Plasma Bullet Research Articles

Propagation of plasma bullet in U-shape tubes

In this work, to better understand the propagation mechanism of plasma bullets in capillary tubes, the propagation of plasma stream in a special designed U-shape tube is investigated. First, it’s observed that the smaller the distance d between the bended tubes is, the shorter the total length of the plasma stream is. Second, when d is reduced to 1 mm, the plasma stream in the lower part of the tube propagates along the upper inner surface of the tube rather than propagating in the whole tube uniformly. Third, high-speed photographs show that the plasma bullet starts to accelerate as soon as the secondary discharge is ignited, and propagating along the upper inner surface of the lower part of the tube. Such behavior is totally different with the propagation of plasma streams in straight tube. According to these results, we can conclude that the surface discharge plays an important role in the propagation of the plasma bullet and the conductivity of the plasma channel is relatively low, which is important for understanding the plasma bullet behavior.

Ion Electrical and Optical Diagnostics of an Atmospheric Pressure Plasma Jet

The characteristics of an atmospheric pressure plasma jet (APPJ) in He discharge are measured with electrical and optical diagnostics methods. The discharge phenomenon in one cycle of the APPJ was diagnosed using intensified charge coupled device (ICCD) imaging. The gate mode images show that the propagation of plasma bullets happens only when the applied voltage on the inner conductor is positive. Moreover, the Schlieren image of the plasma jet shows that the laminar flow is changed into a turbulent flow when the plasma jet is turned on, especially when the gas flow rate increases.

The role of residual charges in the repeatability of the dynamics of atmospheric pressure room temperature plasma plume

To better understand the role of residual electrons in the repeatability of an atmospheric pressure plasma plume, the characteristics of a helium plasma jet from the 1st, 2nd,… until the repeatable discharge pulse are investigated for the first time. It's found that the longest plasma plume is achieved in the 1st discharge pulse. The length of the plasma plume becomes shorter and shorter and reaches a constant value in the 3rd discharge pulse and keeps the same for the following pulses. The dynamics of the 1st discharge pulse show that the plasma bullet appears random in nature. Two photomultiplier tubes are used to distinguish the two potential factors that could result in the stochastic dynamics of the plasma bullet, i.e., stochastic ignition of the plasma and the stochastic propagation velocity. The results show that the stochastic propagation velocity occurs only in the 1st and the 2nd discharge pulses, while the stochastic ignition of the plasma presents until the 100th pulse. The dynamics of the plasma propagation become repeatable after about 100 pulses. Detail analysis shows that the repeatability of plasma bullet is due to the residual electrons density. The residual electron density of 109 cm−3 or higher is needed for repeatable discharges mode.

Pure air–plasma bullets propagating inside microcapillaries and in ambient air

This paper reports on the characterization of air–plasma bullets in microcapillary tubes and in ambient air, obtained without the use of inert or noble gases. The bullets were produced by nanosecond repetitively pulsed discharges, applied in a dielectric barrier discharge configuration. The anode was a tungsten wire with a diameter of 50 µm, centered in the microcapillary, while the cathode was a silver ring, fixed on the outer surface of the fused silica tube. The effects of the applied voltage and the inner diameter of the microcapillary tube on the plasma behavior were investigated. Inside the tubes, while the topology of the bullets seems to be strongly dependent on the diameter, their velocity is only a function of the amplitude of the applied voltage. In ambient air, the propagation of air bullets with a velocity of about 1.25 × 105 m s−1 is observed.

Influence of Atmospheric Pressure Guided Streamers (Plasma Bullets) on the Working Gas Pattern in Air

This paper is devoted to the study of gas flow fields related to helium atmospheric pressure guided streamer (plasma bullet) propagation in the air. For very weak up to moderate helium flows, the modification induced to the gas flow field by the plasma ignition is demonstrated; it is shown that the turbulent flow region is expanded and two conditions must be fulfilled regarding the working gas profile in the air for streamer propagation, i.e., laminar flow and high concentration in this laminar flow region. © 2014 IEEE.

