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In this paper, temporal gas temperature in plasma was measured by Rayleigh scattering in a passive way since synchronization was difficult due to the randomness of current pulses. The plasma was generated between a 10 mm pin-to-plane gap connected to a H.V DC voltage through a 130 MΩ resistor and a skin sample was placed on a grounded plate. Even the plasma can be touched by a human hand without any feeling of warmth, the peak temperature could be 337 K then decrease to 295 K over 60 μs at 1 mm. Moreover, the applied voltage dramatically affects peak current and the peak temperature. Therefore, the transient “high” temperature cannot be touched and the so-called “cold” plasma might not be “cold”.

This work demonstrates a new pathway to the direct on-surface fabrication of surface coatings by showing that application of a plasma can lead to dehalogenative coupling of small aromatic molecules at a catalytic surface. Specifically, we show that a room temperature, atmospheric pressure plasma can be used to fabricate a coating through a surface-confined dehalogenation reaction. Plasma treatments were performed using a dielectric barrier discharge (DBD) technique under pure nitrogen with a variety of power levels and durations. Samples were analysed by optical and helium ion microscopy (HIM), X-ray photoelectron spectroscopy (XPS), optical profilometry, and contact angle measurement. By varying the plasma parameters we could control the chemistry, morphology and roughness of the film. Surface wettability also varied with the plasma parameters, with high-dose plasmas leading to a hydrophobic surface with water contact angles up to 130°.

In this work, we demonstrate a flexible multi-pin plasma generator with movable electrodes, which can change the shape of the electrode array freely, and then provide a large-area uniform plasma for the treatment of surfaces of different shapes. Discharge characteristics including U–I waveforms and discharge images and sterilization performance under three different electrode configurations (flat–flat, flat–curve, curve–curve) are investigated. Very similar results are acquired between the flat–flat configuration and the curve–curve configuration, which is much better than that under flat–curve configuration. This flexible multi-pin plasma generator offers a simple method to treat different irregularly shaped surfaces uniformly with a single device. Moreover, this device provides a foundation for developing a self-adaption large-scale uniform plasma generator by further introducing automatic adjustment of the position of every electrode driven by motors with discharge current feedback in the following study. Thus it will promote the applications of atmospheric-pressure cold plasmas significantly.

A novel three-level coupled rotating electrodes air plasma with nano-sized TiO2 photocatalysts is developed for plasma-catalytic NO x production. The effects of plasma catalysis on NO x production with different air flow rates, different N2 fractions and different humidity levels are evaluated. Final results show the exceptional synergistic effect between TiO2 and three-level coupled rotating electrodes air plasma significantly increases the NO x concentration by 68.32% (from 4952 to 8335 ppm) and reduces the energy cost by 40.55% (from 2.91 to 1.73 MJ mol−1) at an air flow rate of 12 l min−1 and relative humidity level of 12%, which beats the ideal thermodynamic energy limit ∼2.5 MJ mol−1 for the thermal gas-phase process. A possible mechanism for enhanced NO x production with TiO2 is discussed: Highly energetic electrons in plasma contribute to the formations of the electron–hole pairs and oxygen vacancy (Vo) on the TiO2 catalyst surface, which may facilitate the dissociative adsorption of O2 molecules to form superoxide radical groups (like O2 −), and H2O molecules to form surface hydroxyl groups (like OH·), and thus, improving energy efficiency.

Minimizing the breakdown voltage and discharge current required to initiate direct in-liquid discharges, thus lowering power-source requirements and avoiding electrode ablation, is crucial for industrial applications of in-liquid plasmas. Here we demonstrate such considerable reductions by employing movable electrodes, without changing the electrode configuration or increasing the system complexity. The new mechanism is based on electrostatic electrode attraction resulting in a reduction in the discharge spacing by up to 6 times and facilitating a plasma initiation at lower breakdown voltages. The accumulated charges consumed by the discharge revert the electrodes to the initial positions, forming a gliding arc between the enlarged gaps and thus inhibiting current increases and electrode ablation.

Ammonia is one of our most important industrial chemicals supporting the global food supply as the major crop fertilizer. Moreover, it is increasingly being promoted as a promising carbon-free fuel source, and an energy storage and transportation medium. However, the current approach for ammonia synthesis, known as the Haber-Bosch process, requires large-scale infrastructure preventing the designs of decentralization. The process consumes a large fossil fuel input leading it to be a major source of CO2 emissions. Plasma-enabled ammonia synthesis provides a clean, sustainable and flexible alternative, where the process is driven by the use of plasmas that activate the source gas(es). However, atmospheric plasmas are complex due to their highly reactive environment (energetic electrons, reactive oxygen and nitrogen species, UV photons, electric field effects, and others), resulting in a challenging scientific issue for both plasma researchers and chemical engineers. The review summarizes the current state-of-the-art of plasma-enabled ammonia synthesis, and provides insights into the fundamental physio-chemistry of plasma activation, including the excitation, dissociation and ionization of feedstocks, as well as the underlying mechanisms for the reaction dynamics of reactive species in the highly-reactive plasma environment. Finally, the opportunities and challenges for this plasma-enabled technology are outlined to approach a sustainable and flexible ammonia industry.

