Discharges In Dry Air Research Articles

Characterization of High Voltage Cold Atmospheric Plasma Generation in Sealed Packages as a Function of Container Material and Fill Gas

While numerous experiments have demonstrated the efficacy of high voltage cold atmospheric pressure plasmas (HVCAPs) in sealed packages for microbial inactivation, the influence of the package on the emitted species measured during HVCAP discharge is poorly understood. This study elucidates the impact of the package on plasma generation in sealed packages for four separate gases (ambient air, commercial grade compressed air, a helium/air mixture, and nitrogen) placed in commercially available transparent plastic containers and bags representative of the materials used in the food industry. The container and bag individually reduced emission signal intensity by an average of 63 and 45%, respectively, across the measured wavelengths of 200–1100 nm, demonstrating that they acted as broadband absorbers. Neither the container nor bag caused additional emission lines to appear, indicating no significant effect on the types of species generated. Considering the minimum applied voltage necessary to induce a discharge, the power dissipated by the nitrogen and ambient air plasma generated at 72 ± 3.7 kV RMS were comparable to the compressed dry air discharge generated at 80 ± 3.7 kV RMS. The helium discharge at 37 ± 3.7 kV RMS absorbed approximately 92% more power than these gases. Rotational temperatures ranged from 285 K for helium to 479 K for compressed air. These results indicate that the package impacts the intensity distribution but not the presence of the most dominant peaks, although further studies are required to elucidate the impact on less intense peaks.

Modeling and Identification of Electrical Parameters of Positive DC Point-to-Plane Corona Discharge in Dry Air Using RLS Method

In this paper, we present a contribution to the electrical modeling of positive dc point-to-plane corona discharges in dry air (20%O2 and 80%N2) at atmospheric pressure. The electrical equivalent circuit of the proposed model is regarded as a variable resistor in series with a variable capacitor. The evolutions of the electrical parameters are obtained using a mathematical method of identification based on the recursive least squares algorithm. For the same experimental conditions, the comparison between the measured discharge current and the calculated currents obtained using the identified electrical parameters of the proposed equivalent model shows good agreement, which validates the model as well as the method of identification. To validate this work, results and discussions are given in two different operating points.

Branching and path-deviation of positive streamers resulting from statistical photon transport

The branching and change in direction of propagation (path-deviation) of positive streamers in molecular gases such as air likely require a statistical process which perturbs the head of the streamer and produces an asymmetry in its space charge density. In this paper, the mechanisms for path-deviation and branching of atmospheric pressure positive streamer discharges in dry air are numerically investigated from the viewpoint of statistical photon transport and photoionization. A statistical photon transport model, based on randomly selected emitting angles and mean-free-path for absorption, was developed and embedded into a fluid-based plasma transport model. The hybrid model was applied to simulations of positive streamer coaxial discharges in dry air at atmospheric pressure. The results show that secondary streamers, often spatially isolated, are triggered by the random photoionization and interact with the thin space charge layer (SCL) of the primary streamer. This interaction may be partly responsible for path-deviation and streamer branching. The general process consists of random remote photo-electron production which initiates a back-traveling electron avalanche, collision of this secondary avalanche with the primary streamer and the subsequent perturbation to its SCL. When the SCL is deformed from a symmetric to an asymmetric shape, the streamer can experience an abrupt change in the direction of propagation. If the SCL is sufficiently perturbed and essentially broken, local maxima in the SCL can develop into new streamers, leading to streamer branching. During the propagation of positive streamers, this mechanism can take place repetitively in time and space, thus producing multi-level branching and more than two branches within one level.

Measurement of vibrationally excited N2(v) in an atmospheric-pressure air pulsed corona discharge using coherent anti-Stokes Raman scattering

Vibrationally excited N2(v = 1, 2) in an atmospheric-pressure air pulsed corona discharge was measured using coherent anti-Stokes Raman scattering (CARS). In a dry air discharge, the vibrational temperature determined from the ratio N2(v = 2)/N2(v = 0), Tv2, was approximately 500 K higher than that determined from N2(v = 1)/N2(v = 0), Tv1, immediately after the discharge pulse. Both vibrational temperatures reached equilibrium within 100 μs after the discharge pulse by the vibration-to-vibration (V-V) process of N2-N2. The translational temperature was also measured using CARS. The rise in the translational temperature due to vibration-to-translation (V-T) energy transfer was not observed for a postdischarge time of 5 μs–1 ms in the dry-air discharge. However, when the air was humidified, a significant V-T energy transfer was observed. It was due to an extremely rapid V-T process of H2O-H2O following the V-V process of N2-H2O. Measurements showed that the humidification of the ambient air accelerated the decrease in the N2 vibrational temperature and increased the translational temperature. N2(v) was generated mostly in the secondary streamer, not in the primary one, according to estimation from the measured N2(v) density.

