A large-scale cold plasma jet: generation mechanism and application effect

Weisheng CUI (崔伟胜) and Ruobing ZHANG (张若兵)

Shenzhen International Graduate School,Tsinghua University,Shenzhen 518055,People’s Republic of China

Abstract Atmospheric pressure cold plasma jets (APCPJs) typically exhibit a slender,conical structure,which imposes limitations on their application for surface modification due to the restricted treatment area.In this paper,we introduce a novel plasma jet morphology known as the largescale cold plasma jet (LSCPJ),characterized by the presence of both a central conical plasma jet and a peripheral trumpet-like diffuse plasma jet.The experimental investigations have identified the factors influencing the conical and the trumpet-like diffuse plasma jet,and theoretical simulations have shed light on the role of the flow field and the electric field in shaping the formation of the LSCPJ.It is proved that,under conditions of elevated helium concentration,the distributions of impurity gas particles and the electric field jointly determine the plasma jet’s morphology.High-speed ICCD camera images confirm the dynamic behavior of plasma bullets in LSCPJ,which is consistent with the theoretical analysis.Finally,it is demonstrated that when applied to the surface treatment of silicone rubber,LSCPJ can achieve a treatment area over 28 times larger than that of APCPJ under equivalent conditions.This paper uncovers the crucial role of impurity gases and electric fields in shaping plasma jet morphology and opens up the possibility of efficiently diversifying plasma jet generation effects through external electromagnetic fields.These insights hold the promise of reducing the generation cost of plasma jets and expanding their applications across various industrial sectors.

Keywords: diffuse plasma jet,flow field,electric field,surface treatment

1.Introduction

Cold plasma jets,particularly atmospheric pressure cold plasma jets (APCPJs) excited by pulsed power supplies,operate at low gas temperatures and contain an abundance of excited state particles,ultraviolet photons,ozone molecules,free radicals,etc.They hold significant potential for a wide range of applications,including surface modification,disinfection and sterilization,air purification,wastewater treatment,and others [1–8].

The generation of APCPJ typically requires the use of noble gases such as helium (He) as the working gas [9].The presence of He metastable particles plays a crucial role in the initiation of Penning ionization in ambient air,which is essential for the formation of APCPJ.In laminar flow mode,the content of He ejected from the nozzle decreases gradually along the axial progression,which cannot sustain the continued propagation of the ionization wave caused by Penning ionization.Therefore,the APCPJ typically exhibits a slender,conical shape with a decreasing cross-section as it advances along the axial distance [10,11].

Researchers have observed that when the flow velocity is higher,the plasma tail exhibits an unstable “turbulent” form,resulting in a reduction in its length.This phenomenon is attributed to the instability in the variation of He concentration in the gas tail caused by the high-speed turbulence effect.This instability hinders the propagation of the ionization wave,consequently causing a slight reduction in the length of the plasma [12].Additionally,the plasma jet can also influence the transition between laminar flow and turbulent flow,while the interaction is mainly constrained to the axial direction [13–15].

The conical shape of APCPJ and its formation mechanism result in a relatively minor treatment area when applied to surface modification of materials,especially when the surface of the material has a weak electron adsorption capacity or a higher conductivity [16–18].However,the contact of plasma with the surface of the target is essential for effective treatment.To address the issue of the limited scale in plasma jets,researchers have proposed various solutions,such as the large-diameter plasma candle [19] and arrayed plasma jets [20–22].The plasma candle can directly form a larger plasma jet diameter,and arrayed plasma jets achieve an enhancement in the cross-sectional area of the plasma jet through the combination of multiple individual jets.However,these methods are based on the same fundamental formation mechanism of plasma jets and do not alter the morphology of individual APCPJ.Therefore,they cannot effectively increase the treatment area of APCPJ without increasing the consumption of noble gases.Noble gases typically do not interact with other materials,and they are primarily purified from natural gas for industrial production.The high cost of noble gases significantly restricts the applications of APCPJ.

Hence,controlling the morphology of a single-tube plasma jet and increasing its scale is the practical approach to expand the treatment area of the plasma jets and reduce the cost of their application.The dominant Penning ionization process during the plasma jet ionization is primarily related to the interaction between metastable particles in the working gas and ambient impurity gas particles [23].Therefore,the key to regulate the morphology of APCPJ lies in regulating the flow field and controlling the composition of the working gas and ambient gas.The influence of environmental gas compositions on plasma jets has been studied[24–27],but there is still limited research on the impact of flow field variation on plasma jets.