Interactions between effects generated by cold atmospheric-pressure plasmas and water have been widely investigated for water purification, chemical and nanomaterial synthesis, and, more recently, medicine and biotechnology. Reactive oxygen and nitrogen species (RONS) play critical roles in transferring the reactivity from gas plasmas to solutions to induce specific biochemical responses in living targets, e.g., pathogen inactivation and biofilm removal. While this approach works well in a single-organism system at a laboratory scale, integration of plasma-enabled biofilm removal into complex real-life systems, e.g., large aquaculture tanks, is far from trivial. This is because it is difficult to deliver sufficient concentrations of the right kind of species to biofilm-covered surfaces while carefully maintaining a suitable physiochemical environment that is healthy for its inhabitants, e.g., fish. In this work, we show that underwater microplasma bubbles (generated by a microplasma-bubble reactor that forms a dielectric barrier discharge at the gas-liquid interface with the applied voltage of 4.0 kV) act as transport vehicles to efficiently deliver reactive plasma species to the target biofilm sites on artificial and living surfaces while keeping healthy water conditions in a multispecies system. The as-generated air microplasma bubbles and plasma-activated water (PAW) both can effectively reduce the existing pathogenic biofilm load by ∼83 and 60%, respectively, after 15 min of discharge at 40 W and prevent any new biofilm from forming. The generation of underwater microplasma bubbles in a custom-made fish tank for less than a minute per day (20 s per time, twice daily) can introduce sufficient quantities of RONS into PAW to reduce the biofilm-infected area by ∼80-90% and improve the health status of Cichlasoma synspilum × Cichlasoma citrinellum blood parrot cichlid fish. Species generated include hydrogen peroxide, ozone, nitrite, nitrate, and nitric oxide. Using mimicked chemical solutions, we show that the plasma-induced nitric oxide acts as a critical bioactive species that triggers the release of cells from the biofilm and their inactivation.

The twisted behavior of a rotating electrode atmospheric-pressure argon plasma jet is reported and its mechanism is investigated. It is found that the plasma channel turns into a smooth pathway when about 5% of O2 or 3% of N2 are added to the main working gas, argon. Further investigation shows that the rotational frequency of the electrode, which can enhance the turbulence of the gas flow, does not affect the appearance of the plasma channel for both pure argon and argon mixed with different amounts of O2/N2. The optical emission spectra show that the emission intensities of the transitions from seven excited argon levels to two metastable states decrease dramatically when O2/N2 is added. Detailed analyses suggest that when O2/N2 is added, both the concentrations of argon in the two metastable states and their lifetimes decrease significantly, which results in a transition of the discharge channel from a turbulent pathway to a smooth pathway. In short, the metastable state of argon rather than the seed electrons plays the key role in the appearance of the plasma channel.

The influence of gas pressure and electrical parameters on low-pressure plasma has been investigated in this paper. A fast intensified charge-coupled device (ICCD) imaging is utilized to examine the dynamics of plasma bullet and plasma plume length at different pressure and plasma parameters. The experimental results reveal that the gas pressure has great influence on both the length of the plasma and on the propagation velocity of the plasma bullet. By increasing the gas pressure from 15 to 15 000 Pa, both the length of the plasma jet and the velocity of the plasma bullet first increase and then decrease. The maximum value of plasma plume occurred at 100 Pa, in which the propagation velocity gets the maximum value of $5\times 10^{6}$ m/s. Further increasing the gas pressure to 15 000 Pa results in decrease in both the plasma length and bullet velocity. Relative to voltage, the frequency of the applied pulse voltage has a different impact on the plasma propagation at different gas pressures. While a higher voltage induces a longer plasma plume and a faster propagation velocity at both 30 and 2000 Pa, increasing the frequency has little influence on plasma at 30 Pa but reduces the plasma length and propagation velocity observably at 2000 Pa. In addition, an exotic plasma plume structure is observed by ICCD. A possible hypothesis is discussed here to explain this phenomenon. These experiments may lead to a more precise control of the low-pressure plasma, warranted for advanced materials technologies.