Dynamic Evolution of Helium Atmospheric Pressure Plasma Jet With ITO-PET Electrodes

A helium atmospheric pressure plasma jet configured with transparent double ring electrodes is driven by an ac high voltage power, and its dynamic evolution in a whole cycle is carefully investigated with ns scale intensified-charge coupled device images. During the positive current pulse, a plasma bullet appears at downstream of active electrode and donut shape plasma presents inside ground electrode, respectively, and wavepacket-like behavior emerges in Dielectric Barrier Discharge (DBD) zone. However, for negative current pulse, the phenomena with double plasma bullets occur beneath active electrode, and plasma inside ground electrode is formed via overflow of diffuse plasma in DBD zone.

Dual Plasma Bullets Colliding Inside a Hollow Electrode of a Multielectrode Helium Plasma Jet

The dual plasma bullets colliding inside a hollow electrode of a helium plasma jet are observed for the first time. The jet device has a multielectrode configuration, i.e., several active and grounded electrodes are wrapped one by one on a quartz glass tube. The dual bullets are generated on the both sides of negative electrode. They move toward the central region of the hollow electrode, and finally, collide to each other. Some observations are novel, and may be useful for the further unravel of the underlying mechanism of plasma bullet.

Spatio-Temporally Resolved Mapping of Helium Metastable Density in an Atmospheric Pressure Plasma Jet

Tunable diode laser absorption spectroscopy was used to measure the spatio-temporal evolution of helium metastable atoms He (2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> S) in an atmospheric plasma jet. The production of these atoms is clearly correlated to the propagation of an ionization wave, the so-called plasma bullet. Whatever the applied voltage, the spatial structure of the density rapidly evolves from a donut shape to a conical form as the plasma propagates. For applied voltages higher than 4.5 kV, a complex transient structure appears at the tip of the jet.

Effect of Gas Flow Rate on Distribution of Active Species and Dynamics of an Atmospheric RF- (Bio) Plasma Jet

A fast imaging technique applied for RF plasma indicates that the plasma jet propagation dynamics, operating even at megahertz frequency includes the so-called plasma bullet phenomenon. Broadband emission and intensity of lines corresponding to excited Ar and OH are registered. The spatial distribution of ground state OH is also detected by means of laser-induced fluorescence diagnostics.

Photo-ionization and residual electron effects in guided streamers

Complementary experiments and numerical modeling reveal the important role of photo-ionization in the guided streamer propagation in helium-air gas mixtures. It is shown that the minimum electron concentration ∼108 cm−3 is required for the regular, repeated propagation of the plasma bullets, while the streamers propagate in the stochastic mode below this threshold. The stochastic-to-regular mode transition is related to the higher background electron density in front of the propagating streamers. These findings help improving control of guided streamer propagation in applications from health care to nanotechnology and improve understanding of generic pre-breakdown phenomena.

On the Mechanism of Ring‐Shape Structure of Plasma Bullet

Ring‐shape is one of the feature characteristics of plasma bullet. However, regarding the mechanism of the ring‐shape structure of the plasma bullet, it is still not clear. In this paper, a series of special designed experiments are carried out to investigate the mechanism of the ring‐shape structure of the plasma bullet. In these experiments, the effects of the surface discharge and the effect of the surrounding gas diffusion, thus the effect of Penning ionization can be separated. It is found that both the diffusion of the surrounding air and the surface discharge play important role in the forming of the ring‐shape structure.

Computational study of the interaction of cold atmospheric helium plasma jets with surfaces

