Stable Plasma Jet Research Articles

Research on Solid-State Linear Transformer Driver Power Source Driving Atmospheric Pressure Plasma Jet Treatment of Epoxy Resin

The Solid-State Linear Transformer Driver (SSLTD) is a nanosecond pulse power source characterized by its fast rise time and adjustable output waveform. It can generate uniform and stable atmospheric plasma jets, which is suitable for material surface modification. In this study, a 15-stage SSLTD was designed and assembled, which can produce a stable nanosecond pulse voltage up to 15 times the amplitude of the charging voltage at high frequencies, with a rise time of approximately 10 ns. This device can be used to generate stable atmospheric pressure Ar plasma jets with an electron density in the range of 1015~1016 cm−3 and gas temperatures close to room temperature. After the modification treatment by the plasma jets, the content of the C=O groups on the surface of the epoxy resin significantly increased in the wavelength range of 1720~1740 cm−1, and its flashover resistance was noticeably enhanced. The optimal comprehensive modification effect was achieved at a charging voltage of 600 V, pulse width of 50 ns, and pulse frequency in the range of 800~1000 Hz.

A novel triple-cathode plasma torch with hot-wall nozzle for YSZ spherical thin-walled hollow-shell powder preparation

Yttria-stabilized zirconia spherical thin-walled hollow-shell powder (YSZ-STHS) is a highly promising ceramic top-coat materials used in thermal barrier coatings with good homogeneity, a low thickness and no continuous flaws. However, the current technologies for preparing yttria-stabilized zirconia hollow spherical powder (YSZ-HOSP), such as spray drying, inductively coupled plasma spheroidization, and arc plasma spheroidization, face challenges in producing YSZ-STHS due to thick shell thickness and low hollow powder ratio. The triple-cathode plasma torch (TCPT) has characteristics such as high enthalpy and axial powder feeding, making it a promising tool for YSZ-STHS preparation. However, the plasma torch nozzle is often clogged due to powder deposition on the cold anode wall, limiting its application. To address this issue, a novel spheroidization method using a TCPT with hot-wall nozzle is proposed to achieve low shell thickness and high hollow powder ratio in YSZ-STHS powder preparation. Experiments were carried out to investigate the electro-thermal characteristics of the TCPT. The results indicated that the TCPT displayed a high mean enthalpy ranging from 2.37×107 J/kg to 3.36×107 J/kg and a low arc voltage fluctuation of ±1 V, indicating that the proposed TCPT is capable of producing a stable plasma jet with a high temperature. Additionally, experiments and numerical simulations were conducted to reveal the effect of the hot-wall nozzle. The results revealed that the hot-wall nozzle increased the inner wall temperature, decreased the radial temperature gradient of the plasma jet, reduced the nozzle clogging, and extends the high-temperature region and maximum temperature inside the nozzle. Finally, YSZ-STHS preparation by TCPT with hot-wall nozzle was demonstrated through experiments. The results showed that YSZ-STHS with a high spheroidization ratio close to 100%, a high hollow powder ratio of 93.4%, a thin shell thickness with an average value of 3.63 μm, and fully tetragonal phase was successfully prepared, demonstrating the effectiveness of the proposed TCPT with hot-wall nozzle for the preparation of YSZ-STHS.

Mechanism of Hydrogen Peroxide Formation by Alternating Current and Direct Current Plasma Jets

In this work, a discharge device is designed to realize the generation of alternative current (ac) and direction current (dc) cold atmospheric plasma (CAP) jet in the same generator for studying the production mechanism of H2O2 induced by plasma in water. The interaction between the plasma jets and deionized water is decomposed into three pathways by using a special reactor system, including electrochemical pathway, gaseous dissolution pathway, and the integrated pathway. Two kinds of ac plasma jets are generated by adjusting the electrode structure of the plasma generator, meanwhile two kinds of dc plasma jets are generated by adjusting the polarity of the discharge electrodes. Stable plasma jets are generated in all four configurations. The aqueous H2O2 as well as its vital precursor hydroxyl radical (OH) produced by the plasma jets is detected. The results show that the producing pathway of aqueous H2O2 depends on the position where the OH recombination reaction takes place during the treatment, and does not depend on whether the plasma is excited by ac or dc electric power.