Novel plasma-based technologies that offer maximum efficiency at minimal environmental costs are expected to further promote the sustainable societal and economic development. Unique transfer of chemical reactivity and energy from gaseous plasmas to water takes place in the absence of any other chemicals, but results in a product with a notable transient broad-spectrum biological activity, referred to as plasma-activated water (PAW). These features make PAW a green prospective solution for a wide range of biotechnology applications, from water purification to biomedicine. Here, we present a succinct review of how novel, efficient methods based on non-equilibrium reactive plasma chemistries can be applied to low-cost natural water sources to produce a prospective product with a wide range of applications while at the same time minimising the process steps and dramatically reducing the use of expensive and/or hazardous reagents. Despite the recent exciting developments in this field, there presently is no topical review which specifically focuses on the underlying physics and chemistry related to plasma-activated water. We focus specifically on the PAW generation, origin of reactive species present in PAW, its related analytical chemistry and potentially different mechanisms that regulate the bio-activities of PAW in different biotech-applications and their roles in determining PAW efficacy and selectivity. We then review recent advances in our understanding of plasma-water interactions, briefly outlining current and proposed applications of PAW in agriculture, food and biomedicine. Finally, we outline future research directions and challenges that may hinder translation of these technologies into real-life applications. Overall, this review will provide much needed insights into the fundamental aspects of PAW chemistry required for optimization of the biochemical activity of PAW and translation of this environment- and human-health-friendly, and energy-efficient strategy into real life applications.

Recent years have witnessed successful applications of non-thermal plasmas (NTPs) as efficient and flexible tools for direct conversion of liquid alcohols into higher-value energy carriers at mild or even near-room-temperature conditions. Better understanding of the fundamental physical properties of the liquid alcohol discharges is needed for development and optimization of NTP-based applications. Here we investigate the discharge characteristics of a plasma-based liquid ethanol conversion under different process parameters and reveal the general trends of the discharge behavior. Two discharge modes, namely the self-pulsed and direct current (DC) modes, are sustained during the in-liquid discharge. These two modes can be easily interchanged and flexibly controlled by adjusting the processing parameters. The behavior of the bubbles generated during the discharge plays an important role in the transition between the discharge modes. When the discharge is sustained with the diffusion and floating of the bubbles, the DC mode is observed. Otherwise, the discharge develops into the pulse mode. The production rates and composition of the gaseous products are also shown to depend on the discharge modes.

Sustainable and cost effective food preservation techniques are of industrial, environmental and public health significance globally. A promising means for the gentle but efficient sanitation of foods is the application of cold atmospheric plasma. Here, the preservation of fresh-cut apples was investigated using a gas phase surface discharge plasma (SDP) reactor within an exposure chamber. Results show that the microbial load reduction of the fresh-cut apples was found to be strongly dependent on the storage time and preservation method, e.g. refrigeration (control), SDP-room temperature and SDP-refrigeration (SDP-RF). After 6 d of storage, the microbe load on the apple pieces for the SDF-RF treated groups was found to be significantly lower compared to the refrigeration-stored (4 °C) and the SDP only-processed groups, with the lowest bacterial load on the 120 s SDP-RF stored apple pieces (1.76 CFU g-1). Furthermore, the effects of the preservation method on the quality attributes (weight loss, firmness, and physical appearance), and the surface chemistry directly after cutting and SDP processing, as well as the activities of polyphenol oxidase and peroxidase after the different duration of storage were evaluated. This study successfully demonstrates the feasibility of SDP for the effective preservation of fresh-cut apples and contributes to the fundamental understanding of surface plasma-induced effects on the microbial inactivation and postharvest quality of fresh-cut fruits.

Selective conversion of bio-renewable ethanol under mild conditions especially at room temperature remains a major challenge for sustainable production of hydrogen and valuable carbon-based materials. In this study, adaptive non-thermal plasma is applied to deliver pulsed energy to rapidly and selectively reform ethanol in the absence of a catalyst. Importantly, the carbon atoms in ethanol that would otherwise be released into the environment in the form of CO or CO2 are effectively captured in the form of carbon dots (CDs). Three modes of non-thermal spark plasma discharges, i.e. single spark mode (SSM), multiple spark mode (MSM) and gliding spark mode (GSM), provide additional flexibility in ethanol reforming by controlling the processes of energy transfer and distribution, thereby affecting the flow rate, gas content, and energy consumption in H-2 production. A favourable combination of low temperature (< 40 degrees C), attractive conversion rate (gas flow rate of similar to 120 mL/min), high hydrogen yield (H-2 content > 90%), low energy consumption (similar to 0.96 kWh/m(3) H-2) and the effective generation of photoluminescent CDs (which are applicable for bioimaging or biolabelling) in the MSM indicate that the proposed strategy may offer a new carbon-negative avenue for comprehensive utilization of alcohols and mitigating the increasingly severe energy and environmental issues.

