is greater than the thermal kinetic energy of the molecular gas (N2), which is an important characteristic of rf-HCD. During the negative portion of the hollow electrode voltage cycle, electrons mainly follow pendulum movement and produce a large number of ionization collisions in the plasma region. During the positive voltage of the rf cycle, the axial electric field becomes stronger and its direction is pointing to the anode (substrate), therefore the ions move toward the anode (substrate) via the axial electric field acceleration. Compared with dc-HCD, rf-HCD is more suitable for serving as a plasma jet nozzle at low pressure.

Is the negative glow plasma of a direct current glow discharge negatively charged?

A classic problem in gas discharge physics is discussed: what is the sign of charge density in the negative glow region of a glow discharge? It is shown that traditional interpretations in text-books on gas discharge physics that states a negative charge of the negative glow plasma are based on analogies with a simple one-dimensional model of discharge. Because the real glow discharges with a positive column are always two-dimensional, the transversal (radial) term in divergence with the electric field can provide a non-monotonic axial profile of charge density in the plasma, while maintaining a positive sign. The numerical calculation of glow discharge is presented, showing a positive space charge in the negative glow under conditions, where a one-dimensional model of the discharge would predict a negative space charge.