Study on CO2 based thermal plasma torch and its effective utilization for material processing in atmospheric pressure

Reuse of greenhouse gases is one of the promising approaches towards controlling global warming to some extent. As a part of that, an attempt has been made to utilize CO2 gas as one of the primary gases to form a thermal plasma jet for material processing. For that purpose, a unique direct current (DC) non-transferred arc thermal plasma torch was designed and its current-voltage characteristics and electro-thermal efficiency with different discharge currents and gas compositions were studied. The torch is made of rod type cathode and a nozzle type anode, which can produce a stable thermal plasma jet under different gas compositions. CH4 was used as a secondary mixing gas in an Ar-CO2 gas composition to produce a carbon-rich thermal plasma with high electro-thermal efficiency. The maximum electro-thermal efficiency of 73 % were reached while the torch operated at 100 A current with 6 slpm of CH4 flow along with 15 and 20 slpm of Ar and CO2 gas flow, respectively. Torch efficiency decreased to 12 % with increase in discharge current from 100 A to 200 A. Optical emission spectrum of the plasma jet was recorded at the exit of the torch nozzle to understand the emission characteristics of the plasma and estimate temperature and electron density. Temperature of the plasma increased from 13,400 to 17,800 K with respect to the discharge current. Thermodynamical studies were carried out for the given experimental conditions to understand the thermal decomposition of CO2-CH4 gas mixture and select the suitable condition for the material processing. Under the selective conditions, plasma torch was utilized for the spheroidization of tungsten metal powder and carburization of the AISI 316 L stainless-steel coupons in atmospheric pressure without using any shielding gases. Results showed that around 85 % of the metal powders were spheroidized perfectly at 15 kW input power. Plasma carburization results showed that the microhardness of the steel surface reached 605 HV with a short processing time of 3 min which was 4 times higher than the untreated steel surface. This work provides useful insight for the utilization of CO2 as primary plasma forming gas to generate thermal plasma for material processing.

Unveiling the formation and control of unique swirling discharge pattern in helium plasma candle device

The generation of a large cold plasma jet while maintaining the reproducibility and homogeneity of the discharge is one of the major challenges encountered by the plasma community to efficiently apply this technology in the industry. Here, we report on the discharge in a recently developed device called the plasma candle (PC), wherein a stable plasma jet with a diameter of 20 mm can be generated at atmospheric pressure and temperature. Unlike the discharge morphology previously reported for conventional plasma jet devices, the unique configuration of PC device resulted in distinctive discharge patterns. Homogenous discharge was generated in the electrode gap and followed by a swirling discharge toward the tube nozzle. Fast photography and electrical measurements revealed that filament propagation and its morphology form the visually observable swirl discharge. Detailed analysis indicated that residual helium metastable species (Hem) and their penning ionization play an essential role in the discharge mode and its transition, which was verified by changing the feeding gas and the frequency of the applied voltage. For instance, it is found that only filamentary discharge was observed along the entire tube at frequencies less than 3 kHz, at which the time between consecutive discharges was long enough for Hem decay. Consequently, the homogenous discharge pattern was recovered by increasing the pre-ionization levels by adding a trace of impurities (N2, O2 or H2O) to the feeding gas. However, the level of these impurities must be carefully adjusted to achieve a homogenous discharge without negatively affecting the jet properties. A trivial change in the gas impurity, in the range of adsorption and desorption of water from the gas tubing, is sufficient to cause a noticeable change and instability in the discharge mode. This finding is critical to predicting the production of reactive species and plasma-surface interaction for different applications.