APPJs are typically composed of bullet-like plasma volumes travelling with velocities of $10 ^{4-6}\mathrm {m} /\mathrm {s}$, which is close to the propagation velocity of traditional streamers. However, in contrast to the traditional streamers, which propagate in a stochastic manner, plasma bullets have been found to exhibit high repeatability, i.e., propagating the same distance after the same delay time. To understand the high repeatability of the plasma bullets, lots of work has been carried out recently. According to all the studies reported, it is believed that the repeatability of the plasma bullets might be related to background ionization level which is reflected by the density of seed electrons $( n_{seed})$in the gas channel. However, it is not known how high the seed electrons $n_{seed}$is needed for the plasma bullets to propagate in repeatable mode. Besides, when the plasma bullets exhibit random behavior, there are two potential reasons that could result in the unrepeatability, one is the randomness of the ignition and another one is the inconsistence of the propagation speed. This is not clear either.

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.

A plasma jet operated with atmospheric pressure air is presented. Unlike the dynamics of plasma jets working with noble gases, the propagation of the jet that is operated with air is primarily determined by the gas flow. This jet can be generated by applying a continuous, i.e., DC high voltage. However, depending on the applied voltage and gas flow rate a true DC operation can be distinguished from a self-pulsing mode. The gas temperature of the plasma plume when operated in the pulsed mode is lower than for the DC mode. Conversely, emission intensities of atomic oxygen, O, and nitrogen species, N 2 and N + 2 , are much higher for the pulsed mode than observed for DC operation.

Free to read on publisher website Plasma plumes have found a wide range of applications over the recent decade, stimulating studies of characteristics of plasma plumes generated under different conditions. Regardless of whether they propagate within a dielectric tube or different shielding gases, the behavior of these plumes will be affected by the boundary condition of the plume. Yet, at present, little is known about the behavior of plasma plumes of different polarities, especially negative plasma plumes, when propagating under different boundary conditions. To bridge this gap, in this paper, the characteristics of positive and negative plasma plumes propagating within a quartz tube, ambient Ar, and air are studied. The results reveal that the behavior of the positive plasma plume is similar under three different boundary conditions. However, this is not the case for the negative plasma plume, the behavior of which differs significantly between the three cases. Numerical simulation suggests that electron loss due to the drift in the radial direction impacts significantly the characteristics of the negative plasma plume.

A chiral plasma plume has recently been generated inside a dielectric tube without the use of an external magnetic field. In this paper, we seek to further study the key properties of such a chiral plume to improve our understanding of how this interesting structure is generated and controlled. The chiral plume is generated by externally mounting a stainless steel helical coil or a ring onto the dielectric tube. By changing the pitch of the helical coil, the pitch of the plasma plume can be controlled, with the shape of the plume following the shape of the helical coil. The addition of the helical coil significantly expands the range of parameters under which the chiral plasma plume appears. When the frequency of the applied voltage increases, additional stable discharge channels appear between the adjacent helices. The addition of two helical coils results in the formation of two chiral plasma plumes, which follow the shape of the helical coils. When a metal ring is placed on the outside of the tube, there is no chiral plasma plume between the high voltage electrode and the ring; however, a chiral plasma plume appears on the right side of the ring if the distance between the ring and the high voltage electrode is small. These findings suggest that the chiral plasma can be effectively modulated and guided using an externally mounted helical coil, which acts as the floating/actual ground to reduce the impedance of the discharge and as such contributes to the emergence of the chiral plasma plume behavior.

The propagation behavior of atmospheric-pressure plasma plumes has recently attracted lots of attention. In this paper, five different methods are used to measure the propagation velocity of an atmospheric-pressure plasma plume. The first method, named the ?current method,? obtains the propagation velocity of the plasma plume by measuring the currents carried by the plasma plume at different positions. The second method, named the ?voltage method,? obtains the plume propagation velocity by measuring the plasma plume voltage potential at different positions along the plasma jet with a voltage divider. The third method, called the ?charge method,? which significantly interferes with the plume propagation, estimates the plume propagation velocity by measuring the charges deposited on the surface of a quartz tube. The fourth method, which is the noninterference method, obtains the plume propagation velocity by capturing the dynamics of the plasma plume with an intensified charge-coupled device camera. Finally, the fifth method estimates the plume propagation velocity based on the temporal optical-emission intensity measurement of the selected species by using a spectrometer. The advantage and disadvantage of each method are discussed. The experimental results show that plasma plume velocities obtained from the five methods have reasonable agreement with each other. They are all in the range of 104 m/s.

