【作者】 杨林；

【导师】 李公平； 戴晶怡； 谈效华；

【作者基本信息】 兰州大学 ， 粒子物理与原子核物理， 2014， 博士

【摘要】 真空弧等离子体具有电离度高的特点,已经被广泛用于镀膜沉积、离子注入和离子引出加速等应用中。目前国内外研究报道了大量采用直流、kA量级大电流放电模式的真空弧等离子体,而对于低电流下脉冲真空弧等离子体特性的研究报道较少。本文主要针对低电流下脉冲真空弧放电等离子体的光学现象和物理特性,建立一套基于等离子体自发辐射诊断脉冲真空弧放电等离子体特性参数的方法。通过开展发射光谱实验研究,揭示了脉冲真空弧等离子体的非平衡态特性的特点以及形成原因,并分析了不同放电条件对等离子体特性的影响规律。通过开展高速摄影实验,获得了脉冲真空弧放电的瞬态过程图像和产生等离子体的放电形式,并对多斑放电现象的特性和等离子体射流特性进行了分析。本文工作内容和主要结论体现在以下几个方面：1.研究了脉冲真空弧等离子体光谱的特点,获得等离子体内部各类粒子的非平衡态特征。通过多通道光谱仪测量了脉冲低电流下真空弧等离子体的发射光谱发现：本论文中钛阴极金属离子的最高价态为二价离子,200-300nm波段主要为二价离子谱线,300-490nm波段主要为一价金属离子谱线,490-525nm主要为金属原子谱线。同时还出现了其它电极材料杂质的谱线,如氢原子alpha线、C2分子Swan谱等。利用多条谱线强度的Boltzmann斜率法获得了离子和原子的平衡态特征,一价离子处于局部热平衡态,原子处于非平衡态。利用软件Specair拟合C2分子Swan谱发现,气体转动温度远小于振动温度,因此等离子体整体处于非平衡态。2.建立了一套氢原子Stark展宽理论数据插值方法,获得了脉冲真空弧等离子体的电子密度和离子温度。通过分析氢原子谱线Stark展宽的经典Griem理论和最精确的Gig-Card理论结果,建立了相应的理论数据插值方法。理论数据的插值结果表明,氢原子Stark展宽会随原子折合质量的增大而减小,也会随电子温度的增加而减小。在脉冲真空弧等离子体实验中充入氢同位素气体,然后对氢原子alpha谱线进行展宽去卷积拟合解谱。基于Gig-Card理论结果,建立了一种氢同位素原子双峰Stark展宽同时诊断电子密度和折合质量的方法。结果表明,脉冲真空弧等离子体中氢原子折合质量受离子温度的影响而发生变化,这在国内外尚未见其它公开报道。3.建立了脉冲真空弧等离子体中修正非平衡态中原子能级分布的方法,并在此基础上发展了计算脉冲真空弧等离子体中原子和一价离子相对浓度的方法。通过分析非平衡态效应对粒子能级布居的影响,由此建立了一种矫正非平衡态中原子能级布居的方法。结果表明,脉冲真空弧等离子体中由原子和一价离子谱线强度获得的电子温度是相同的,原子处于电离态等离子体环境。利用传统电离平衡Saha方程线性拟合谱线强度得到电离温度,发现电离温度等于电子温度,而得到的电子密度远高于Stark展宽结果。因此,原子和一价离子的光学厚度不同是影响原子电离平衡的主要原因,由此可以获得脉冲真空弧等离子体中原子和一价离子的相对浓度。4.研究了不同放电条件下的脉冲真空弧等离子体特性,获得了放电条件对等离子体的影响规律当增加放电电流时,脉冲真空弧等离子体中原子和离子光强及分子谱光强都会出现了局部极大值。离子激发温度表现出了相同的变化趋势,而原子激发温度则只出现了轻微的趋势,电子密度与放电电流近似线性增加关系。另外,增大电流会使转动温度增加至稳定值,同时使振动温度降至稳定值。电流增大也使得各类粒子谱线发生了明显地多普勒偏移,表明等离子体中的各类粒子定向速度随放电电流增大而增加。当通入微量氢同位素气体后,离子温度也出现了对应的局部极大值现象。由此可以推断,等离子体中电子和离子的温度变化是引起局部光强变化的主要原因,在大电流放电研究中没有出现此类现象。增大放电间距后发现,原子和一价离子的光强明显变弱,而杂质氢原子光强明显提高。电子温度和气体转动温度及电子密度明显变大,但分子振动温度降低。增加放电电流后,钛离子、碳原子和和碳一价离子的谱线光强先增加后减小,但氢原子和碳二价离子光强近似线性增加。一价钛离子离子激发温度不受电流的影响,而二价钛离子激发温度出现了先增加后减小的趋势。电流增大使得分子转动温度增大并趋于定值,而振动温度则与二价钛离子变化趋势一致。利用氢原子alpha线Stark展宽测量电子密度后发现,电子密度也出现了先增加后减小的变化趋势。观察各类粒子谱线的多普勒偏移发现,大间距放电会使粒子定向速度出现了很强的不规则波动现象。改变背景气压会直接影响脉冲真空弧等离子体特性,并且对于不同间距的放电过程会产生不同的影响。对于小间距放电,增大气压的过程中,粒子光强会先增加后减小；而增大间距后发现,中性气体粒子光强会先增加后减小,而离子光强则会先减小后增大。无论间距大小,激发温度都会先减小后增大,但气体转动温度在小间距时是一直增大,而在大间距时是一直减小。对比电子密度结果发现,小间距时电子密度出现轻微的先增加后减小的趋势,但在大间距时随着气压的增大而一直减小。5.研究了真空击穿现象的物理过程,获得了脉冲真空弧放电的产生机制和等离子体射流的特点高速摄影实验发现,两电极间的高压真空击穿过程起始于阴极烧蚀,绝缘材料表面的二次电子倍增过程是形成两极击穿的主要因素。阳极光斑下移的过程,其实是等离子体通道由阴极产生的扩散等离子体替代绝缘材料表面倍增等离子体的过程,也是等离子体通道阻抗逐渐减小的过程。最终,电极之间形成稳定的弧通道。高速摄影结果表明,低电流下脉冲真空弧等离子体来自于电极表面的多个阴极斑。沿面闪络的特点是只存在一条等离子体通道,即使电极表面出现多个烧蚀斑,来自多个阴极斑的等离子体也会共用一条闪络通道。对比阴极斑的光强和位置的变化后发现,可以获得阴极斑的平均寿命和迁移速率,同时发现阴极斑倾向出现在已烧蚀过的表面区域。观察等离子体射流时发现,随着放电时间的持续,轴向上的光强峰值在减小,同时峰位在远离边缘。原因主要在于阴极斑在沿边缘以一定的速度扩散,径向上逐渐呈现出中心弱两侧强的变化趋势。尽管射流起始位置处分布很不均匀,但在一定距离后射流会融合在一起,最终径向光强变为对称的近高斯分布。更多还原

【Abstract】 Cathodic arc with highly ionized metal plasma, has been widely used for thin film synthesis, ion implantation and ion injection into accelerators. Up to now, many workers have measured and reported proper parameters for their vacuum arc systems in which DC arc sources with high current (>kA) were used to generate the discharge. However, few reports about pulsed arc sources in low-current discharge can be found. In this thesis, we mainly focus on the basis optical phenomenon and physical characteristics in pulsed low-current vacuum arc plasma, and set up a series of diagnostic methods based on spontaneous emission from the vacuum arc plasma. By carrying out the experiment of emission spectrum, it has been found that the characteristics and the causes of non equilibrium properties in pulsed vacuum arc plasma. Additionally, the influence of different discharge conditions on the plasma characteristics has also been studied. Through the high-speed photography experiment, we obtained the transient processes and discharge types in the pulsed vacuum arc discharge, and the characteristics of the multi-spot discharge and the plasma jet are analyzed. The results of these studies can be concluded as follows:1. The characteristics of pulsed vacuum arc plasma spectrum have been investigated, and the