Dynamics of plasma bullets by nanosecond pulsed micro-hollow cathode discharge within air

In this paper,the air plasma jet produced by micro-hollow cathode discharge(MHCD)is investigated.The discharge is powered by a positive nanosecond pulse high voltage supply.The waveforms of the discharge,the images of the jet,the evolution of the plasma bullet and the reactive species are obtained to analyze the characteristics of the MHCD plasma jet.It is found that the length of the plasma jet is almost proportional to the air flow rate of 2–6 slm.Two plasma bullets appear one after another during a single period of the voltage waveform,and both of the two plasma bullets are formed during the positive pulse voltage off.The propagation velocity of the two plasma bullets is on the order of several hundred m/s,which is approximate to that of the air flow.These results indicate that the gas flow has an important influence on the formation of this MHCD plasma jet.

N_2 Mole Fraction Dependence of Plasma Bullet Propagation in Premixed He/N_2 Plasma Needle Discharge at Atmospheric Pressure

In this work, a computational modeling study on the mechanism of the acceleration behavior of a plasma bullet in needle-plane configuration is presented. Above all, in our model, two sub-models of time-dependent plasma dynamics and laminar flow axe connected using a oneway coupled method, and both the working gas and the surrounding gas around the plasma jet are assumed to be the same, which are premixed He/N2 gas. The mole fractions of the N2 （NMF） ingredient are set to be 0.01%, 0.1% and 1% in three cases, respectively. It is found that in each case, the plasma bullet accelerates with time to a peak velocity after it exits the nozzle and then decreases until getting to the treated surface, and that the velocity of the plasma bullet increases at each time moment with the peak value changing from 0.72×10^6 m/s to 0.80×10^6 m/s but then drops more sharply when the NMF varies from 0.01% to 1%. Besides, the electron impact ionizations of helium neutrals and nitrogen molecules are found to have key influences on the propagation of a plasma bullet instead of the penning ionization.

Formation Mechanism of the Ring-shaped Structure of Plasma Bullet

The formation of plasma bullets’ring-shaped structure in atmospheric pressure helium is analyzed by using a coupled fluid model.The model consists of a two-dimension neutral gas flow module and a one-dimension plasma dynamics module.The obtained radial distributions of the electron’s number density and the nitrogen’s metastable number density have different structures under different types of reactions or air contents in the model.It shows that total electron impact ionization plays an important role in sustaining the discharge,and together with Penning process,they lead to the shifted-off structure of electron number density.Meanwhile,the ring structure of plasma bullets forms mainly due to the excitation reaction of nitrogen molecule on air contents.

Effects of Gas Flow Rate on the Discharge Characteristics of a DC Excited Plasma Jet

A direct current（DC） source excited plasma jet consisting of a hollow needle anode and a plate cathode has been developed to form a diffuse discharge plume in ambient air with flowing argon as the working gas.Using optical and electrical methods,the discharge characteristics are investigated for the diffuse plasma plume.Results indicate that the discharge has a pulse characteristic,under the excitation of a DC voltage.The discharge pulse corresponds to the propagation process of a plasma bullet travelling from the anode to the cathode.It is found that,with an increment of the gas flow rate,both the discharge plume length and the current peak value of the pulsed discharge decrease in the laminar flow mode,reach their minima at about1.5 L/min,and then slightly increase in the turbulent mode.However,the frequency of the pulsed discharge increases in the laminar mode with increasing the argon flow rate until the argon flow rate equals to about 1.5 L/min,and then slightly decreases in the turbulent mode.

Mechanisms of Streamer Propagation Affected by Driven Voltage Polarity in a Cold Atmospheric Pressure Plasma Jet

A two-dimensional self-consistent fluid model is used to investigate the effects of DC-voltage polarity in plasma initiation and propagation of helium plasma jet.The simulation results indicate that the difference in initial breakdown for the positive jet and negative jet leads to a difference in the electron density of about 4 orders of magnitude,even with the same initial electric field,which also influences the subsequent propagation.In the propagation process of negative jets,the ionization process exists in a longer gas channel behind the streamer head.In addition,the drift process to the infinite grounded electrode driven by the electric field results in higher energy consumption in the ionization process.However,in the positive jet,the ionization process mainly exists in the streamer head.Therefore,the differences in the initial breakdown and propagation process make the electric field intensity and the ionization weaker in the streamer head of the negative jet,which explains the weaker and shorter appearance of the negative jet compared to the positive jet.Our model can adequately reproduce the experimental results,viz.a bullet-like propagation in the positive jet and a continuous plasma plume in the negative jet.Furthermore,it also indicates that the streamer velocity shows the same variations as the electron drift velocity for both positive and negative jets.