In recent years, the use of shielding gas to prevent the diffusion of the ambient air, particularly oxygen and nitrogen species, into the effluent of the atmospheric pressure plasma jet, and thus control the nature of chemical species used in the plasma treatment has increased. In this paper, the radial propagation of a plasma jet in ambient Ar is examined to find the key determinants of the polarity of plasma jets. The dynamics of the discharge reveal that the radial diffusion discharge is a special phenomenon observed only at the falling edge of the pulses. The radial transport of electrons, which is driven by the radial component of the applied electric field at the falling edge of the pulse, is shown to play an important role in increasing the seed electron density in the surrounding Ar. This result suggests a method to provide seed electrons at atmospheric pressure with a negative discharge. The polarity of the plasma jet is found to be determined by the pulse width rather than the polarity of the ap...

In contrast to other atmospheric pressure plasma jets, two perpendicular jet-like plasmas generated in ambient air by a special designed plasma device are reported. High-speed photographs taken by an ICCD camera with exposure time of 2 ns show that the horizontal part of the plasma is actually ignited by the vertical part of the plasma. Both the vertical and horizontal parts of the plasma are in fact a small bullet like volume of plasma traveling along different directions at high velocities. The horizontal plasma volume velocity is about half of the vertical plasma volume velocity at the same instant.

Dynamics of atmospheric-pressure plasma plumes have recently attracted significant attentions. However, the nature of the ldquoplasma bulletrdquo behavior is still not clearly understood. The plasma bullets have some feature of a cathode-directed streamer. However, there are several notable differences between the streamerlike plasma plumes and the cathode-directed streamers. One of the differences is the repeatability (reproducible in time and space) of the two types of the discharges. All reports on the plasma bullet behavior suggested that the plasma bullets are very repeatable. This feature is different with that of the cathode-directed streamers, which are typically unrepeatable due to the stochastic nature of their initiation. In this paper, a simple plasma jet device is used to study the plasma bullet behavior. It is found that, when the applied voltage is 8 kV or lower, the plasma bullets are not repeatable. On the other hand, when the applied voltage is 9 kV or higher, they are very repeatable. This characteristic is similar with that of the cathode-directed streamer discharge.

It has recently been demonstrated that pulsed direct-current (dc) voltages show better performance in generating diffuse plasmas under various conditions. However, it still remains unclear whether the pulsewidth or the rising and falling times of the voltage pulse play the essential role in the improvement of the performance of the dielectric barrier discharges (DBDs). In this paper, we focus on the effect of pulsewidth. Pulsed dc voltages with pulsewidth varying from 0.2 mus to about 1 ms are used to drive the DBDs. High-speed photographs show that diffuse Ar plasmas can be generated by pulsed dc voltages with pulsewidths covering the entire investigated range. It is found that the pulsewidths of the applied voltages affect the discharge current durations significantly when the pulsewidth is shorter than 600 ns or the break between the two consecutive pulses is shorter than 600 ns.

The application of cold plasma in sterilization of a root canal of a tooth has recently attracted great attention. In this paper, a reliable and user-friendly plasma-jet device, which can generate plasma inside the root canal, is reported. The plasma can be touched by bare hands and can be directed manually by a user to place it into root canal for disinfection without causing any painful sensation. When He/O2(20%) is used as working gas, the rotational and vibrational temperatures of the plasma are about 300 K and 2700 K, respectively. The peak discharge current is about 10 mA. Preliminary inactivation experiment results show that it can efficiently kill enterococcus faecalis , one of the main types of bacterium causing failure of root-canal treatment in several minutes.

Atmospheric-pressure non-equilibrium plasma jets have recently received a major attention due to their widespread applications in surface and materials processing, biomedical engineering, and medicine. The discovery of fast-propagating plasma bullets opens an ex citing possibility to control sm all bits of plasma sover sub-microsecond and possibly even nanosecond t ime scales. However, this fascinating phenomenon is still challenging to control and is far from being understood despite of a few mechanisms proposed.

The breakdown delay time of a closed plasma plume excited by a high-voltage pulse is investigated. The visible monochromatic light of 404, 532, and 662 nm wavelength and narrow-waveband light at a central wavelength of 400, 430, 450, 470, 500, 530, 570, 610, and 630 nm are used to pre-ionize the gas. It is found that the breakdown delay time decreases when the visible light illuminates the discharge tube. The light is most effective when it is applied at the position near the high-voltage electrode. Besides, the tube material and size are important for enhancing the effect. The jet using quartz tube and larger inner diameter make the effect stronger. The effect of visible light is found to inversely relate to the wavelength, manifested by the longer breakdown delay times for longer wavelengths. With increasing the frequency and the pulse width of the voltage, the visible light shortens the delay time more effectively. These observations can be explained by the visible light-enhanced generation of free electrons before the ignition. The proposed mechanisms of free-electron generation are the optically stimulated exoelectron emission from the inner surface of the discharge tube wall and the vibrational excitation of nitrogen molecules. The effects of visible light weaken with the addition of oxygen as a result of electron affinity to oxygen.