non-equilibrium effects of particles have been studied.Through the multi-channel spectrometer, we measured the emission spectrum of the pulsed low-current vacuum arc discharge. The highest charge of titanium metal ions in this work is+2, and these lines are present in ultra violet band200-300nm. The lines emitted from+1charged ions and atoms are mainly in the band300-490nm and490-525nm, respectively. Some lines of hydrogen atom and the C2molecular Swan spectrum have also been found. Based on the Boltzmann-slope method, the+1charged ions are in in local thermal equilibrium state, but the atoms are in non-equilibrium state. By fitting the C2Swan spectrum using the software Specair, the rotational is far less than the vibrational temperature, so the plasma is in the non-equilibrium state.2. The interpolation algorithm for the discrete-points tables of hydrogen atomic line Stark broadening can be constructed, and the electron density and ion temperature are determined.Based on the basic theory principles and computational results of Griem and Gig-Card calculations, a simple and precise interpolation algorithm for the discrete-points tables can be constructed to obtain the traditional ne-Te diagnostic maps of hydrogen atomic line Stark broadening. The results show that hydrogen alpha line Stark broadening increases with the atomic reduced mass and with the electron temperature decreases. After a little hydrogen isotope gas was injected into the chamber, the convoluted fitting of hydrogen atomic alpha spectral line have been performed to obtain Stark broadening. Based on the computational results of Gig-Card calculation, a corrected method is set up to determine the electron density and ion temperature. The variation of the reduced mass pair due to the non-equilibrium effect contributes to the difference of the results derived from two hydrogen isotope alpha lines.3. A method has been introduced to correct the atomic non-equilibrium effect in vacuum arc plasma, and to determine the relative concentration of atoms and single charged ions.Based on the analysis of the non-equilibrium effects on the particle level population, a modified method for determining the electron temperature is established. The results show that the electron temperature derived from the intensity of atomic and ionic lines is the same, and atoms are in ionizing plasma environment. The ionization temperature has been determined through Saha-Boltzmann plots, and it is found that the ionization temperature is equal to the electron temperature, but electron density from Saha equation is much more than the value from Stark broadening results. It indicates that the main cause of Saha non-equilibrium is the different optical thickness of particles, then we can obtain the relative population of atoms and single charged ions.4. The characteristics of pulsed vacuum arc plasma under different discharge conditions have been investigated, and the influence of different discharge conditions on vacuum arc plasma have been studied. When increasing the discharge current, all the intensity of the atomic and ionic lines and C2molecular Swan (0,0) line have the local maximum value in the increasing trendency. The ionic excitation temperature shows the same trend, but the atom has just a slight variation. The electron density increases linearly with the discharge current. Additionally, the increasement of current lead to the increasing trend of the rotational temperature and the decreasing varation of the vibration temperature, both temperatures have fixed value at last. The obvious Doppler shifts of the lines emitted from different particles show that the directional velocity of all kinds of particles in the plasma increases with arc current. After the injection of the hydrogen isotope gas, it has been found that the ion temperature also has the corresponding local maximum value. Therefore, the variation of the electron