【作者】 刘富成；

【导师】 王德真；

【作者基本信息】 大连理工大学 ， 等离子体物理， 2013， 博士

【摘要】 近年来,大气压冷等离子体射流由于其独特的优势和广泛的应用前景而受到人们格外的关注。和传统的大气压非平衡等离子体源相比,大气压冷等离子体射流最大的优势就是能够将等离子体产生区域和工作区域在空间中分离开来,在保持放电稳定性的同时,还能保持较强的等离子体化学活性。尽管大气压冷等离子体射流目前已被广泛应用于材料加工和表面改性以及等离子体医学等各个应用领域,但是一些根本性的问题还没有得到很好的解决。因此,需要对其作进一步深入研究。在本论文中,主要开展了以下工作：建立了一个1.5维等离子体流体力学模型,数值研究了针板型放电结构下等离子体射流子弹的产生、传播以及熄灭过程；结果表明：等离子体子弹的传播机制是一种类似于流注的电离波的传播。在其传播过程中,电子的碰撞电离是最主要的电离机制,而潘宁电离可以为其传播提供足够的种子电子。驱动电压的极性决定了等离子体射流的动力学行为,在正脉冲电压驱动下,等离子体子弹头部内存在一个具有高斯型分布的净电荷区域,空间电荷在该区域内激发强电场,电离剧烈,发光比较强。等离子体子弹的传播速度与最大空间电场的变化规律一致,都是首先增加,达到最大值后开始下降。而在负脉冲驱动下,等离子体射流中没有准中性区域的存在,且在阴极附近存在着一个非常窄的阴极位降区。电场在整个射流范围内均比较高,电离也都比较强,没有类似等离子体子弹的结构存在。另外,等离子体射流的传播速度比正脉冲射流的传播速度低,且随时间不断减小。研究了环境空气对氦气冷等离子体射流放电性质的影响。结果发现由于氧的附着效应使得等离子体射流中的电子密度降低了。氦亚稳态原子与空气之间的潘宁电离作用促进了电离反应,加快了射流的传播速度。在不考虑空气扩散效应的情况下,当空气含量小于1%时,等离子体射流的最大传播速度随着空气含量的增加而增大；而当空气含量大于1%时,随着通道内空气的增多,射流的传播速度将不断地降低。等离子体射流的长度随着空气含量的增加不断减小。考虑了空气扩散的作用,利用一维径向流体力学模型研究了等离子体子弹横截面上环形结构的形成机理。结果表明由于气体成分的不均匀空间分子造成的电子直接碰撞电离变化是环状结构形成的根本原因。基于一维等离子体流体力学模型,研究了气流对平板型等离子体射流中心放电放电特性的影响。在亚微秒单极脉冲驱动下,气流对第一次放电的影响比较显著,而对第二次放电几乎没有影响。随着气体流速的增加,第一次放电的放电电流不断减小,且放电时刻也不断延迟,这是由于种子电子在气流的作用下移出放电区域所造成的；在交流驱动下,由于空间电场的存在,电子的迁移输运比对流输运高很多,因此气流的输运主要体现在正离子的对流输运上。气流的存在造成了正负半周内的放电上下不对称。同时由于表面电荷的作用,使得相邻两次放电之间的时间间隔不再相等,而是呈现长短交替的变换规律。当气流超过某一临界值,系统会经历倍周期分岔过程而进入一个二倍周期的放电状态。基于自洽的二维等离子体流体力学模型,研究了直流脉冲驱动下放电参数(例如脉冲频率、上升沿时间、介电常数等)对放电性质的影响。模拟结果显示：随着脉冲重复频率的增加,两次放电电流的大小均不断减小,且第一次放电电流的峰值时刻不断提前。在正脉冲放电电流峰值时刻,瞬时阴极附近的最大电子密度随着脉冲重复频率的增加而不断减小,其峰值对应的位置不断向瞬时阴极靠近。而在负脉冲放电电流峰值时刻,虽然瞬时阴极附近的最大电子密度也随着脉冲重复频率的增加而不断减小,但是其峰值对应的位置却不断向瞬时阳极靠近。随着上升沿时间的增大,两次放电电流的大小也都不断减小。下降沿时间的长短对对第二次放电电流影响明显。下降沿时间越长,第二次放电电流越小。基于自洽的二维等离子体流体力学模型,研究了大气压氦气冷等离子体射流在自身环境气体中以及在介质管中的传播问题。得到了电子密度、电离速率、空间电场以及电子温度等参量的时空分布规律,分析了介质管大小以及介质管介电常数对射流放电性质的影响,得到了一种提高电子密度和射流尺寸的新方法。更多还原

