Normal Glow Regime Research Articles

Gliding arc/glow discharge for CO2 conversion: Comparing the performance of different discharge configurations

We studied the use of low current (hundreds of milliamperes) gliding arc/glow discharges for CO2 dissociation, at atmospheric pressure, in three different configurations. All of these are based on the gliding arc design with flat diverging electrodes. The discharge is mainly in the normal glow regime with contracted positive column. The CO2 gas is injected from a nozzle, at the closest separation between the electrodes. A pair of quartz glasses is placed on both sides of the electrodes, so that the gas flow is restricted to the active plasma area, between the electrodes. For two of the tested configurations, an external magnetic field was applied, to create a magnetic force, both in the direction of the gas flow, and opposite to the gas flow. In the first case, the arc is accelerated, shortening the period between ignition and extinction, while in the second case, it is stabilized (magnetically-stabilized). We studied two quantities, namely the CO2 conversion and the energy efficiency of the conversion. Generally, the CO2 conversion decreases with increasing flow rate and increases with power. The energy efficiency increases with the flow rate, for all configurations. The magnetically-stabilized configuration is more stable and efficient at low gas flow rates, but has poor performance at high flow rates, while the non-stabilized configurations exhibit good conversion for a larger range of flow rates, but they are generally more unstable and less efficient.

Generation and Diagnostics of Ambient Air Glow Discharge in Centimeter-Order Gaps

Re-pulsing glow discharges in gaps up to 2 cm were experimentally investigated in atmospheric pressure air. Discharge was ignited as a sequence of spark and glow discharge and sustained in a glow regime after ignition. It was confirmed that the discharge with defined parameters could be ignited without any adjustments after initiation of the discharge. The discharges were characterized by a high-speed camera, current and voltage measurements, and optical emission spectrometry (OES). Typical structure of the glow discharge consisting of cathode glow, dark faraday space, positive column, and anode glow was observed. Transition from subnormal to normal glow mode was confirmed by analysis of current and voltage waveforms. It was found by OES that the type of discharge strongly affects the ratio of produced reactive species. It was confirmed that with change of discharge from subnormal to normal glow regime, rotational temperature could vary across a wide range (from 1650 to 3500 K). The developed device allows glow discharge to be ignited with defined parameters, which makes the control of the gas temperature and ratio of reactive species possible by presetting the discharge parameters. Additionally, it was demonstrated that corona discharge or repeating spark discharges could also be generated using the developed device without additional manipulations of the experimental setup by adjusting the discharge parameters.

Comparison of deionized and tap water activated with an atmospheric pressure glow discharge

Liquid-cathode discharge at atmospheric pressure has been excited by a direct current voltage above the surfaces of deionized and tap water. Gap voltage decreases with increasing discharge current, which suggests that the discharges above both the liquids belong to a normal glow regime. For a given discharge current, gap and power voltages of the discharge above deionized water are higher than those above tap water. The optical spectrum obtained from the water surface reveals that there are abundant active species related to oxygen, nitrogen, and water vapor, which leads to changes in the pH value, conductivity, and concentrations of NO3−, NO2−, and H2O2 in liquid. Therefore, these parameters are investigated in the two liquids as functions of discharge current and plasma activation time. For both of them, the results show that the pH value decreases, while conductivity increment and concentrations of NO3−, NO2−, and H2O2 increase with increasing discharge current or activation time. Compared to activated tap water, deionized water has a lower pH value, while higher conductivity increment and concentrations of NO3−, NO2−, and H2O2 are observed after the same activation time and discharge current. Finally, these parameters are compared for different salt types and salt concentrations after plasma activation.