Investigation of the Electron Energy Distribution Function in Capacitively Coupled (13.56 MHz) Radio Frequency Discharge in Dry Air

In this study, the electron energy distribution function in capacitively coupled radio frequency dry air discharge in three different regions along discharge tube was measured. It was observed that all regions do have Fermi electron energy distribution function which increases in magnitude with increasing the applied power, and decreasing with increasing the internal pressure. This function magnitude reaches its highest value near the powered electrode, while it decreases towards the earthed one.

Ozone generation by negative direct current corona discharges in dry air fed coaxial wire-cylinder reactors

An analytical study was made in this paper for calculating the ozone generation by negative dc corona discharges. The corona discharges were formed in a coaxial wire-cylinder reactor. The reactor was fed by dry air flowing with constant rates at atmospheric pressure and room temperature, and stressed by a negative dc voltage. The current-voltage characteristics of the negative dc corona discharges formed inside the reactor were measured in parallel with concentration of the generated ozone under different operating conditions. An empirical equation was derived from the experimental results for calculating the ozone concentration generated inside the reactor. The results, that have been recalculated by using the derived equation, have agreed with the experimental results over the whole range of the investigated parameters, except in the saturation range for the ozone concentration. Therefore, the derived equation represents a suitable criterion for expecting the ozone concentration generated by negative dc corona discharges in dry air fed coaxial wire-cylinder reactors under any operating conditions in range of the investigated parameters.

Plasma chemistry in repetitively pulsed discharges in air at low pressures**This paper was presented as an invited talk at the 30th International Conference on Phenomena in Ionized Gases, held in Belfast, UK, 28 August–2 September 2011.

Repetitively pulsed discharges in dry air (N2–20%O2) at low pressures (1 Torr) are studied by means of a time-dependent kinetic model. Simulations are carried out for both pulse and afterglow phases, accounting for the temporal evolution of the composition of the gas between each pulse. Modelling results are computed for a dc current of 40 mA, pulse durations between 200 µs and 10 ms, and pulse repetition rates ranging from 20 up to 1000 Hz. This work shows that the vibrational distribution function of N2 molecules becomes more populated by increasing the pulse duration or the pulse repetition rate. A similar behaviour is also predicted for the final concentrations of NO(X), N(4S) and O(3P). It is shown that [NO(X)] increases with the duty cycle ratio independently of the pulse repetition rate, as observed experimentally under similar conditions. The influence of the pulse duration, repetition rate and number of pulses in the overall kinetics is analysed and discussed in detail.

Assessment of ozone generation in dry air fed silent discharge reactors

Two equations have been derived in this paper for expecting the ozone generation inside dry air fed silent discharge reactors. The derived equations have been built on theories of the silent discharges and their applications, reported in the literature. The two equations have been investigated individually with experimental results for ozone generated by silent discharges in coaxial cylindrical reactors. The reactors were stressed with an ac voltage and fed by dry air flowing with constant rates at atmospheric pressure and room temperature. The results, that have been calculated by the two equations individually, have agreed with the experimental results over the whole range of the investigated parameters. Subsequently, the two derived equations represent suitable criteria for expecting the ozone concentration generated by silent discharges in dry air fed coaxial cylindrical reactors under any discharge conditions in range of the investigated parameters.

Electrical Analysis of Pulsed Positive Multipoint Corona Discharges in Air at Atmospheric Pressure

Positive corona discharges in dry air at atmospheric pressure operating with a pulsed voltage supply are electrically analyzed for a multitip configuration using an increasing number of tips, from one up to six. The peak value of the instantaneous discharge current as a function of the number of tips and the modification of the branching structure are discussed.

Primary and Secondary Streamer Dynamics in Pulsed Positive Corona Discharges

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Preliminary comparisons between numerical simulation and experimental results are obtained under pulsed applied voltages in the case of positive point to plane corona discharges in dry air at atmospheric pressure. The experimental conditions correspond to 7 mm interelectrode distances with a tip radius of 20 <formula formulatype="inline"><tex>$\mu\hbox{m}$</tex></formula>. The calculations are based on a first-order 2-D electrohydrodynamic model including the main charged-particle/gas reactions occurring in dry air discharge. The development of the corona discharge is analyzed using streak camera pictures. The transition from the branching structure to the monofilamentary one is described. An interesting comparison is performed between the streak camera pictures and the spatiotemporal evolution of the calculated ionization rate. This shows a qualitative agreement in the development of both primary and secondary streamers. </para>

Influence of a Bias Voltage on the Characteristics of Surface Discharges in Dry Air