This paper introduces a unique plasma jet consisting of both a central conical and a peripheral trumpet-like diffuse form plasma,referred to as a large-scale cold plasma jet(LSCPJ) (patent pending).This study investigates the impact of doped gas in the working gas and ambient gases on the plasma jet and delves into the formation mechanism of LSCPJ from both flow field and electric field perspectives.Finally,silicon rubber was used as an example to validate that LSCPJ,when applied to material surface treatment,can increase the treatment area by more than 28 times that of APCPJ,thus effectively reducing the application cost of plasma jets.

2.Experimental setup

In this study,a semi-enclosed chamber was designed as the discharge chamber,which allows for the adjustment of the ambient gas composition.A buffer chamber used for gas doping is set on the sidewall of the discharge chamber.A quartz tube connects the discharge chamber and the buffer chamber,which is used for generating the plasma jet.A ringshaped gas outlet is located on the sidewall opposite the quartz tube,with inner and outer diameters of 130 mm and 150 mm,respectively,and its axis aligns with the plasma jet tube.There is an environmental gas regulation inlet on the sidewall near the plasma jet tube to control the ambient gas composition.The quartz tube has inner and outer diameters of 8 mm and 10 mm,respectively.A tungsten needle with a diameter of 1.5 mm is positioned in the center of the quartz tube as the high-voltage electrode,and a copper ring with a diameter of 10 mm serves as the ground electrode attached to the outside of the quartz tube.The high-voltage electrode is positioned 10 mm from the outlet of the quartz tube,and the center of the ground electrode aligns with the high-voltage electrode.

The experiments were conducted using a nanosecond pulsed power supply (HVP-20P,Xi’an Smart Maple Electronic Technology) with a maximum amplitude of 20 kV and a maximum frequency of 100 kHz.This pulsed power supply is capable of generating square wave pulses with adjustable amplitude,frequency,duty cycle,and rising/falling times.The discharge voltage and the current during the experiment were measured using a high-voltage probe (P6015A,Tektronix) and a current transducer(CP0030H,Cybertek),respectively.Additionally,the optical images of plasma jet morphology were acquired using an SLR camera (EOS 5D Mark IV,Canon),and the ionization wave propagation dynamics during the discharge process were diagnosed using a high-speed ICCD camera (PI-MAX3,Princeton Instruments).

The He (purity of 99.999%) with air doping was used as the working gas,and it flowed out of the quartz tube after purging the discharge chamber with the same gas composition.The ambient gas composition in the discharge chamber was adjusted through the environmental gas regulation inlet.Mass flow controllers with ranges of 30 L min-1and 0.5 L min-1(CS200-A,Sevenstar) were used to achieve highprecision customized flow rates of He and air.The flow rate of working gas is set to 4 L min-1,and the ratios of air doping and air regulation were calculated in percentages relative to the flow rate of working gas.The discharge voltage was set at 7 kV,the frequency at 1 kHz,the duty cycle at 0.001,and the pulse rise and fall times at 500 ns and 100 ns,respectively.

In addition,the flow field simulation and electric field simulation for the plasma jets were performed using Fluent software (version 2022 R2) and ANSYS Maxwell 3D software (version 2020 R1),respectively.The flow field simulation used the steady-state model to obtain the stable flow field distribution,and the electric field simulation used an electrostatic solver to get the transient electric field distribution at specific moments during the formation process of the plasma jet.

3.Results and discussions

To facilitate comparative analysis,the plasma jets with pure He (APCPJ) and He with 3.75% air doping ratio (APCPJ-1)were generated in an open environment.The air doping and air regulation ratios of LSCPJ were set to 3.75% and 5%,respectively.The schematic diagram of the experimental setup and plasma jet morphology are shown in figure 1.

Figure 1.(a) Schematic diagram of the LSCPJ generation system.(b) Comparison of the generation morphologies between the APCPJ and the LSCPJ.The APCPJ and the APCPJ-1 are generated using the working gas of pure He and He doped with 3.75%air,respectively.The LSCPJ uses the working gas of He with an air doping ratio of 3.75% and introduces an air regulation ratio of 5%in the ambient gas.The exposure time for the morphology of APCPJ,APCPJ-1,and LSCPJ is set to 1 s.