Concise characterization of cold atmospheric pressure helium plasma jet

This article provides a concise methodology for the development of a cold atmospheric pressure plasma jet and its characterization. To optimize the plasma jet parameters for biological and industrial applications, it is highly necessary to thoroughly understand its characteristics. The major emphasis of this work is to utilize simple and advanced diagnostics systematically with low complexity in the post-data analysis and to obtain in situ information of plasma jet parameters. The detailed optimization methods and the effect of the applied voltage and gas flow rate to achieve the stable plasma jet of the desired dimensions are discussed. In addition, the effects of the gas flow rate on the discharge current profiles and filament behavior are provided. Moreover, optical techniques, such as optical emission spectroscopy and time-resolved fast imaging, are used for the characterization of plasma parameters, i.e., Texc and ne, in a simple way. The gas temperature along the length of the plasma jet is estimated using a K-type thermocouple. The discussed simple characterization techniques and range of parameters of our designed plasma source will be useful for the development and optimization of plasma jet sources for various biological and industrial applications. Furthermore, we have also discussed various applications where we can use the discoursed diagnostics for the system development as well as for characterization. As the characterization of cold atmospheric pressure plasma jets is a multiphysics study, this concise characterization report on the cold atmospheric pressure plasma aims to provide necessary information for early researchers.

Towards launching a stable wide plasma jet from a single tube: I. The importance of controlling the gas dynamics

The success of plasma jets in broad fields is raising the demand to enlarge the jet size to be used in practical and industrial applications. However, conventional upscaling approaches that use multi-tubes have considerable drawbacks resulting from fluid-dynamics and electrical interactions among the jets. Therefore, developing a new upscaling approach to launch a wide plasma jet from a single tube is a great of interest. Thus, this study aims to unveil and address the challenges faced in obtaining a stable, uniform, and wide plasma jet from a single tube with a diameter of up to 30 mm. The focus is on the dynamic flow of the effluent gas, where its modification showed a significant influence on the jet formation in wide tubes. It was discovered that the presence of a sudden expansion flow at the inlet of wide tubes has a remarkable negative effect on the jet formation, owing to the increase in the air entrainment. Therefore, no discharge could be ignited at relatively low flow rates, and only an unstable jet with a brush-like shape could be launched at higher flow rates. Meanwhile, the mitigation of the sudden expansion influences by controlling the geometric parameters such as expansion ratio and length-diameter ratio at different Reynolds numbers indicated a significantly positive effect on the discharge ignition and stability of the formed jet. Furthermore, a new approach to address the challenges of upscaling the plasma jet is presented; in this approach, a porous material (a sponge) is used as a gas diffuser and embedded at the tube inlet. The gas diffuser greatly modified the gas flow inside the tube, thereby a stable, uniform plasma jet could be launched from a wide tube at relatively low flow rates. Finally, the interaction of the jet with different targets is presented to show the applicability of the launched jet. The treatment patterns from a wide single tube are similar to that observed using a narrow tube, where a wide diffused discharge spot found over a dielectric target, whilst intense discrete spots were created on a floating metallic target.

Towards launching a stable wide plasma jet from a single tube: II. Scaling parameters from the tube geometry and electrical characteristics

In this study, the characteristics of the plasma jet launched from a single-wide tube (diameter of up to 30 mm) are investigated under different operating conditions and electrode arrangements in order to derive empirical scaling equations, which can be used as guidelines for enlarging the jet diameter. The effect of these selected parameters on the jet formation was pronounced at wider tubes, due to the significant effects of the tube diameter (D) on the electric field as well as the formation and loss of plasma species. For instance, no jet was launched from tubes with D > 10 mm at frequencies less than 5 kHz, wherein a higher reduction in the plasma species. Moreover, the discharge had a chaotic behaviour in wide tubes, and the discharge stability was greatly improved by reducing the electrode gap (g); however, the jet was dismissed at a considerable smaller gap (g < 30 mm). Therefore, an optimum gap was found to launch a stable plasma jet, and its value increased linearly with D, particularly at D > 10 mm. Furthermore, a threshold velocity and power were required to launch a stable jet, and their values also increased linearly with increasing the tube diameter. Interestingly, it is found that there is an effective residence time for the gas in the discharge gap, wherein a stable jet can be launched from a tube regardless of its diameter. Finally, the role of the investigated parameters was analysed to formulate scaling equations for designing wide plasma jet devices from a single tube.

