Plasma Bullet Propagation Research Articles

Contrasting the characteristics of atmospheric pressure plasma jets operated with single and double dielectric material: physicochemical characteristics and application to bacterial killing

This study reports an experimental comparison of two types of atmospheric pressure plasma jets in terms of their fundamental plasma characteristics and efficacy in bacterial sterilization. The plasma jets are fabricated by inserting a high voltage electrode inside a one-end closed (double DBD plasma jet) or both ends open (single DBD plasma jet) quartz tubes which are further enclosed inside a second quartz tube containing a ground electrode. Both plasma jets are operated in contact with water surface by using a unipolar pulsed DC power supply with helium as the working gas. Results from electrical and time-resolved imaging show that the single DBD configuration induces 3–4 times higher accumulation of charges onto the water surface with significantly faster propagation of plasma bullets. These results are accompanied by the higher discharge intensity as well as stronger emissions from short-lived reactive species which were analyzed through optical emission spectroscopy at the plasma-water interface. The rotational temperature for the single DBD configuration was observed to be higher making it unsafe for direct treatments of sensitive biological targets. These characteristics of the single DBD configuration result in the production of more than two times higher concentration of H2O2 in plasma activated water. Shielding of the HV electrode reduces the plasma potential which in turn reduces the electric field & electron energy at the plasma-water interface. The reduced electric field for the double DBD configuration was lower by ≈463 Td than the single DBD configuration. The bactericidal efficacy of the two configurations of the plasma jets were tested against Escherichia coli, a well studied Gram-negative bacterium that can be commensal and pathogenic in human body. Our results demonstrate that although single DBD plasma jet result in stronger antibacterial effects, the double DBD configuration could be safer.

Optical diagnostics of the characteristics of a square unipolar nanosecond pulse-driven atmospheric pressure helium plasma jet

Compared with the traditional sinusoidal voltage source, a short rising nanosecond voltage source can generate a high electron current for a short rising time. This paper investigates how the nanopulse parameters such as the voltage amplitude, pulse duration, and repetition frequency affect the radical generation and the plasma bullet propagation in an atmospheric pressure helium plasma jet. An intensified charge-coupled device was used to observe the bullet propagation in the nanosecond gate mode. The plasma bullet’s propagation speed is mainly affected by the applied voltage and externally biased electrodes rather than the pulse duration or the driving frequency. In contrast, optical emission spectroscopy diagnostics estimate that the radical density inside the atmospheric pressure plasma jet mainly increases with the repetition frequency. At the same time, the population of high-energy electrons can be controlled with the unipolar voltage amplitude. Thus, unipolar nanosecond pulses make it possible to control the emitting charges and the generated radicals independently.

Acceleration of Plasma Bullets by Grooved Dielectric Rod

In this work, we studied hollow cylindrical plasma bullets propagating along the grooved glass rod in a single electrode configuration. The plasma source was a cylindrical shell consisted of two coaxial glass cylinders. The surface of the inner cylinder was structured by creating grooves on its surface to investigate the plasma bullet acceleration through the traveling shell. The corresponding plasma jet was driven by a 6.2-kHz sinusoidal waveform using helium gas. Optical and electrical characteristics proved that the plasma jet was working in a stable bullet mode. The propagation speed of the hollow cylindrical bullet was evaluated along with the downstream of the jet axis inside and outside of the plasma carrier shell. The results showed that the charge associated with the bullets and corresponding root-mean-square (rms) value of the alternative current (ac) electric field were increased by creating the grooves on the path of the bullets. It was observed that the radiation intensity of the plasma was increased around each groove, too. The structured inner cylinder revealed that the dielectric properties play an essential rule in the plasma bullet propagation through tubes.