It can be seen from the figure that APCPJ exhibits the typical slender conical shape with a gradual reduction in the plasma jet cross-section.In contrast,LSCPJ presents a largescale branched morphology with both a central conical and a peripheral trumpet-like diffuse plasma jet.The conical plasma jet is shorter and thicker,while the trumpet-like diffuse plasma jet expands outward,resembling the shape of morning glory.The generation scale of LSCPJ is larger than that of APCPJ,and its unique branched structure has distinct morphological characteristics compared to APCPJ,suggesting the presence of a different generation mechanism.Diagnosis of voltage and current waveforms revealed that the current amplitude of the LSCPJ is larger than that of the APCPJ,as well as the energy consumption for each pulse.

By varying the air doping and air regulation,the changes in LSCPJ morphology were observed,as illustrated in figure 2.It has been seen that air doping and air regulation have different effects on the morphology of LSCPJ.When the air doping ratio is 3.75% and the air regulation ratio is 2.5%,there is no central plasma jet in the LSCPJ.Increasing the ambient air regulation ratio results in the appearance of a conical plasma jet,gradually strengthening,while the intensity of the trumpet-like diffuse plasma jet is visible weakens.When the air regulation reaches 7.5%,the diffuse plasma jet almost disappears.On the other hand,when the air doping ratio is 1.25% and the air regulation ratio is 5%,the central conical plasma jet exhibits a higher intensity,and the peripheral diffuse plasma jet bends back toward the ground electrode.As the air doping ratio increases,the intensity of the central conical plasma jet weakens,and when the air doping exceeds 6.25%,the central conical plasma jet disappears.Simultaneously,the angle between the trumpet-like plasma jet and the conical plasma jet gradually decreases,shifting from a T-shape to a V-shape.Based on the variation law of plasma jet morphology,it is inferred that air doping in the working gas is responsible for the formation of the trumpetlike diffuse plasma jet,while the introduction of ambient air ratio plays a crucial role in the formation of the conical plasma jet.

The analysis suggests that the formation of LSCPJ is the result of the combined effects of air doping in the working gas and the introduced ambient air regulation.The flow field simulation is performed and reveals the distributions of flow field vector and the He molar ratio for APCPJ in an open environment and LSCPJ in a semi-enclosed environment,as shown in figure 3.

From figure 3(a),it is known that the high-speed ejection of the working gas from the quartz tube leads to a decrease in pressure at the interface between the working gas flow and the ambient air.This results in the flow of ambient air to the jet in an open environment.As the working gas progresses forward,it gets continuously diluted by the ambient air,causing a continuous reduction in the He concentration.The molar ratio distribution of He shows that in the open environment,the molar ratio of He in the working gas flow exhibits a gradual decrease both axially and radially.According to the references [11,28],the breakdown field strength for air is approximately four to five times that of He.The rapid decrease in radial He concentration significantly increases the breakdown field strength,limiting plasma generation to the axial region with a high He molar ratio,resulting in a cone-shaped plasma jet.

Figure 2.The changes in LSCPJ morphology as a function of gas parameters.(a)–(e) The air doping is fixed at 3.75%,and the introduced ambient air ratio ranges from 2.5%,3.75%,5%,6.25%,to 7.5%.(f)–(j) The introduced ambient air regulation ratio is fixed at 5%,and the air doping ranges from 1.25%,2.5%,3.75%,5%,to 6.25%.The exposure time for the morphology of the plasma jets is set to 1 s.

In contrast to the open environment,the ambient gas flowing to the jet has an approximately equal composition with the working gas within the discharge chamber.Due to the significantly lower flow rate of air regulation compared to working gas,it has minimal impact on the overall flow field distribution.The introduced air flows towards the quartz tube under the influence of the spatial flow field.Figure 3(b) reveals that the molar ratio of He shows minimal changes in most regions of the discharge chamber and a visible decline at the boundary of the working gas flow and near the bottom of the quartz tube.This decline can be attributed to the dilution effect as the introduced ambient air moves toward the jet.Analyzing the line distribution of the He molar ratio at different axial distances from the quartz tube,as shown in figure 3(c),it can be observed that near the nozzle of the tube,there is a region where the He molar ratio decreases rapidly by 7% near the interface between the working gas flow and the ambient gas.As the distance from the quartz tube nozzle increases axially,the decrease extent of the He molar ratio gradually diminishes.