Characterization of a kHz sinusoidal Argon plasma jet impinging on water using Thomson scattering and fast imaging

The electron dynamics in a stable and non-filamentary Argon plasma jet, generated using AC excitation at kHz frequencies and interacting with a liquid surface either at floating potential or electrically grounded were examined using laser Thomson scattering. In the case of a floating liquid, two discharge events were observed during each half-cycle of the applied sinusoidal voltage. In the grounded liquid case only one discharge event was observed, which occurred during the positive half period. Through spatio-temporal imaging of the discharge, its repetitive breakdown behavior was analyzed and divided into pre-, main-, and post-breakdown phases. The dynamics and presence of the various phases differed depending upon the grounding of the liquid. Thomson scattering measurements revealed maximum electron densities and temperatures of 6.0–6.3 × 1014 cm−3 and 3.1–3.3 eV for the floating liquid case and 1.1 × 1015 cm−3 and 4.3 eV in the grounded liquid case. Electron-driven reactions are the primary source of reactive chemical species in a plasma jet. Therefore, the electrical characteristics of the liquid sample can impact the fundamental physicochemical processes at play in the discharge, ultimately influencing its chemical composition.

Investigation on the Microwave Excited Plasma Filament at Atmospheric Pressure

A high-efficiency microwave atmospheric pressure plasma jet (APPJ) has been developed in previous work. Long and stable plasma filament is excited directly in the open air with 2.45-GHz microwave power and double-channel argon flow by a double-coaxial structure. Unlike those plasma jets that strongly rely on a high flow rate to shape the plasma jet, this method produces a stable filamentous plasma jet at a low flow rate whose length and diameter are in a linear relationship with absorbed microwave power. This study aims to investigate the fundamental parameters of this special plasma filament under different power levels and gas flow rates by optical emission spectroscopy (OES), including the electron temperature and the spectrum. The electron temperature is determined to be 0.71–0.81 eV. It decreases slightly as the power increases, which is different from other high-voltage or radio frequency excited plasma jets under high power. The electron temperature does not change significantly with the change of microwave power and argon flow rate, more manifested as a change in the length, which indicates that this special plasma filament is related to the complex interaction of the microwave field and plasma. This study provides basic parameters for further investigation of this kind of plasma jet for theoretical and numerical simulation.

Observations of a plectonemic configuration in a stable magnetized plasma jet

Astrophysical jets are collimated high-speed outflows emerging from spinning and accreting matter around celestial objects and may spontaneously result from self-organized processes. Magnetic self-organization is commonly observed in laboratory plasma physics experiments; however, they require close-fitting flux conservers to constrain and stabilize the toroidal or cylindrical structures. Here we report the first observations of a long, stable, free-boundary plasma jet far from chamber walls, embedding a double-helix magnetic structure resembling a force-free plectonemic Taylor state. The jets arise from an experimental setup that mimics an accretion disk and has no close-fitting solid flux conserver. The results support the hypothesis that self-organization could be a universal, intrinsic explanation for jet formation, collimation, and stability and may help explain double-helix features in celestial observations.

Plasma-jet sterilization enhanced by ion wind

This work demonstrates that a needle-ring discharge device combined with an ion-wind generator can realize a plasma-jet system of self-inlet gas, replacing huge, unportable carrier-gas systems. First, a needle-ring atmospheric-pressure plasma-jet device is designed and manufactured. The effects of a dielectric tube and the discharge voltage on the jet characteristics are analyzed. The results show that a needle-ring discharge device with a dielectric tube can produce a long, stable plasma jet under a high DC applied voltage. Second, such a device and a 10-stage ion-wind device are combined into an integrated device, and the discharge characteristics are studied. The results show that an ion wind with a speed of 2.62 m/s can be generated when a DC voltage of −13 kV is applied to the ion-wind device. The plasma jet produced by the integrated device is longer and brighter when the ion wind is included. Finally, the ion wind is found to enhance the sterilization effect of the jet and greatly increase the sterilization range.

Benefits of Hydrogen in a Segmented-Anode Plasma Torch in Suspension Plasma Spraying

Suspension plasma spraying (SPS) enables the production of various coating microstructures with unique mechanical and thermal properties. Aeronautical manufacturers have been working for fifty years to improve the thermal barrier coating (TBC) performances in gas turbines. Commercial plasma torches with a segmented anode that are characterized by stable plasma jets should enable a better control of the TBC microstructure. The addition of diatomic gases such as hydrogen in the plasma-forming gas affects the plasma jet formation and causes some instabilities. However, it enhances the thermal conductivity of the gas flow, the plasma mass enthalpy and the heat transfer to particles. This study aims to characterise and describe the coating microstructure changes of yttria-stabilised zirconia when gradually adding hydrogen with argon into the plasma gas mixture. The effect of hydrogen is weighted out due to the gas mass enthalpy, mean velocity at the nozzle exit and “hot zone” length of the plasma jet. The coating microstructures, which depend on these plasma jet parameters, will be mapped from feathery and porous to dense and cracked deposits depending on the spraying conditions.