We describe a computational modeling study of a cold atmospheric pressure plasma jet interacting with a dielectric surface placed normal to the jet axis. The plasma jet is generated by the application of a nanosecond pulse voltage applied to a dielectric tube through which the jet issues into ambient air. A base fluid flow field is pre-computed using a Navier–Stokes model for the helium jet impinging on the dielectric target surface with a two-species description for laminar diffusional mixing of the helium and ambient air streams. A self-consistent, multiple species, two-temperature model is used to describe the non-equilibrium plasma discharge dynamics in the presence of the base jet flow field. A single nanosecond pulse discharge event starting from initial breakdown in the dielectric tube, to propagation into the open gap, and finally the interaction with the dielectric surface is simulated. Initially, the plasma forms within the dielectric tube and propagates along the tube surface as a surface discharge driven by large induced electric fields produced by trapped charge on the dielectric surface. When the discharge reaches the end of the dielectric tube, the discharge transitions to a constricted fast ionization wave that propagates along the helium–air interface. The fast ionization wave eventually reaches the dielectric target surface where charged species are deposited as the discharge propagates parallel to the wall as a surface driven discharge. The surface driven discharge ceases to propagate once the quantity of air to helium is sufficient enough to quench the hot electrons and prevent further ionization. Due to the low speed of the flow discharge and the short life times of the radical species such as O, most of the radical species delivered to the surface are a result of the surface discharge that forms after the plasma bullet impinges against the surface. It is found that factors such as the thickness of the target dielectric and the profile of the stagnation helium–air jet significantly impact the net quantity of reactive particles delivered to the surface.

Interpretation of the gas flow field modification induced by guided streamer (‘plasma bullet’) propagation

Atmospheric-pressure non-equilibrium plasmas of noble gases in the form of ‘bullets’ have attracted considerable attention, against cold low-pressure or thermal atmospheric-pressure plasmas, for multidisciplinary scientific fields such as material science and biomedicine, due to their unique compatible features. A key factor for the efficiency of most of these systems is the interaction between the noble-gas channel, where the ‘bullets’ (streamers) propagate, and the plasma itself. It is the object of this paper to demonstrate this interaction and to provide the explanation on the gas flow field modification induced by the plasma ignition. A three-dimensional numerical model incorporating most of the governing equations, schlieren imaging and UV–visible high-resolution optical emission spectroscopy are applied. In accordance with the present results, the mechanism leading to the flow field alteration is clearly related to the electrohydrodynamic force, while it is demonstrated that the gas temperature plays a minor role.

Production of active species in cold helium–air plasma jets

Evaluation of the composition of chemically active species produced in the plasma bullet mode of cold atmospheric-pressure helium–air plasma jets is performed. Distributions of species densities along the jet, both in the region of streamer propagation and in the relaxation zone, are given. Dominating channels for the production of major active species (O, OH, NO, O3) are revealed. Effects of variation of the governing parameters (the frequency and amplitude of applied voltage pulses, the content of air in the helium–air mixture, the air humidity) on the plasma composition are discussed.

Influence of voltage magnitude on the dynamic behavior of a stable helium atmospheric pressure plasma jet

Effects of voltage magnitude on the development of a stable helium atmospheric pressure plasma jet are investigated by current measurements and high temporal-resolution streak images. Generated by a coaxial dielectric barrier discharge structure, the entire discharge can be classified into three regions: discharges in the tube gap, downstream jet, and up-streamer. The discharge morphologies of each region are analyzed. In the positive discharge phase, there are two discharges in the tube gap between the electrodes; the first one is ignited as corona and then developed into streamer corona, and the second one is similar with positive glow. The downstream jet is ignited independently from the discharge in the tube gap. Referred as “plasma bullet,” the dynamic behavior of the jet can be well described as a positive streamer. Under specific applied voltage, the jet is found to be composed by double bullets in which case the jet length decreases since that less charge is carried by the first bullet. The up-streamer can be captured as long as the discharge in the tube gap is activated. Propagating with velocity of ∼4 km/s, the up-streamer can be regarded as the extension of the first discharge in the tube gap. In the negative discharge phase, the discharge is confined in the tube gap with nearly symmetrical morphology with the positive one. Besides, with the rising of voltage, the negative discharge is initially intensified and then turns weaker after surpassing certain voltage, which may provide suitable condition for the occurrence of double-bullet phenomenon.

The isolated head model of the plasma bullet/streamer propagation: electric field-velocity relation

A model of the isolated streamer head based on Meek's criterion of the avalanche to streamer transition is applied for description of the plasma bullet propagation in a helium/air admixture. According to the model previously proposed by Kulikovsky for streamers in air, along with the knowledge of one of three parameters: electric field, ionization integral or the width of the space charge layer, the other two parameters could be determined. Furthermore, using the streamer current or radius, it is possible to determine the electric field-streamer velocity functional dependence. Obtained results showed satisfactory agreement with both the results of the fluid model from the literature and the experimental results of plasma jet studies. Finally, for the sake of comparison, streamer velocity dependence on the electric field strength range of 10–250 kV cm−1 is determined for helium, argon and air.