The influence of background ionization on the ignition dynamics of the pulsed plasma plume is studied. The ignition delay time of each pulse is investigated by recording the voltage signal and the light emission signal. By changing the frequency, the relationship between the pulse-off time and the ignition delay time is revealed. This indicates that residual active species produced in the previous discharge play a role in the next one. With the decrease in the frequency, both time delay and ignition delay time increase. This is due to the decay of the reactive species densities in the pulse-off time. Lower concentrations of these species lead to a longer ignition delay time. The functions for calculating the ignition delay time are utilized to explain the effect of residual species. The independent data of each discharge also evidence the impact of the previous pulse. The exotic relationship between the ignition delay times of the first two pulses may be due to the electrode configuration used in this work. For a pin-to-plane electrode structure, the active species produced during the breakdown are accumulated around the anode (pin-point) where the discharge initiated for the asymmetrical electric field distribution.

In order to maximize the O concentration generated by the atmospheric pressure plasma jets (APPJs), several different methods of adding O2 additive to working gas have been proposed. However, it is not clear, which method is capable of generating the highest concentration of O atom. In this paper, the concentration of O atoms in an APPJs by adding O2 to (1) the working gas, to (2) the downstream inside the tube, and (3) to the shielding gas is investigated by two-photon absorption laser-induced fluorescence spectrometry. The results clearly demonstrate that the highest O density is achieved when 1.5% of O2 is added to the working gas rather than the other two methods. In other words, the most effective way to generate O atoms is by premixing O2 with the working gas. Further investigation suggests that O atoms are mainly generated around the electrode region, where the electric field is highest. In addition, when O2 is added to the working gas, if in the meantime extra O2 is added to the downstream inside ...

One of the significant differences between the traditional streamers and the plasma jets is the repeatability of their propagation. In this paper, the effect of the seed electron density on the repeatability of the plasma jets is investigated. The seed electron density plays an essential role in the propagation of plasma plume which is in either repeatable mode or random mode depending on the frequency of the applied voltage and the mixture percentage of the working gas. By measuring the propagation velocities and the ignition delay time, it is found that the propagation velocities of the plasma plume are independent of the seed electron density. However, the jitter of the ignition delay time strongly depends on the frequency of the applied voltage and the mixture percentage of the working gas. After detailed analyzing of the experiment results, it is concluded that the minimum seed electron density required for the plasma bullet to propagate in repeatable mode is on the order of 108 cm−3 for gas pressure of 2 × 104 Pa. The minimum required seed electron density for the gas pressure of 4 × 103 Pa is on the order of 107 cm−3. Further analysis shows that, at one atmospheric pressure, the required minimum seed electron density for repeatable mode is on the order of 109 cm−3.

In this paper, first, steady state of the plasma jet at different operating conditions is investigated through Schlieren photography with and without applying shielding gas. Second, the dynamic process for the plasma impacting on the gas flow field is studied. When the discharge is ignited, reduction in laminar flow occurs. However, when the gas flow rate is too low or too high, this phenomenon is not obvious. What is more, both frequency and voltage have significant impact on the effect of plasma on the gas flow, but the former is more significant. Shielding gas provides a curtain for plasma to propagate further. High speed camera along with Schlieren photography is utilized to study the impact of plasma on the gas flow when plasma is switched on and off. The transition of the gas flow from laminar to turbulent or vice versa happens right after the turbulent front. It is concluded that appearance and propagation of turbulence front is responsible for the transition of the flow state.

A low-temperature He microplasma plume generated in a micro quartz tube with the inner diameter decreasing from 245 μm to 6 μm is reported. The microplasma plume has a length of around 1.5 cm and reaches the position where the tube diameter is 10 μm. Though the inner diameter of the tube is in sub-millimeter, the cross section of the tube is not fully filled with the plasma only until the tube inner diameter is down to 30 μm. The electron density estimated from Hα stark broadening increases as the inner diameter of the tube decreases. The ignition voltage increases from 11 kV to 40 kV as the diameter of the inner quartz tube decreases from 245 μm to 10 μm. Further analysis shows that, in order to ignite a non-equilibrium plasma plume in 1 μm diameter tube, the applied voltage of about 65 kV is needed and the plasma density could be as high as ∼1018 cm−3.

Plasma jets generated with different working gases and propagating in different surrounding gases are studied. He plasma jets propagating in ambient Ar and N 2 show featherlike structures. Ar plasma jet diffuses in ambient He. In the same ambient gas, He plasma jets are longer than Ar plasma jets.