【Abstract】 Atmospheric pressure cold plasma jets (APCPJs) have recently been of an enormous interest due to their unique advantages and numerous potential applications. Compared with traditional atmospheric pressure non-equilibrium plasma sources, one of the most prominent features of the atmospheric pressure cold plasma jets is the spatial separation of plasma generation from their working regions, which can simultaneously achieve both high discharge stability and efficient reaction chemistry. Although APCPJs have been applied to several practical applications such as material processing, surface modification, and plasma medicine, many fundamental mechanisms still remain unknown. Therefore, further detailed investigations on these issues are needed. In this paper, the following works have been done:The plasma bullet creation, propagation and inhibition in a needle-to-plane discharge have been investigated based on a l.5D plasma fluid model. It is found that the propagation of plasma bullet is similar to the propagation of an ionizing wave. The collision ionization is the most important ionization mechanisms during the bullet propagation, while the Penning ionization can provide sufficient seed electrons ahead of it. The dynamics of the plasma jets are determined by the voltage polarity. When the plasma jets arc driven by a positive pulsed voltage, there is a net space charge domain with Gaussian-shaped distribution in the head of the plasma bullet. In this domain, the electric filed induced by space charge is strong enough, and the ionization is intensive. The bullet velocity has the same evolution as that of peak field. It starts to accelerate as soon as it is launched, and then slow down after reaching its maximum velocity at some distance. In contrast to the positive plasma jet, there are some specific features in the plasma jet driven by a negative pulse. The notable features are the existence of a narrow cathode fall region near the tip electrode and the absence of quasi-neutral ionized channels. Moreover, the propagation velocity of plasma jet in negative pulses, which is lower than that in the positive pulses, decreases all the time during its propagation.The influence of ambient air on the atmospheric pressure helium plasma jets has been studied. It is demonstrated that the electron density in the plasma jets are reduced due to the electron attachment by oxygen molecules. The Penning ionization between metastable helium atoms and air molecules facilitates the volume ionization and accelerates the propagation of the plasma jets. Without taking air diffusion into account, the maximum of streamer velocity increases with the increasing air content, and reaches its maximum when the air impurity level is increased to1%, and then decreases with the further increasing air content. Meanwhile, the plasma jet length is decreased exponentially as the air impurity level increases. In addition, the forming mechanism of ring-shaped structure on the cross-section of plasma jet is investigated by using a one-dimensional fluid model that including the air diffusion effects. It is shown that the formation of ring structure is governed mainly by the difference of direct ionization rate between helium and air.The influence of gas flow on the discharge characteristics in the dielectric barrier discharge with parallel electrodes at atmospheric pressure was investigated by a one-dimensional self-consistent kinetic model. When the discharge is driven by a sub-microsecond pulsed dc voltage, two discharge current pulses, the positive one and the negative one, are operated in a normal glow mode and a sub-normal glow mode, respectively. It is shown that the gas flow has a significant impact on the discharge characteristics, especially on the positive discharge pulse. The spatial distribution of electrons is affected by the gas flow through the convection transport mechanism. When the discharge is driven by ac voltage, the convective transport mechanism is mainly governed by positive ions. The presence of gas flow results in the formation of asymmetric discharge in a whole driving period. Associated with the surface charge, the gas flow makes the intervals between two consecutive discharge events no longer remains constant, but exhibits a alternative fashion. As the gas flow exceeds some critical value, the system will undergoes a double period bifurcation and transits into a Period2discharge pattern.The effects of discharge parameters (pulsed driving frequency, the rising time, permittivity, etc) on the atmospheric pressure dielectric barrier discharge excited by repetitive voltage pulses have been numerically studied by using a two-dimensional fluid model.It is demonstrated that both the two discharge currents decrease as the driving frequency is increased. The time delay between the igniting event and the current peak of the first discharge becomes shorter and shorter. The maximum of electron density near the instantaneous cathode at the positive discharge current peak moment decreases, and the corresponding position moves towards the instantaneous cathode when the driving frequency increases. However, although the maximum of electron density near the instantaneous cathode at the positive discharge current peak moment decreases, but the corresponding position moves towards the instantaneous anode when the driving frequency increases. As the rising time is increased, both he two discharge currents decrease. The falling time plays an important role in the second discharge. The longer the falling time is increased, the smaller current peak of second discharge becomes.Based on a self-consistent two-dimensional plasma fluid model,we investigated the propagating problems in the cold atmospheric pressure helium plasma jets that surrounded by helium itself and thin dielectric tube, respectively. The spatio-temporal distributions of electron density, ionization rate, electrical field, and electron temperature were obtained. It is found that both the radius and the permittivity of dielectric tube have an impact on the discharge characteristics. A new method of improving the electron density and plasma jet size were also proposed.更多还原