Time-resolved imaging and spectroscopy of atmospheric pressure plasma bullet propagation and RONS production

An atmospheric pressure plasma jet visually appears as a continuous plasma, but is actually comprised of a series of fast moving ionization waves, commonly called plasma bullets. Plasma bullets are the visible front of ionization streamers. Studying their spatiotemporal formation and propagation can help understand the plasma dynamics. In this work, the formation and lifetime of a high voltage pulsed dc plasma jet and the subsequent bullets were observed using time-resolved synchronized measurements with an intensified charge-coupled device (ICCD) camera and an ICCD spectrometer. The operating voltage, helium flow rate, and pulse width were varied to observe changes in bullet behavior. The measured velocities of the plasma bullets largely depended on voltage but showed that flow rate can have an effect. The formation of OH and excited N2 (2+) was measured at the nanosecond timescale. It was seen that the formation of these species begins at the visible bullet, but the peak emission and decay occur ∼50–100 ns after bullet passage, indicating a finite time of reaction, excitation, and emission. Observing the temporal formation of plasma bullets and reactive species provides insight into some of the reactions occurring in the discharge and how to tailor the plasma operating conditions.

Plasma bullet propagation and reflection from metallic and dielectric targets

In this paper, we discuss the results from the computational investigation of the effect of a metal grounded target, metal target at a floating potential and dielectric targets (conductive and non-conductive) on the plasma bullet propagation and reflection. We show that the intensity of the primary ionization wave (IW) is the highest for the metal target, while it is significantly lower for the non-conductive dielectric. For the conductive dielectric, the wave intensity is greater than that for the non-conductive dielectrics, but lower than for a metal target. After the primarily forward IW touches the target, the reflected waves are observed for all the targets under investigation. For a metal target the reflected IW changes its direction and transforms into the secondary forward wave. We did not observe secondary forward IWs for dielectric targets. For dielectric targets, the reflected waves gradually decayed without changing their direction. The ‘stopping’ path for the reflected wave is introduced and the increase/decrease of this path is discussed in dependence on the target properties.

Propagation of Plasma Bullet in Impurity-Controlled Working Gas: from Standard to Ultrapure Atmospheric Pressure Plasma

Propagation of Plasma Bullet in Impurity-Controlled Working Gas: from Standard to Ultrapure Atmospheric Pressure Plasma

Numerical simulation of capillary helium and helium−oxygen atmospheric pressure plasma jets: propagation dynamics and interaction with dielectric

Atmospheric pressure plasma jets (APPJs) can be generated in capillary tubes flowing with pure helium and with admixtures of oxygen into the pure helium. The jet exiting the tube can be used for a variety of applications through surface interaction. In this study, a two-dimensional axi-symmetric model has been developed to provide insight into the evolution of capillary helium plasma jets with and without oxygen admixtures and their interaction with a dielectric surface placed normal to the jet axis. The model considers the gas mixing of helium and ambient air and the analytical chemistry between helium, nitrogen and oxygen species. Experiments were performed in conditions similar to those of the simulations in order to get qualitative agreement between them. The numerical and experimental results show that the evolution of the helium plasma jet is strongly affected by the introduction of oxygen admixtures. In particular, it was observed that the addition of oxygen admixtures in the helium gas promotes plasma bullet propagation on the axis of symmetry of the tube (in contrast to off-axis propagation for the pure helium plasma jet). On the other hand, the presence of the dielectric surface (a slab placed in front of the tube exit) forces the plasma bullet to spread radially. Furthermore, the plasma bullet speed decreases when the helium plasma jet is operated in the presence of oxygen admixtures. The numerical results also showed that He/O2 plasma jets induced much higher electric fields on the dielectric surface in comparison to the pure helium plasma jet.