Figure 5.The plasma bullet propagation characteristics for (a) the APCPJ and (b) the LSCPJ.The exposure time for each frame is 50 ns,and the accumulation is set to 2000 for both plasma jets.

The formation of atmospheric-pressure He plasma jet,i.e.,the propagation of ionization waves,is primarily based on the Penning ionization between He metastable particles and impurity gas molecules [29,30].In an open environment,the impurity gas molecules are oversupplied,and the propagation of ionization waves primarily depends on the distribution of He concentration.In contrast,the He molar ratio in the discharge chamber exceeds 0.95,resulting in abundant supplies of He.In this case,the propagation of ionization waves is mainly influenced by the distribution of impurity gas particles rather than He.According to the plasma density [6,16],the ionization rate of atmosphericpressure cold plasma jets is typically below 0.1%,which indicates that changes in the molar ratio of impurity gas molecules exceeding 7% can significantly impact the propagation of ionization waves,consequently altering the formation characteristics of the plasma jet.As the maximum change amplitude of the He molar ratio in figure 3(c) can be influenced by the relative proportion of air in the working gas and the ambient gas,it conforms to the conclusion that air doping and air regulation affect the formation of the conical and the trumpet-like diffuse plasma jet,respectively.

The Penning ionization of metastable He particles can promote the generation and propagation of ionization waves,while the distribution of the electric field can influence the direction of motion of the free electrons generated by Penning ionization,thereby affecting the direction of ionization wave propagation.In an open environment,the breakdown electric field strength of air is significantly higher than that of the He flow region,restricting the propagation of ionization waves to the radial low He concentration region.However,the He concentration exceeds 0.9 in most regions within the discharge chamber.Therefore,the gas breakdown voltages in different regions are relatively equal,and the electric field distribution may have an impact on the propagation direction of the ionization wave.

The electric field distribution of the LSCPJ during the formation process of the plasma jet is obtained,as shown in figure 4.The generated plasma during the propagation of the ionization wave is substituted by the conducted fluid.It has been seen that there are axial and radial electric field components in the front of the ionization wave during its propagation.As the ionization wave propagates within the quartz tube,it is confined by the wall of the quartz tube.When the ionization wave exits the quartz tube,the radial electric field component can take effect and influence the development path of the ionization wave.Due to the steep drop in He concentration near the nozzle,some of the electrons generated by Penning ionization are restricted and continue to propagate axially,forming a cone-shaped plasma jet,while others propagate radially under the influence of the electric field,forming a trumpet-like diffuse plasma jet.Furthermore,the electric field strength near the external ground electrode of the quartz tube reaches 2.2×104V cm-1,surpassing the breakdown field strength of He [31],leading to surface discharge outside the quartz tube.Surface discharge extends the effective area of the ground electrode,increases the electric field strength near the tube nozzle,and facilitates the radial propagation of ionization waves,resulting in the formation of the trumpet-like diffuse plasma jet.

The diagnostic of the dynamic propagation of plasma bullets (i.e.,ionization waves) in APCPJ and LSCPJ was carried out using ICCD cameras,as shown in figure 5.The start of the capture was synchronized with the rising time of the voltage waveform.

It has been seen from the figure that the plasma bullet of the APCPJ appears as a hollow circular structure,being consistent with previous research [23,28,32].As the plasma bullet exits the quartz tube,it continues to propagate axially and gradually evolves into a solid structure.The transition process is related to He diffusion downstream of the flow[33,34].In contrast,the plasma bullet of the LSCPJ exhibits entirely different characteristics in appearance and propagation behavior.When the initial hollow plasma bullet was generated within the quartz tube,surface discharge plasma was created simultaneously on the outer side of the tube.The surface discharge plasma extends with the propagation of plasma bullets in the tube.After extending to a certain distance,the surface discharge weakens gradually.The inner plasma bullet continues to move forward and split into two paths: one develops axially,forming the conical plasma jet,while the other diverges radially to create a trumpet-like diffuse plasma jet.As mentioned in the theoretical analysis,there is an abrupt “decline” in the He molar ratio in the split region of the plasma bullet near the tube nozzle.When the plasma bullet reaches the tube nozzle,the synchronized impact of axial and radial electric field vectors at the front of the plasma leads to simultaneous propagation along two paths,resulting in the coexistence of conical and divergent plasma jets in LSCPJ.Therefore,the dynamics of plasma bullets of APCPJ agrees with the earlier analysis.In addition,it has also been found that the propagation of plasma bullet for the LSCPJ is much slower than that of the APCPJ,which is inferred due to the reduction of the acceleration process of Penning ionization from ambient gases [26].