Measurement of Electron Density and Ion Collision Frequency with Dual Assisted Grounded Electrode DBD in Atmospheric Pressure Helium Plasma Jet

The properties of a helium atmospheric-pressure plasma jet（APPJ）are diagnosed with a dual assisted grounded electrode dielectric barrier discharge device.In the glow discharge,we captured the current waveforms at the positions of the three grounded rings.From the current waveforms,the time delay between the adjacent positions of the rings is employed to calculate the plasma bullet velocity of the helium APPJ.Moreover,the electron density is deduced from a model combining with the time delay and current intensity,which is about 10^（11）cm^（-3）.In addition,The ion-neutral particles collision frequency in the radial direction is calculated from the current phase difference between two rings,which is on the order of 10~7 Hz.The results are helpful for understanding the basic properties of APPJs.

Hydrogen Peroxide Formation in Saline Solution with Atmospheric He/H2O Plasma Jet

Electrical discharges in or in contact with liquid can produce H2O2effectively.However,wound treatment requires not only high energy yield,but also low temperature,stability,safety and reproducibility in H2O2generation.Thus a method of producing H2O2in saline solution reacting with He/H2O plasma jet is described in this paper.Two working modes(bullet and arc modes)are stable while keeping at low temperature in the H2O2production.The production is much faster under the arc mode,but has rather high production rate and energy efficiency under the bullet mode.Plus,the energy efficiency increases with treatment time and higher moist helium flow rate under both modes,but especially the bullet mode.Moreover,55 min after the plasma treatment,there is only 6%degradation of H2O2concentration in the saline solution,and this indicates the potential of He/H2O plasma jet in plasma pharmacy.

A Brief Study on the Ignition of the Non-Thermal Atmospheric Pressure Plasma Jet from a Double Dielectric Barrier Configured Plasma Pencil

To understand the self sustained propagation of the plasma jet/bullet in air under atmospheric pressure, the ignition of the plasma jet/bullet, the plasma jet/bullet ignition point in the plasma pencil, the formation time and the formation criteria from a dielectric barrier configured plasma pencil were investigated in this study. The results were confirmed by comparing these results with the plasma jet ignition process in the plasma pencil without a dielectric barrier. Electrical, optical, and imaging techniques were used to study the formation of the plasma jet from the ignition of discharge in a double dielectric barrier configured plasma pencil. The investigation results show that the plasma jet forms at the outlet of the plasma pencil as a donut shaped discharge front because of the electric field line along the outlet＇s surface. It is shown that the required time for the formation of the plasma jet changes with the input voltage of the discharge. The input power calculation for the gap discharge and for the whole system shows that 56% of the average input power is used by the first gap discharge. The estimated electron density inside the gap discharge is in the order of 1011 cm^-3. If helium is used as a feeding gas, a minimum 1.48× 10^-8 C charge is required per pulse in the gap discharge to generate a plasma jet.

Characteristic of Atmospheric Pressure Plasma Jet in Coplanar Dielectric Barrier Discharge

Non-thermal plasma jet at atmospheric pressure has recently attracted lots of attention because of its applications in plasma bullet or plasma plume.Thus,we studied on generating plasma jet by coplanar dielectric barrier discharge in a device driven by sinusoidal voltage.The processes of plasma discharges in both positive and negative half cycles were recorded using a high-speed ICCD(intensified charge-coupled device)camera;based on the results we estimated the velocity of plasma propagation,and investigated the influence of gas flow on the plasma development.It is shown that the plasma bullets,which have velocity in the order of 103～104m/s,exist only outside the cathode.APPJ(atmospheric pressure plasma jet)is created by the electron beam from the cathode,and then sustained by a strong radial electric field near and outside the cathode.The gas flow influences the APPJ length in air but not the APPJ discharge,while the discharge is affected significantly by the applied voltage.