Confluence or independence of microwave plasma bullets in atmospheric argon plasma jet plumes

Plasma bullet is the formation and propagation of a guided ionization wave (streamer), normally generated in atmospheric pressure plasma jet (APPJ). In most cases, only an ionization front produces in a dielectric tube. The present study shows that two or three ionization fronts can be generated in a single quartz tube by using a microwave coaxial resonator. The argon APPJ plumes with a maximum length of 170 mm can be driven by continuous microwaves or microwave pulses. When the input power is higher than 90 W, two or three ionization fronts propagate independently at first; thereafter, they confluence to form a central plasma jet plume. On the other hand, the plasma bullets move independently as the lower input power is applied. For pulsed microwave discharges, the discharge images captured by a fast camera show the ionization process in detail. Another interesting finding is that the strongest lightening plasma jet plumes always appear at the shrinking phase. Both the discharge images and electromagnetic simulations suggest that the confluence or independent propagation of plasma bullets is resonantly excited by the local enhanced electric fields, in terms of wave modes of traveling surface plasmon polaritons.

Simulation of a non-equilibrium helium plasma bullet emerging into oxygen at high pressure (250–760 Torr) and interacting with a substrate

A two-dimensional computational study of a plasma bullet emanating from a helium gas jet in oxygen ambient at high pressure (250–760 Torr) was performed, with emphasis on the bullet interaction with a substrate. Power was applied in the form of a trapezoidal +5 kV pulse lasting 150 ns. A neutral gas transport model was employed to predict the concentration distributions of helium and oxygen in the system. These were then used in a plasma dynamics model to investigate the characteristics of the plasma bullet during its propagation and interaction with a substrate. Upon ignition, the discharge first propagated as a surface wave along the inner wall of the containing tube, and then exited the tube with a well-defined ionization front (streamer or plasma bullet). The plasma bullet evolved from a hollow (donut-shaped) feature to one where the maximum of ionization was on axis. The bullet propagated in the gap between the tube exit and the substrate with an average speed of ∼2 × 105 m/s. Upon encountering a metal substrate, the bullet formed a conductive channel to the substrate. Upon encountering a dielectric substrate, the bullet turned into an ionization wave propagating radially along the substrate surface. For a conductive substrate, the radial species fluxes to the surface peaked on the symmetry axis. For a dielectric substrate, a ring-shaped flux distribution was observed. The “footprint” of plasma-surface interaction increased either by decreasing the gap between tube exit and substrate, decreasing the relative permittivity of an insulating substrate, or decreasing pressure. As the system pressure was lowered from 760 to 250 Torr, the discharge was initiated earlier, and the plasma bullet propagation speed increased. A reverse electric field developed during the late stages of the ramp-down of the pulse, which accelerated electrons forming a brief backward discharge.

N2 Mole Fraction Dependence of Plasma Bullet Propagation in Premixed He/N2 Plasma Needle Discharge at Atmospheric Pressure**supported by National Natural Science Foundation of China (No. 11465013), the Natural Science Foundation of Jiangxi Province, China (No. 20151BAB212012), and in part by the International Science and Technology Cooperation Program of China (No. 2015DFA61800)

In this work, a computational modeling study on the mechanism of the acceleration behavior of a plasma bullet in needle-plane configuration is presented. Above all, in our model, two sub-models of time-dependent plasma dynamics and laminar flow are connected using a oneway coupled method, and both the working gas and the surrounding gas around the plasma jet are assumed to be the same, which are premixed He/N2 gas. The mole fractions of the N2 (NMF) ingredient are set to be 0.01%, 0.1% and 1% in three cases, respectively. It is found that in each case, the plasma bullet accelerates with time to a peak velocity after it exits the nozzle and then decreases until getting to the treated surface, and that the velocity of the plasma bullet increases at each time moment with the peak value changing from 0.72×106 m/s to 0.80×106 m/s but then drops more sharply when the NMF varies from 0.01% to 1%. Besides, the electron impact ionizations of helium neutrals and nitrogen molecules are found to have key influences on the propagation of a plasma bullet instead of the penning ionization.