On the mechanism of acceleration behavior of plasma bullet

Two special experiments are designed to study the mechanism of the acceleration behavior of a plasma bullet when it exits a nozzle. First, a T-shape device is used to simulate the air diffusion when a plasma plume exits the nozzle. It is found that adding just 1% of N2, O2, or air to the main working gas He results in the acceleration of the plasma bullet. Second, materials of different permittivity are added to the left part of the outside of the tube. The experimental results show that the plasma bullet accelerates at the moment when it enters into the right part of the tube where there is no extra material on the outside of the tube. These two experiments confirm that the acceleration behavior of the plasma bullet when it exits the nozzle is due to the air diffusion, hence Penning ionization, and the permittivity change when the bullet exits the nozzle, for example, from a tube with high permittivity to air with low permittivity. Besides, electric field measurements show that the electric field in the bullet head increases when the plasma bullet accelerates. This confirms the electric field driven nature of the plasma bullet propagation.

Two-dimensional numerical study of two counter-propagating helium plasma jets in air at atmospheric pressure

In this paper, a computational study of two counter-propagating helium plasma jets in ambient air is presented. A two-dimensional fluid model is applied to investigate the physical processes of the two plasma jets interaction (PJI) driven by equal and unequal voltages, respectively. In all studied cases, the PJI results in a decrease of both plasma bullets propagation velocity. When the two plasma jets are driven by equal voltages, they never merge but rather approach each other around the middle of the gas gap at a minimum approach distance, and the minimal distance decreases with the increase of both the applied voltages and initial electron density, but increases with the increase of the relative permittivity. When the two plasma jets are driven by unequal voltages, we observe the two plasma jets will merge at the position away from the middle of the gas gap. The effect of applied voltage difference on the PJI is also studied.

The influence of the geometry and electrical characteristics on the formation of the atmospheric pressure plasma jet

An extensive electrical study was performed on a coaxial geometry atmospheric pressure plasma jet source in helium, driven by 30 kHz sine voltage. Two modes of operation were observed, a highly reproducible low-power mode that features the emission of one plasma bullet per voltage period and an erratic high-power mode in which micro-discharges appear around the grounded electrode. The minimum of power transfer efficiency corresponds to the transition between the two modes. Effective capacitance was identified as a varying property influenced by the discharge and the dissipated power. The charge carried by plasma bullets was found to be a small fraction of charge produced in the source irrespective of input power and configuration of the grounded electrode. The biggest part of the produced charge stays localized in the plasma source and below the grounded electrode, in the range 1.2–3.3 nC for ground length of 3–8 mm.

Influence of Penning effect on the plasma features in a non-equilibrium atmospheric pressure plasma jet

Non-equilibrium atmospheric pressure plasma jet (APPJ) is a cold plasma source that promises various innovative applications. The influence of Penning effect on the formation, propagation, and other physical properties of the plasma bullets in APPJ remains a debatable topic. By using a 10 cm wide active electrode and a frequency of applied voltage down to 0.5 Hz, the Penning effect caused by preceding discharges can be excluded. It was found that the Penning effect originating in a preceding discharge helps build a conductive channel in the gas flow and provide seed electrons, thus the discharge can be maintained at a low voltage which in turn leads to a smaller propagation speed for the plasma bullet. Photographs from an intensified charge coupled device reveal that the annular structure of the plasma plume for He is irrelevant to the Penning ionization process arising from preceding discharges. By adding NH3 into Ar to introduce Penning effect, the originally filamentous discharge of Ar can display a rather extensive plasma plume in ambient as He. These results are helpful for the understanding of the behaviors of non-equilibrium APPJs generated under distinct conditions and for the design of plasma jet features, especially the spatial distribution and propagation speed, which are essential for application.