AbstractSummary: Surface discharges (SDs) in a three‐electrode system have been investigated. The electrode system consisted of an upper plane electrode (to apply a bias voltage) and a SD arrangement with metallic strip electrodes on the upper side of a dielectric plate, while the reverse side was covered with a metallic layer. When high voltage was applied to the metallic layer with grounded strip electrodes, SDs appeared in between the strips on the dielectric. The characteristics of SDs have been already the subject of a number of investigations with respect to different plasma technologies. In the present work, the influence of a dc bias voltage on the SD characteristics has been investigated, especially on current, active power, discharge structure and ozone formation in a flow of dry air. When applying dc voltages up to 20 kV of both polarities, a rising direct current appeared with densities up to more than 50 μA · cm−2. This effect was more pronounced with negative polarity. An influence of the air flow rate on the direct current was also registered. The structure of the SD observed with a CCD camera became more diffuse with a dc bias voltage. Furthermore, it depended on the kind of dielectric material under investigation. With negative dc voltages of more than 7 to 9 kV, filamentary discharge structures appeared in the gap between the plates. The structure of these discharges and the conditions of their appearance also depended on the dielectric material.Image of discharges on a rough pyroceramic surface (the dark strips mark the position of the electrode).magnified imageImage of discharges on a rough pyroceramic surface (the dark strips mark the position of the electrode).

Ozone Production by Primary and Secondary Streamers in Pulsed Positive Corona Discharge

AbstractThe energy efficiencies of ozone production by the primary and secondary streamers are studied in pulsed positive corona discharge. In a 20-kV dry air discharge, the two-dimensional distribution of ozone density shows that most of ozone is distributed in the secondary streamer channel, not in the primary one. It demonstrates that most of ozone is produced by the secondary streamer. However, the energies consumed by the primary and secondary streamers are estimated to be approximately equal in the 20-kV discharge. This result indicates that the energy efficiency of ozone production by the secondary streamer is much higher than that by the primary one. It is shown that high voltage operation is favorable for ozone production because the rise in voltage increases the ratio of energy consumed by the secondary streamer to that consumed by the primary one.

A Global Model of Chemical Vapor Deposition of Silicon Dioxide by Direct-Current Corona Discharges in Dry Air Containing Octamethylcyclotetrasiloxane Vapor

A model of the electron distribution in direct current corona plasmas is combined with a global chemistry model and a two-dimensional transport model to predict the rate of chemical vapor deposition of silicon dioxide on the discharge wire in both positive and negative discharges in dry air containing octamethylcyclotetrasiloxane. The gas-phase chemistry includes reactions to form atomic oxygen (O) and additional global reactions to form gaseous silicon dioxide precursors by the impact reactions of electrons and atomic oxygen with silicone molecules. Surface chemistry is approximated by a single step global reaction from gaseous to solid silicon dioxide. The rate coefficient between atomic oxygen and octamethylcyclotetrasiloxane is estimated from prior experiments to be on the order of 10 −12 cm 3 /molecule-s. The effects of discharge polarity, current, wire radius and air velocity (Peclet number for mass transfer) on the deposition rate are considered. Deposition rates can be minimized by using positive coronas instead of negative coronas for Peclet number less than 18.5. At higher Peclet numbers, the deposition rate is slightly higher in positive corona discharges, but devices used indoors should continue to use the positive corona in order to minimize the production of ozone. The deposition rate in the positive corona is relatively insensitive to air velocity for velocities from 0.044 to 10 m/s −1 . However,it may be minimized by operating the corona with the lowest current that provides adequate performance (e.g., particle charging) and the smallest wire that provides adequate mechanical strength.

Characterisation of low molar mass siloxanes extracted from crosslinked polydimethylsiloxanes exposed to corona discharges

Crosslinked polydimethylsiloxanes were exposed to corona discharges in dry air at normal pressure. Short-time solvent extraction and subsequent analysis of the extractables, by gas chromatography, mass spectrometry and size exclusion chromatography showed that, oligomers consisting mainly of cyclics with 4–9 repeating units were formed during corona exposure. The size distribution of the oligomers was independent of the crosslink density and corona exposure time. The amount of oligomers located at the surface increased, with increasing storage time, after the corona exposure in qualitative agreement with the ongoing hydrophobic recovery process. Longer extractions penetrated deeper into the samples, and, in addition to the cyclic oligomers, higher molar mass species (∼50,000gmol−1 for unexposed samples) were detected. Samples exposed to corona, treated in this way, showed a broadening of the high molar mass peak towards lower molar masses.

Surface Discharge Studies with Uniform Field Electrodes at Low Pressures

An investigation of power frequency (50 Hz) surface partial discharges in dry air, using 21r/3 Rogowski profile electrodes in the low pressure range of 0.067 to 91.333 kPa, shows that for the discharges occurring symmetrically around the electrodes and just outside the uniform field region, the breakdown voltages are 20 to 30% lower than those accounted for by the usual Paschen values. Emphasis, therefore, has been given to modified values of breakdown voltages for any useful calculations. The effect of reduced pressure on inception voltage has been discussed and an attempt has been made to explain the difference between the observed and calculated values on the basis of a pressure-dependent secondary ionization coefficient. It is shown that increasing the insulation thickness in a critical pressure range (0.067 to 0.400 kPa) does not allow any significant increase in the discharge free working stress of the insulation system. At higher pressures (>0.400 kPa) the increase in inception voltage with thickness and pressure follows an equation which is expected to hold for other insulating materials as well.