常压空气中大间隙介质阻挡放电特性

常压空气中大间隙介质阻挡放电是一种低成本的等离子体产生方法。为此,研究了采用针-板介质阻挡放电(DBD)装置和光电倍增管对常压空气中大间隙介质阻挡放电的特性。结果表明,随着外加电压的变化,存在电晕放电和等离子体羽放电2种放电模式。电晕放电发生在针尖处很小的区域,而等离子体羽放电发生在针-板电极间的较大区域,且等离子体羽长度随外加电压呈阶段性变化。对等离子体羽不同位置的发光信号进行了空间分辨测量,发现针尖附近为连续放电,而远离针尖处为等离子体子弹放电。每次放电等离子体长度随电压峰值的增长关系与每个脉冲的起始电压随外加电压峰值的变化关系一致。

大气压氦气冷等离子体射流放电一维数值模拟

为了明确大气压冷等离子体射流的形成机理,采用一维流体模型研究了针-板电极下大气压氦气冷等离子体射流中等离子体子弹的传播过程,得到并分析了等离子体子弹的空间结构。模拟结果表明,等离子体子弹实际上就是等离子体射流头部的电离区域,其传播过程即为该电离区域的推进过程。Penning电离可以为等离子体射流的传播提供种子电子。驱动电压的极性对等离子体射流的性质有着重要的影响,与正脉冲驱动等离子体射流不同,负脉冲驱动下的等离子体射流具有一个较窄的阴极位降区,且内部没有准中性区域。

N_2含量对大气压He等离子射流特性影响的数值模拟研究

该文采用2D对称流体力学模型,研究N_2含量对大气压He等离子体射流的管外放电结构和粒子组分的影响。工作气体为He和He-N_2混合气体,混合比为x(He)/x(N_2)=99.85/0.15,x(He)/x(N_2)=99.7/0.3和x(He)/x(N_2)=99/1。工作气体中混合N_2增强等离子体放电,并且x(He)/x(N_2)=99.7/0.3混合气体中的等离子体放电最强。1%N_2含量内潘宁电离的促进作用强于电子激发N_2分子反应的削弱作用,但是He混合1%N_2对应的电子激发反应消耗的能量也起很大作用。混合N_2能够加速等离子体子弹从环形结构向实心结构的转变。N_2含量对介质管外等离子体射流中的离子组成有很大影响:纯He等离子射流中He_2^+密度最大;工作气体为x(He)/x(N_2)=99.85/0.15和x(He)/x(N_2)=99.7/0.3混合气体时,N_2^+和N_4^+密度明显高于He^+和He_2^+;工作气体为x(He)/x(N_2)=99/1混合气体时,N_4^+完全主导正离子电荷。

超长射流等离子体在介质管内的传播机制

利用交流驱动的单针射流等离子体装置,在介质管内产生了可以沿着介质管任意弯曲的超长射流等离子体,且可以喷射到介质管外的空气当中。利用高速相机对介质管内等离子体的传播过程进行拍照,研究了这种超长等离子体在介质管内的传播机制。研究发现,该装置在大气压下产生的等离子体长度可以达到约85 cm。在外加电压的正、负半周期,介质管内的等离子体具有不同的形貌和传播机制。正半周等离子体是以"等离子子弹"的方式向前传播,而负半周是以连续模式向前传播。分析发现,放电形成的空间电荷与介质管壁上的壁电荷之间形成的电场,是影响介质管内等离子体传播的主要因素。

介质阻挡放电中等离子体子弹的速度

利用针板介质阻挡放电装置,采用光学方法对大气压长间隙空气放电的等离子体羽特性和等离子体子弹速度进行了研究.结果表明:改变外加电压等离子体羽长度会发生变化.利用光电倍增管采集光信号对等离子体子弹速度进行了研究,发现等离子体子弹的速度与针板间距、外加电压以及针尖直径有关.利用光谱仪采集放电的发射光谱,发现放电等离子体中存在OH自由基、氧原子等多种活性粒子,表明该放电在生物医疗领域具有重要的应用价值.

氦等离子体射流子弹及活性粒子时空分布特征研究

重点研究了脉冲激励的大气压氦等离子体射流子弹及活性粒子的时空分布特征。通过电学测量和光学测量等手段，研究了不同放电参数对子弹传播特性的影响规律，并一步得到等离子体子弹中活性粒子（He、N2＋和O）的分布特征。实验结果表明：1）脉冲上升沿和下降沿各产生一次放电，其等离子体子弹呈现不同形状，这与空间电荷的作用相关：2）放电电压和放电频率对子弹传播影响较大，射流长度和子弹速度随施加电压增加而增加，但电压过高会发生湍流现象，导致射流长度减小。子弹速度同样随频率增加而增加，但在高频率下，其速度衰减更快；3）N2＋粒子的时空分布与等离子体子弹一致，传播距离较远，He、O粒子传播距离较短，位于石英管出口附近。