Using the preionization of an electron beam of 13 keV, the electron avalanche process in argon at a high pressure of 20 kPa was studied. A bright conical plasma plume, which looks like an electron avalanche, is captured when electrons are accelerated by a dc voltage of 8 kV.

Atmospheric pressure nonequilibrium plasmas have recently obtained considerable attention for several emerging applications such as surface modification and materials processing, biological and chemical decontaminations of media, thin film deposition, synthesis of nano materials, and water decontamination. On the other hand, because atmospheric pressure plasma jet devices can generate plasmas that are not spatially bound or confined by electrodes, they have obvious advantages over the traditional electrodes configuration. Because of these advantages, they are very desirable in many situations such as biomedical applications.

The fundamentals on the propagation of the atmospheric pressure plasma jets are not yet well understood. Teschke et al (2005), Lu et al (2006, 2008), Sands et al (2008), Shi et al (2008), and Ye et al (2008) studied the dynamics of the various plasma jets generated by different devices and sustained by different driving voltages. They all found that the plasma plumes are not a continuous volume of plasma; rather the plumes are more like a bullet formed by a small and well-confined plasma volume that travels from the exit aperture and terminates somewhere in the surrounding air. The speed of the “plasma bullets” varies from ∼ 104 – 105 m/s, which is several orders of magnitude higher than the gas velocities. Further analysis shows that the C-APPJs are electrically driven, which is quite similar to positive streamer discharges. However, there are several significant differences between the bullet-like plasma plumes and the widely studied positive streamers (cathode-directed streamer). For a positive streamer, the streamer head is connected to the power electrode by a highly conducting channel, which acts as a metallic “needle” protruding from the power electrode: the field at the end of the streamer is greatly enhanced. Obviously, this is not the case for the “plasma bullet”, which looks like isolated from the power electrode. Nevertheless, more studies are needed to have a better understanding of the plasma bullet behavior.

In this work, for better applications of atmospheric pressure plasma jets, the physics of plasma streamers in a glass tube with a part of it covered by a conductor is investigated. To better understand the propagation mechanism of plasma bullets in capillary tubes passing through a curved or narrow passage for some biomedical or material applications, the propagation of plasma streamers in a tube covered by a floating conductor is investigated. For a plasma streamer propagating in a tube covered by a conductor, the plasma streamer is suppressed and becomes shorter, and a secondary streamer is generated in the tube at the downstream end of the conductor. The larger the area covered by the conductor, or the thinner the tube, the stronger the plasma streamer is inhibited. The electric potential of the conductor is measured to be as high as 6 kV. On the other hand, a higher voltage applied on the HV electrode, or a higher gas flow rate will make the secondary plasma streamer longer. It is found that the capacitor formed by the conductor outside the tube and the wall of the tube plays an important role in inhibiting the original plasma streamer and generating the secondary streamer. Moreover, the active species generated by the original plasma play important role in generating a secondary plasma streamer.

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.

The dynamics of an atmospheric-pressure plasma plume generated in a controlled gas environment are studied. It is found that, at a particular pulse width, two or three plasma bullets are generated in each voltage pulse for both positive and negative voltage pulses. The velocities of the multiple bullets are more than four and less than fourteen times those in the case of a single bullet, for positive and negative voltage pulses, respectively. A detailed analysis shows that the charges carried by the bullets probably play an important role in the propagation of plasma bullets and in the formation of multiple bullets. It is found that the electrode polarity also plays an important role in the generation and propagation of multiple plasma bullets.

Up to now, all studies on the dynamics behavior of non-equilibrium plasma plumes were focussed on noble gas plasma plumes. It was found out that they are electrically driven rather than gas flow dependent. Our study on the dynamics of a non-equilibrium N2 plasma plume reveals that the propagation velocity of the N2 plasma plume is several orders magnitude lower than those previously reported and further studies show that it is close to the gas flow velocity. The gas flow has a significant effect on the length of the plasma plume, and the results provide some fundamental knowledge about atmospheric pressure plasma jets.

To better understand the variation in the “plasma bullet” velocity, the dynamics of an atmospheric pressure plasma plume driven by positive and negative pulses are investigated in detail. It is found that, before the plasma exits the nozzle, the plasma propagates at a speed of about 30 km/s for both positive and negative pulses. As soon as the plasma exits the nozzle, the plasma propagation speed increases dramatically for both cases. The peak velocity for the case of the positive pulse is much higher than that of the negative pulse, it is approximately 150 km/s and 70 km/s, respectively. According to the optical emission spectra, the acceleration behavior of the plasma bullet when it exits the nozzle is due to the increase in the N2+ concentration.