Deposition and oxidation behavior of atmospheric laminar plasma sprayed Mo coatings from 200 mm to 400 mm under 20 kW: Numerical and experimental analyses

A stable laminar plasma jet with a jet length of above 600 mm was adopted to deposit molybdenum coatings to investigate the behavior of high-melting point metal, molybdenum, and particles during laminar plasma spraying process. The temperature, velocity, and species distribution of the plasma jet were calculated by numerical simulation. In the experiment, the temperature and velocity distribution of the in-flight particles were measured, and the microstructure and properties of the coatings were analyzed. Combining the results of the simulation and experimental results, the deposition behavior and oxidation mechanism of the coatings prepared at the spray distance from 200 mm to 400 mm were discussed. The results indicate that both the laminar plasma jet and injected particles exhibit high temperature and low velocity, and low temperature and velocity gradients. The microstructure of the coatings shows a high similarity and hardly changes when the spray distance exceeds 250 mm. When the spray distance is 200 mm, post-deposition oxidation occurs due to the heating effect of the long plasma jet on the substrate. In contrast, when the spray distance is longer, the particles undergo severe in-flight oxidation, and the oxide formed is uniformly distributed in the coating improving the hardness from 500 HV0.3 to 700 HV0.3.

Effect of Electromagnetic Boundary Conditions on Reliability of Plasma Torch Models

The cascaded-anode plasma torch makes it possible to get a longer and more stable plasma jet with higher specific enthalpy than conventional plasma torches. It is now used widely, but there are still few models of the cascaded-anode plasma torch. This study developed a 3-D time-dependent model that couples the gas phase and electrodes by encompassing the electromagnetic and heat equations both in the electrodes and gas phase. The model was applied to a commercial plasma spray gun equipped with a single cathode, single cylindrical anode and an inter-electrode insert to fix the average arc length. This paper examines the effect of the boundary conditions for the magnetic vector potential and electric current density on the electromagnetic, velocity and temperature fields of the plasma jet. The model predictions showed that, for such plasma torches where the arc is close to walls, the Biot and Savart formalism is required at the domain boundaries for the magnetic vector potential. They also showed that similar plasma fields could be obtained by imposing an electric current density profile at the cathode tip or by including the electrodes in the computational domain. However, this profile has to be chosen according to the specific design of the cathode, which is not obvious when the cathode has a design different from that of conventional plasma torches with sharp conical tip or rounded tip.

Dynamic Behavior of Plasma in a Cascade Plasma Torch with External Magnetic Field

The effect of operating conditions on the arc voltage fluctuations and dynamic behavior of plasma were investigated for a cascade plasma torch with an external magnetic field. The key factors that affect the fluctuations of the arc voltage were studied, which is useful for generating stable plasma jets. A ring-shaped permanent magnet was used to produce an external magnetic field having a flux density of approximately 0.27 T at the center of the magnet. The operating current, gas flow rate, and anode–cathode distance were varied in the range of 40-160 A, 10-40 L/min, and 0-10 mm, respectively. The width of the ring-shaped anode was set to 1 mm or 3 mm. The end-on images of the plasma were captured by high-speed photography, and the associated arc voltage was measured simultaneously. When an anode of 1 mm width was used, a periodic sine-like voltage waveform with low amplitude was observed at a low gas flow rate. In that condition, an arc root rotated along the anode wall continuously with no observable breakdown process. The rotational frequencies of the arc exceeded 1 kHz and were strongly dependent on the square root of the operating current.