Spatial–Temporal Evolution and Plasma Parameters’ Diagnosis of a Transverse Glow Discharge in Atmospheric Pressure Air

A stationary transverse discharge excited by an alternating current voltage is generated downstream of two needle electrodes perpendicular to flowing air lower than 4.1 L/min. The results show that under a constant peak current, the discharge length increases with increasing the air flow rate. Under a constant flow rate, it becomes fatter and brighter with increasing the peak current. Through the voltage–current characteristics, it is found that the transverse discharge operates in a normal glow regime under a lower peak current and an abnormal glow regime under a higher peak current. The critical peak current from the normal glow transiting to the abnormal glow increases with increasing the air flow rate. By fast photography, it is found that, no matter which regime it is in, an emission channel always bridges the two electrodes. However, its emission intensity varies with time during one voltage cycle. This suggests that abundant charges and active species always exist in the channel. It is also found that characteristic regions can be observed for the normal glow discharge, which consist of a positive column, a Faraday dark space (FDS), and a negative glow (NG) at the voltage peak moment. NG alternately appears in the vicinities of the two electrodes. Compared with the normal glow discharge, FDS is absent, and the anode glow is more noticeable for the abnormal glow discharge. Based on the optical emission spectroscopy, the electron temperature, the vibrational temperature, the rotational temperature, and the electron density are investigated.

Numerical investigation of trichel pulse of negative corona discharge in N2–O2 mixture

Trichel pulse of negative corona discharge in atmospheric air is investigated numerically using a 2D fluid model. The model consists of a hyperbolic cathode tip and a plane anode, and considers 11 kinds of particles and the most important interactions among them. The spatio-temporal evolution of charged species and the electric field are evaluated during the pulse process. During the pulse rising edge, the positive ions accumulate ahead of the tip forming the temporal cathode sheath, significantly enhancing the local field. In the pulse decay edge, the temporal sheath collapses and the discharge falls back to a low-current mode. In the pulse interval, the discharge does not cease but sustains weakly until the next pulse. The location of the temporal sheath is independent of the averaged value during the Trichel pulse regime and also the same with that in a normal glow regime, which determines a nearly constant pulse rising time at given configurations. However, a smaller tip radius will lead to their decrease. The effect of negative ions on the pulse process is studied by adjusting the attachment rates. It indicates that the negative ions are actually not necessary in the Trichel pulse process, but will influence the pulse waveform significantly.

Three modes of a direct-current plasma jet operated underwater to degrade methylene blue

A direct-current air plasma jet operated underwater presents three stable modes including an intermittently-pulsed discharge, a periodically-pulsed discharge and a continuous discharge with increasing the power voltage. The three discharge modes have different appearances for the plasma plumes. Moreover, gap voltage-current characteristics indicate that the continuous discharge is in a normal glow regime. Spectral lines from reactive species (OH, N2, N2+, Hα, and O) have been revealed in the emission spectrum of the plasma jet operated underwater. Spectral intensities emitted from OH radical and oxygen atom increase with increasing the power voltage or the gas flow rate, indicating that reactive species are abundant. These reactive species cause the degradation of the methylene blue dye in solution. Effects of the experimental parameters such as the power voltage, the gas flow rate and the treatment time are investigated on the degradation efficiency. Results indicate that the degradation efficiency increases with increasing the power voltage, the gas flow rate or the treatment time. Compared with degradation in the intermittently-pulsed mode or the periodically-pulsed one, it is more efficient in the continuous mode, reaching 98% after 21 min treatment.

Trichel pulse in various gases and the key factor for its formation

We confirm in this paper that Trichel pulse of negative corona is a common phenomenon that can occur in various gases rather than only in electronegative gases as considered in the last 70 years since G W Trichel firstly reported. Trichel pulse is exactly a mode transition between low-current Townsend and high-current normal glow regime, two pulseless stages of negative corona with different operating conditions and ion flux threshold. The rising of the pulse corresponds to the breakdown and formation of temporal glow discharge, the decay corresponds to the destroy of temporal sheath, and the interval (or repetition frequency of pulses) is determined by the re-building of the positive ion cloud to enhance significantly the local electric field for glow discharge to form again. The positive ions play a predominant role for the pulse formation and the mode transition, while the negative ions in electronegative gas are not necessary even if they affect greatly the pulsing process.