temperature and ion temperature is the main cause of the local intensity changes.When the gap between two electrodes becomes large, the line intensity of atoms and ions is weak, but the intensity of hydrogen atomic alpha line becomes more strongger. The electron temperature and electron density and gas rotational temperature becomes larger, but the molecular vibrational temperature decreases. Increasing the discharge current, the intensity of titanium ions, hydrogen alpha line, double charged carbon ions increase at first and then decrease finally, but the carbon atoms and carbon single charged ions increase linearly. The excitation temperature of single charged titanium ion is not affected by the current, and but the excitation temperature of double charged titanium ion appeared to the same variation as its line intensity. The rotational temperature increases with the current till up to the constant value, while the vibration temperature has the same variation of double charged titanium ion. Additionally, the electron density has the same tendency as the hydrogen alpha line intensity. Observation of Doppler shifts shows that more irregular fluctuation of directional speed in vacuum arc discharge with the large gap.The background pressure can directly affects the pulsed discharge processes, and perform different effects on different discharge gaps. For small spacing discharge processes, all the line intensity increases at first and then decreases. For large spacing discharge processes, the intensity of C2molecular Swan (0,0) line has the same variation while the ionic line intensity decreases at first and then increases finally. Regardless of gap size, excitation temperature has the same variation as the ionic line intensity, but the gas rotational temperature is always increasing in small gap and decreasing in large gap. Additionally, the electron density slightly increases firstly and then decreases in the small space, but always reduce in the large space.5. The physical processes of vacuum breakdown have been investigated, and the mechanism of pulsed vacuum arc discharge and the characteristics of plasma jet have been studied.The results of high-speed photography experiment show that, vacuum breakdown processes between two electrodes under high voltage began with the cathodic erosions, the secondary electron on the surface of the insulation material is the main contribution to the formation of the original arc channel. The movement of anode light spot is actually the processes of plasma produced from cathode spot replacing the diffusion plasma channel supplied by the insulation, and it is also the process of decreasing the impedance of the plasma channel. Finally, the stable arc channel is established.The results of high speed photography show that many cathode spots simultaneously supply the plasma in low-current pulsed vacuum arc discharge. The character of surface flashover is the only one plasma channel. Even if many spots appear simultaneously on the electrode surface, the plasma from cathode spots shares one flashover channel between two electrodes. Compared with the changes between the location and light intensity of cathode spots, the averaged life time and the apparent spot velocityof cathode spots have been obtained. Meanwhile, it has been found that the new cathode spot would appear around the ablated surface region.It has been found that the maximum light intensity in the axial distribution of plasma jet decreases with the discharge duration. Meanwhile, the position of the maximum light intensity is leaving away from the edge. It is contributed that cathode spots are moving along the edge at a certain speed and the light intensity in the radial distribution is becoming valley shape. Although the intensity distribution of the initial jet is unhomogeneous, but it may converge in a certain distance and become the symmetric Gauss distribution.更多还原