沿面型介质阻挡放电喷枪的发光特性及气体温度研究

利用氩气作为工作气体,采用正弦电压驱动沿面型等离子体喷枪,在大气压空气环境中产生了均匀的等离子体羽。电学和光学测量结果表明,等离子体羽放电只存在于外加峰值电压的正半周期,并且正半周期的放电脉冲个数随气体流量的增加而增加。通过对正半周期不同位置的发光脉冲信号进行比较,发现等离子体羽均按子弹形式传播,其中每一个发光脉冲均对应一次等离子体子弹传播过程。通过对比放电电流和等离子体羽的发光信号,发现等离子体羽的发光脉冲滞后于放电电流脉冲,且该延迟时间基本服从正态分布。该延迟时间随着外加电压峰值及气体流量的增大而减小。利用光纤测温仪测量了等离子体羽的气体温度,发现气体温度随外加峰值电压的增大而升高,随工作气体流量的增大而降低。通过分析放电过程,对上述现象进行了定性解释。

脉冲波形对氦等离子体射流子弹传播特性的影响

本文研究脉冲波形对大气压等离子体射流子弹传播特性影响,并进一步阐述电场强度与等离子体射流时空演变特性之间的关系。实验表明:不同脉冲波形对等离子体射流子弹传播特性产生影响,但影响机制不同。较短纳秒脉冲上升沿产生过电压击穿,子弹半径较大且传播速度快。脉冲宽度和下降沿通过残余电荷形成反向电场以影响射流整体电场强度。当脉冲宽度小于10μs时,射流长度随脉冲宽度增加而减小;当脉冲宽度大于10μs时,残余电荷累积饱和,脉冲宽度不再对射流特性发生影响。当下降沿增加时,上升沿子弹消散速度较慢,且有一个明显的拖尾,脉冲下降沿可能通过残余电荷形成的反向电场影响子弹头部的空间电场方向和强度,进而影响脉冲等离子体子弹传播。最后使用谱线比方法计算等离子体射流宏观电场强度,其变化趋势与等离子体羽流长度和子弹分布相一致。

脉冲电压对大气压脉冲放电等离子体射流的影响

在固定脉冲频率及占空比的情况下,研究脉冲电压对大气压脉冲放电等离子体射流长度的影响。分析脉冲放电的电流电压波形、等离子体射流的光发射强度的时空演化过程以及等离子体子弹速度的变化。结果表明:等离子体子弹随脉冲电压的增大由单子弹变为双子弹再变为单子弹;射流长度相应地先增长然后趋于稳定,射流长度由增长转变为稳定的转变电压正好处于形成双子弹的脉冲电压附近。

电场与流场夹角对大气压等离子体羽动力学的影响

通过设计新型的交流激励氩气等离子体射流,在棒电极的下游产生了大气压等离子体羽.该射流与平行场射流和交叉场射流不同,它的电场与流场夹角可以在一定范围内改变.结果表明,在同一外加电压时,下游羽长度随着夹角的增大而减小.对于不同的夹角,外加电压的正、负半周期各对应着一个放电脉冲,并且放电脉冲强度随着夹角的增大而减小.利用高速影像,研究了夹角对等离子体羽动力学的影响.发现一个放电脉冲对应着一个子弹的传播过程,子弹的传播速度先增大后减小,并且其最大传播速度随着夹角的增大而减小.基于碰撞辐射模型,利用谱线强度比的方法研究了电子激发温度的空间分布,它反映出了净电场的空间分布.结果表明它与子弹传播速度的空间分布一致.

单针电极放电的发光特性

采用单针电极放电装置在氩气中产生了稳定的均匀等离子体。利用光学方法对单针放电特性进行了研究,结果表明单针放电等离子体的长度随外加电压峰值、气压的增大而增大,随空气含量的增大而减小。利用光电倍增管对单针电极放电等离子体羽的发光信号进行了空间分辨测量,发现靠近单针电极的羽头和等离子体羽其余部分放电行为不同。其中羽头源于针尖附近的电晕放电,它仅在外加电压负半周期的发光较强,正半周期发光几乎探测不到。而等离子体羽其余部分的放电在外加电压正、负半周期均存在,且正半周期强于负半周期。研究表明,外加电压正半周期的等离子体羽是源于发光光层(等离子体子弹)的传播,而负半周期等离子体羽的不同位置几乎同时放电。