Study on the breakdown characteristics of the trigatron spark gap triggered by plasma jet

The breakdown characteristics of a trigatron spark gap triggered by a plasma jet are studied in this paper; the development of a plasma jet and the influence of the gap distance, working coefficient, and plasma jet ejection properties on breakdown characteristics are compared. The results show that the plasma jet ejecting process can be divided into expanding, steady, and dissipating phases. The electric field distortion induced by the plasma jet is different during the expanding and dissipating phase, in which the plasma jet length increases and the radius of the curvature of the plasma jet front decreases, respectively. As the two processes have different variation rates, a faster increasing rate of the breakdown delay can be obtained during the dissipating phase. A detailed physical mode that explains the effects on the triggered breakdown process is presented. The shape of a plasma jet induced by polyethylene and ceramic surface discharge changes to a great extent during the dissipating phase; a more stable plasma jet can be obtained when induced only by polyethylene, resulting in a smaller breakdown delay under a low working coefficient. The ultraviolet radiation (UV) generated by the plasma jet is effective in reducing the critical breakdown electric field, while the electric field distortion induced by the plasma jet is the main reason behind the triggered breakdown under different working coefficients; the UV could play a limited role in the triggered breakdown process.

Control of the Arc Motion in DC Plasma Spray Torch with a Cascaded Anode

Two common concerns in DC plasma torches are stability of plasma jet and anode erosion. The challenge is how to get a stable plasma jet with minimal anode erosion. This study tackles this question by using either a swirling gas injection or an external axial magnetic field applied to the Oerlikon SinplexPro™ plasma torch. A 3-D, time-dependent MHD model of the plasma torch operation was used to predict the value of the external magnetic field and its effect on the heat flux to the anode and plasma jet stability. The special feature of the model is to couple the gas phase and electrodes that makes it possible to follow the anode temperature evolution. For specific operation conditions (anode of O9 mm, 500 A, Ar 60 NLPM), the model predicted that the maximal value of the azimuthal self-magnetic field inducted by the arc current was 0.055 T; it also showed that an external magnetic field of 0.05 to 0.1 T could make it possible to limit the anode erosion without noticeably disturbing the plasma jet issuing from the plasma torch. We expect this approach to help to better understand the arc behavior in commercial plasma torches and control anode erosion

Study of generation characteristics of glow-type atmospheric-pressure plasma jet based on DC discharge in air

In order to form an atmospheric-pressure plasma jet without airflow, a needle–ring electrode structure is proposed in this paper. When heteropolar potentials are applied to a needle and a ring, a marked electric field strength enhancement around the needle’s pointed end has been found. When the same potential is applied to both the needle and the ring, the lateral electric field strength for the needle can be weakened. By using the above two methods, an increase of the difference between the pointed end electric field strength and the lateral one is achieved and stable plasma jets are formed. A symmetrical space electric field distribution is established at the pointed end of the needles when several sets of heteropolar needle–ring electrodes are uniformly arranged, which is conducive to forming a uniform array plasma jet. Under DC discharge conditions, a safe and stable plasma jet of high density and an array plasma jet are successfully achieved.

Numerical simulation of the flow characteristics inside a novel plasma spray torch

A novel direct current non-transferred arc plasma torch that can generate a long, stable and silent plasma jet of over 350 mm in length into ambient air is studied by numerical modelling. Numerical simulation of the plasma torch operating in the current of 160 A with a mixture of 70% nitrogen and 30% argon in volume at three different gas flow rates (8.5 SLPM, 10 SLPM and 14 SLPM) are performed in a 3D domain. The renormalization group method is employed to model the turbulent flow inside the plasma torch, particularly the torch’s novel channel structure. The results show that a narrow circular gap at the boundary layer of the cathode led to a region of high flow constriction and large pressure and velocity gradients. The anode counts with a novel trumpet-like structure that separates the flow in the channel and induces turbulent fluctuations in the direction transverse to the flow, which can disrupt and decrease the cold boundary layer around the arc column. The arc attachment position that obtained in simulation is in agreement with the experiment observation. The maximum velocity at the torch nozzle in simulation is increased and the length of the plasma jet in experiment is decreased with the increasing of the total gas flow rates from 8.5 SLPM, 10 SLPM to 14 SLPM. However, the maximum temperature at the torch nozzle in three different gas flow rates are varied slightly. Our results suggest that generation of a long and stable plasma jet downstream of the nozzle exit should not only focus on a relatively low gas flow rate, but depend instead on the aerodynamic characteristics of the channel flow, particularly in the anode region and downstream of the anode.