Suppression of self-pulsing regime of direct current driven microplasma discharges

An instability suppressor circuit for self-pulsing direct current (DC) driven microplasma discharge is proposed and experimentally tested over a range of pd values for helium feed gas. The external circuit configuration suppresses self-pulsing of the discharge, extending the normal glow regime to lower currents. The negative differential resistance (NDR) region was observed to shift further left in the voltage–current parametric space (i.e., lower current), and the slope of the NDR region was decreased substantially. In addition, the suppressor element decreased the pulsing frequency as well as the static positive resistance of the discharge in the shifted NDR region. Modeling of the discharge configuration indicated that the inductor element of the suppressor circuit increases the time lag of the plasma response, which increases the stable region of operation. A stability map in terms of the external circuit parameters is introduced.

A dc glow discharge as a source of electrons for a portable mass spectrometer: characterization of the electron current intensity and electron kinetic energy distribution

A dc glow discharge cell with planar electrodes operating at about 1 Torr has been characterized with an inlet flow from ambient air. It will be used as a source of electrons to further ionize molecules by electron impact in a portable mass spectrometer.The Paschen curve has been measured for six values of the pressure around the minimum of the curve. The voltage-current characteristic, observed until 7 mA for different values of the pressure, increases with current in the normal glow regime. The temporal evolution of the discharge current has been recorded over long periods greater than one day for different insulator materials. The best stability has been obtained with Macor and Nylon. The available electron current for ionization has been measured at about 0.6 µA and the electron kinetic energy distribution has been found to be between 0 and a few tens of eV.

Characteristics of a Normal Glow Discharge Excited by DC Voltage in Atmospheric Pressure Air

Atmospheric pressure glow discharges were generated in an air gap between a needle cathode and a water anode. Through changing the ballast resistor and gas gap width between the electrodes, it has been found that the discharges are in normal glow regime judged from the current-voltage characteristics and visualization of the discharges. Results indicate that the diameter of the positive column increases with increasing discharge current or increasing gap width. Optical emission spectroscopy is used to calculate the electron temperature and vibrational temperature. Both the electron temperature and the vibrational temperature increases with increasing discharge current or increasing gap width. Spatially resolved measurements show that the maxima of electron temperature and vibrational temperature appeared in the vicinity of the needle cathode.

Effect of limiting the cathode surface on direct current microhollow cathode discharge in helium

This paper describes how to light several microdischarges in parallel without having to individually ballast each one. The V-I curve of a microhollow cathode discharge is characterized by a constant voltage in the normal glow regime because the plasma is able to spread over the cathode surface area to provide the additional secondary electrons needed. If one limits the cathode surface area, the V-I characteristic can be forced into an abnormal glow regime in which the operating voltage must increase with the current. It is then possible to light several microdischarges mounted in parallel without ballasting them individually.

DC normal glow discharges in atmospheric pressure atomic and molecular gases

DC glow discharges were experimentally investigated in atmospheric pressure helium, argon, hydrogen, nitrogen and air. The discharges were characterized by visualization of the discharges and voltage and current measurements for current of up to several milliamperes. Significant differences are seen in the gas temperature; however all the discharges appear to operate as temperature and pressure scaled versions of low pressure discharges. In the normal glow discharges, features such as negative glow, Faraday dark space and positive column regions are clearly observable. In hydrogen and to a lesser degree in helium and argon standing striations of the positive column were visible in the normal glow regime. Normal glow characteristics such as normal current density at the cathode and constant electric field in the positive column are observed although there are some unexplained effects. The emission spectra for each of the discharges were studied. Also the rotational and vibrational temperature of the discharges were measured by adding trace amounts of N2 to the discharge gas and comparing modeled optical emission spectra of the N2 2nd positive system with spectroscopic measurements from the discharge. The gas temperatures for a 3.5 mA normal glow discharge were around 420 K, 680 K, 750 K, 890 K and 1320 K in helium, argon, hydrogen, nitrogen and air, respectively. Measured vibrational and excitation temperatures indicate non-thermal discharge operation. Mixtures of gases achieved intermediate temperatures.