Dynamics of Plasma Bullets in a Microsecond-Pulse-Driven Atmospheric-Pressure He Plasma Jet

In this paper, an atmospheric-pressure helium plasma jet driven by a home-made microsecond pulse generator was studied. The discharge was characterized by monitoring the voltage–current waveforms and head-on/side-on intensified charge coupled device (ICCD) images. The effects of gas flow rate and pulse frequency on the plasma bullet propagation were studied. The plasma bullets showed a donut shape in the beginning of discharge and then became solid. This was due to the curling up of the plasma plume. A smaller gas flow rate was beneficial for a homogeneous discharge and a higher bullet peak velocity. The pulse frequency had no influence on the bullet peak velocity, but a higher frequency induced a faster bullet speed decay.

The density and velocity of plasma bullets propagating along one dielectric tube

This study shows that the propagation of plasma bullets along one dielectric tube is strongly affected by many discharge parameters, such as the waveform of applied voltage (AC or pulsed DC), peak voltage, He flow rate, and the frequency of AC voltage. Analysis indicates that the density and velocity of plasma bullets are mainly determined by the electric field at the front of plasma bullets. These discharge parameters may significantly influence the distribution of plasma potential along the tube, thus control the electric field at the front of plasma bullets and their propagation. An increase in the pulsed DC voltage with its rise time of <40-50 ns can lead to an obvious improvement in the electric field at the front of plasma bullets, resulting in generation of a plasma in the high density gas and a fast propagation of plasma bullets. He flowing through the tube can contribute to the surface diffusion of charged species, and greatly increase the electric field at the front of plasma bullets. During the propagation of plasma bullets, their density is decreased due to the surface recombination of charged species, such as electrons and ions.

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.

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.

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.

Propagation of plasma bullets in helium within a dielectric capillary—influence of the interaction with surfaces

This paper reports an experimental study on the propagation of an ionization wave inside a thin capillary, called ‘plasma bullet’. A low-frequency sinusoidal voltage is applied and several bullets are emitted during each cycle. The delay between two successive discharges is short enough so the propagation of one bullet is influenced by the charges deposited on surfaces by the previous one. It was shown that the propagation length increases during the cycle. An interesting result is that a discontinuity during the propagation of a bullet has been clearly pointed out. When it crosses the discontinuity, the bullet has been shown to increase its light emission and its propagation velocity. It was explained by a transition between regions of different potential, due to charges deposited by the previous bullets.

From short pulses to short breaks: exotic plasma bullets via residual electron control

Plasma plumes with exotically segmented channel structure and plasma bullet propagation are produced in atmospheric plasma jets. This is achieved by tailoring interruptions of a continuous DC power supply over the time scales of lifetimes of residual electrons produced by the preceding discharge phase. These phenomena are explained by studying the plasma dynamics using nanosecond-precision imaging. One of the plumes is produced using 2 – 10 μs interruptions in the 8 kV DC voltage and features a still bright channel from which a propagating bullet detaches. A shorter interruption of 900 ns produces a plume with the additional long conducting dark channel between the jet nozzle and the bright area. The bullet size, formation dynamics, and propagation speed and distance can be effectively controlled. This may lead to micrometer- and nanosecond-precision delivery of quantized plasma bits, warranted for next-generation health, materials, and device technologies.

Effects of seed electrons on the plasma bullet propagation

The effects of the seed electrons nseed of background gas of a plasma jet on the propagation of the plasma bullet are investigated. It is found out that the plasma bullet appears stable when the pulse repetition frequency is reduced from 10 kHz to 0.25 kHz. However, when the pulse frequency is further decreased to 0.2 kHz, the plasma bullet appears randomicity. Simulation results on the nseed for pulse frequency of 0.25 kHz immediately before the following pulse show that the nseed is on the order of 105 cm−3 near the nozzle, which is about five orders lower than the presumption of reported. Besides, the effect of the strong electric field in the detachment of O2− ions is investigated. It is found out that, whether the strong electric field considered or not in the detachment of O2− ions, there are no obvious differences of the spatial distribution of seed electrons nseed between them.