Cold atmospheric-pressure plasma plumes are generated in the ambient air by a single-electrode plasma jet device powered by pulsed dc and ac sine-wave excitation sources. Comprehensive comparisons of the plasma characteristics, including electrical properties, optical emission spectra, gas temperatures,plasma dynamics, and bacterial inactivation ability of the two plasmas are carried out. It is shown that the dc pulse excited plasma features a much larger discharge current and stronger optical emission than the sine-wave excited plasma. The gas temperature in the former discharge remains very close to the room temperature across the entire plume length; the sine-wave driven discharge also shows a uniform temperature profile, which is 20–30 degrees higher than the room temperature. The dc pulse excited plasma also shows a better performance in the inactivation of gram-positive staphylococcus aureus bacteria. These results suggest that the pulsed dcelectric field is more effective for the generation of nonequilibrium atmospheric pressure plasma plumes for advanced plasma health care applications.

Cold plasmas have recently received great attention. In this paper, optical and electrical diagnostics are carried out on a reliable and user-friendly plasma plume. A simple electrical model is used to simulate the electrical characteristics of the device. The plasma is represented by a resistor connected in parallel with a capacitor, an inductor, and another resistor, which are connected in series. The simulated current-voltage waveforms have very good agreement with experimental measurements. Besides, the emission spectra of the plasma are also studied. It shows that, when Ar is used as working gas, there is strong OH (hydroxyl radical) emission and the emission intensities of the N2 emission bands are more than three times higher than that of He. On the contrary, when He is used as working gas, the emission intensities of N2+ band are much stronger. Detail analyses on these observations are presented.

Rather than using noble gas, room air is used as the working gas for an atmospheric pressure room-temperature plasma. The plasma is driven by submicrosecond pulsed directed current voltages. Several current spikes appear periodically for each voltage pulse. The first current spike has a peak value of more than 1.5 A with a pulse width of about 10 ns. Emission spectra show that besides excited OH, O, N2(C–B), and N2+(B–X) emission, excited NO, N2(B–A), H, and even N emission are also observed in the plasma, which indicates that the plasma may be more reactive than that generated by other plasma jet devices. Utilizing the room-temperature plasma, preliminary inactivation experiments show that Enterococcus faecalis can be killed with a treatment time of only several seconds.

The propagation behavior of cold atmospheric pressure plasma jets has recently attracted lots of attention. In this paper, a cold He plasma jet generated by a single plasma electrode jet device is studied. The spatial-temporal resolved optical emission spectroscopy measurements are presented. It is found that the emission intensity of the He 706.5 nm line of the plasma behaves similarly both inside the syringe and in the surrounding air (plasma plume). It decreases monotonously, which is different from the emission lines, such as N2 337.1 nm line, N2+ 391.4 nm line, and O 777.3 nm line. For the discharge inside the syringe, the emission intensity of the He 706.5 nm line decays more rapidly than that of the other three spectral lines mentioned above. The N2 337.1 nm line behaves a similar time evolution with the discharge current. For the N2+ 391.4 nm line and the atomic O 777.3 nm line, both of them decay slower than that of the He 706.5 nm and the N2 337.1 nm. When the plasma plume propagates further awa...

The effects of various discharge parameters and ambient gas on the length of He atmospheric plasma jet plumes expanding into the open air are studied. It is found that the voltage and width of the discharge-sustaining pulses exert significantly stronger effects on the plume length than the pulse frequency, gas flow rate, and nozzle diameter. This result is explained through detailed analysis of the I-V characteristics of the primary and secondary discharges which reveals the major role of the integrated total charges of the primary discharge in the plasma dynamics. The length of the jet plume can be significantly increased by guiding the propagating plume into a glass tube attached to the nozzle. This increase is attributed to elimination of the diffusion of surrounding air into the plasma plume, an absence which facilitates the propagation of the ionization front. These results are important for establishing a good level of understanding of the expansion dynamics and for enabling a high degree of control of atmospheric pressure plasmas in biomedical, materials synthesis and processing, environmental and other existing and emerging industrial applications.

The “plasma bullet” behavior of atmospheric pressure plasma plumes has recently attracted significant interest. In this paper, a specially designed plasma jet device is used to study this phenomenon. It is found that a helium primary plasma can propagate through the wall of a dielectric tube and keep propagating inside the dielectric tube (secondary plasma). High-speed photographs show that the primary plasma disappears before the secondary plasma starts to propagate. Both plumes propagate at a hypersonic speed. Detailed studies on the dynamics of the plasma plumes show that the local electric field induced by the charges on the surface of the dielectric tube plays an important role in the ignition of the secondary plasma. This indicates that the propagation of the plasma plumes may be attributed to the local electric field induced by the charges in the bulletlike plasma volume.