Modeling of direct current micro-plasma discharges in atmospheric pressure hydrogen

Numerical simulations and experimental studies were conducted to characterize direct current (dc) hydrogen discharge for a pin plate electrode configuration having an inter-electrode separation distance of 400 µm. A self-consistent two-dimensional hybrid model was developed to simulate the atmospheric pressure dc hydrogen micro-discharges. The discharge simulation model considered consists of momentum and energy conservation equations for a multi-component gas mixture, conservation equations for each component of the mixture (electrons, ions, excited species and neutrals) and state relations. The model uses a drift–diffusion approximation for the electron and the ion fluxes. The species considered include H, H2, H+, , , , H2 v=1 and the electrons. The electric field is obtained from the solution of Poisson's equation. Numerical simulations and experimental measurements indicated some of the key features of a normal glow discharge: flat voltage–current characteristics and constant cathode current density. Basic plasma properties such as electron number density, gas temperature, electric field and electron temperature were studied. The model predicted a constant current density of ∼22 A cm−2 in the normal glow regime. The normal current density was found to be a temperature scaled value of a low pressure normal current density. The ion Joule heating and Frank–Condon heating were found to be the dominant gas heating mechanisms. The peak gas temperature of ∼500 K indicated the discharge to be a non-thermal non-equilibrium discharge. Predictions from the model compares favorably well with the experimental measurements.

Spectroscopic studies and rotational and vibrational temperature measurements of atmospheric pressure normal glow plasma discharges in air

DC normal glow (NG) discharges were created in atmospheric pressure air for a pin to plate type geometry. The rotational and vibrational temperatures of the discharge were measured by comparing modelled optical emission spectra with spectroscopic measurements from the discharge. The temperatures were measured as a function of discharge current, ranging from 50 µA to 30 mA, and discharge length, ranging from 50 µm to 1 mm. Rotational temperatures from 400 to 2000 K were measured over this range. Vibrational temperatures vary from 2000 K to as high as 5000 K indicating a non-equilibrium plasma discharge. Spectroscopic measurements were compared using several different vibrational bands of the 2nd positive system of N2, the 1st negative system of and the UV transitions of NO. NO and transitions were also used to determine the electronic temperature and density. The discharge temperature appears to be controlled by two cooling mechanisms: (1) radial conductive cooling which results in an increase in temperature with increasing discharge current and (2) axial cooling to the electrodes which results in a temperature saturation with increase in discharge current. The measured discharge temperature initially increases rapidly with discharge current then becomes nearly constant at a higher discharge current. Thus, radial cooling appears to dominate at lower discharge currents and the axial cooling at higher discharge currents. The vibrational temperature decreases with increasing rotational temperature due to increased vibrational to translation relaxation but the discharge remains non-thermal and stable over the range studied. The discharge appears to have a maximum vibrational temperature at the low current limit of the NG regime.

Characterization of a dc atmospheric pressure normal glow discharge

Atmospheric pressure dc glow discharges were generated between a thin cylindrical anode and a flat cathode. Voltage–current characteristics, visualization of the discharge and estimations of the current density indicate that the discharge is operating in the normal glow regime. Emission spectroscopy and gas temperature measurements using the 2nd positive band of N2 indicate that the discharge forms a non-equilibirum plasma. Rotational temperatures are 700 K and 1550 K and vibrational temperatures are 5000 K and 4500 K for a 0.4 mA and 10 mA discharge, respectively. The discharge was studied for inter-electrode gap spacing in the range of 20 µm–1.5 cm. It is possible to distinguish a negative glow, Faraday dark space and positive column regions of the discharge. The radius of the primary column is about 50 µm and is relatively constant with changes in electrode spacing and discharge current. Estimations show that this radial size is important in balancing heat generation and diffusion and in preventing thermal instabilities and the transition to an arc.