Figure 6 illustrates the surface modification effect using silicone rubber as an example for APCPJ and LSCPJ.As the treated area cannot be measured directly with the opaque material,it is calculated based on the diameter of the spot due to the approximately circular spread of plasma on the surface.It has been seen from figures 6(a)–(f) that the LSCPJ with different air regulations can achieve significantly larger treatment areas than that of the APCPJ in processing the surface of silicone rubber.At an applied voltage of 4 kV,APCPJ’s treatment area is relatively limited,measuring approximately 0.5 cm2.As the voltage increases to 8 kV,APCPJ’s treatment area expands to 1.3 cm2.In contrast,the LSCPJ achieves a treatment area exceeding 4 cm2when operated at 4 kV.With an increase in voltage to 8 kV,the LSCPJ consistently provides treatment areas surpassing 20 cm2.In the case with a doping gas ratio of 1.25% and an air regulation of 5.0%,the LSCPJ reaches a treatment area of 22.13 cm2at 6 kV,exceeding APCPJ’s treatment area by more than 28 times at the same voltage.

Figure 6.The surface treatment effect of silicone rubber for (a) the APCPJ and (b)–(f) the LSCPJ with air doping of 1.25% and air regulation range from 2.5% to 12.5%.(g) The trends in the treatment area for APCPJ and LSCPJ under different conditions.The exposure time for (a)–(f) is set to 1 s.

It is inferred that in APCPJ,the diffusion of the working gas near the treated surface results in a rapid decrease in the concentration of He particles,leading to the inability to sustain the ionization wave.In contrast,within LSCPJ,He particles exist in a state of oversaturation,which minimizes the impact of gas diffusion on He particle ratio near the treated surface.Consequently,the ionization wave can propagate over a much larger distance on the surface of silicon rubber until the decrease of electric field strength causes the extinction of the ionization wave.Therefore,LSCPJ can achieve significantly larger treatment areas compared to APCPJ,and the treated area increases effectively with the boost of applied voltage.

4.Conclusions

In summary,an LSCPJ with a unique morphology has been introduced,simultaneously featuring a conical and a trumpetlike diffusion form.Theoretical analysis demonstrates that under conditions of elevated He concentration,the morphology of the plasma jet is not governed by He particles.Instead,the spatial electric field distribution,which has limited influence in APCPJ,plays a crucial role in shaping the formation of the LSCPJ.In addition,by regulating the impurity gases in the working gas and the ambient environment,it becomes possible to effectively control the appearance of LSCPJ and increase its treatment area when applied to surface modification.

We want to emphasize that the LSCPJ represents a novel way of forming plasma jets,where the propagation of ionization waves primarily depends on the distributions of the spatial electric field and the impurity gas concentration rather than the working gas flow as the APCPJ.Consequently,it is possible to further diversify the generation effects of plasma jets through external magnetic and electric fields.While the semi-enclosed environment of the LSCPJ may introduce some limitations in terms of application flexibility,its connection to atmospheric pressure allows for the tailoring of flexible treatment strategies to specific scenarios.Therefore,the LSCPJ may have the potential to reduce the application costs of plasma jets,which could play a role in its future large-scale industrial applications.

Acknowledgments

This work was supported by the Guangdong Basic and Applied Basic Research Foundation (No.2023A1515011505),Shenzhen Science and Technology Program (Nos.JCYJ 20220530142808020 and JSGG20220606140202005),China Postdoctoral Science Foundation (No. 2023 M731878),and Project (No.SKLD22KM17) by State Key Laboratory of Power System Operation and Control.