氩气微腔放电中特性参数的数值模拟研究

本文采用二维自洽完全流体模型,针对阳极为通孔的高气压微腔放电结构,研究了微腔放电的参数特性.数值计算得到了氩气压强为100 Torr,放电稳态时的电势分布、电子数密度分布和电子温度分布等重要参数.模拟结果表明放电区存在显著的阴极鞘层结构,电子数密度的峰值达到10²⁰ m^-3,电子温度的量级为几个eV至十几eV,该结论与实验结果相一致.数值模拟合理的解释了微腔放电的基本原理. 关键词： 微腔放电 等离子体模拟 流体模型     

强流电子束源阴极等离子体的粒子模拟

 模拟了强流电子束源阴极表面附近区域数密度约1014 cm-3的等离子体的膨胀过程,观察到等离子体膨胀速度约为1 cm/μs。通过观察不同时刻阴极附近电子和离子的相空间分布、数密度分布和轴向电场分布,分析了等离子体膨胀过程。结果表明:等离子体的产生使得阴极表面电场增强,进而增大阴极的电流发射密度,电流密度增加使得空间电荷效应增强,并使等离子体前沿处的电场减小,当等离子体前沿处的电场减小到零时等离子体向阳极膨胀。讨论了等离子体温度、离子质量、束流密度和离子产生率对等离子体膨胀速度的影响。结果表明:等离子体的膨胀速度随着等离子体温度升高而增大,随离子质量增大而减小,但膨胀速度不等于离子声速;等离子体产生率越小,等离子体膨胀速度越小。     

四阳极直流辉光放电正柱区数值分析

分析了具有中间开窗电极四阳极直流辉光放电装置的氦气放电中阴极与较近阳极间放电正柱区的电子密度、电子温度和等离子体碰撞频率, 及其空间分布随放电气压变化规律。结果表明电子密度在径向上呈类似于抛物线的分布, 而碰撞频率和电子温度在整个放电管内近似均匀分布。探讨了不锈钢阳极座和矩形窗口对这些参数的影响, 为进一步进行装置仿真和实验研究提供参考。     

微空心阴极放电的流体模型模拟

采用流体模型研究了微空心阴极放电(MHCD)的特点，对放电中电场的形成，电子和离子的密度分布，电子能量分布进行了数值模拟.该计算是针对高气压，圆筒形阴极结构下的He放电.结果表明放电中存在空心阴极效应，从电子能量分布可以看出，放电中存在高能电子，放电空间的电场分布主要表现为径向电场.此外，通过改变气压，阴极孔径等参数计算出它们对放电的影响.分析表明减小孔径有利于负辉区更充分的重合.提高气压将缩短阴极位降区. 关键词： 微空心阴极放电 流体模型     

微空心阴极维持辉光放电的时空特性

利用流体模型在氩气环境下模拟得到了微空心阴极维持辉光放电的电势、电子密度、离子密度和电场等放电参数的时空分布特性.模拟结果表明,微空心阴极维持辉光放电在不同的时刻表现出不同的放电模式.放电的初始阶段为汤生放电模式;第二阶段为汤生放电模式向空心阴极效应放电模式过渡阶段,微空心阴极维持辉光放电得到初步发展;第三阶段为空心阴极效应放电模式,微空心阴极维持辉光放电区逐渐形成;第四阶段为放电的稳态阶段.在稳态放电状态下,空心阴极腔内的电子和离子密度峰值达到10~(15)cm~(-3),位于空心阴极腔的中心位置,维持辉光放电区电子密度可以达到10~(13)cm~(-3).研究结果同时表明,微空心阴极放电促进了微空心阴极维持辉光放电的形成;同时微空心阴极维持辉光放电也促进了微空心阴极放电的发展.另外,实验研究表明,第二阳极对微空心阴极腔内外的电势、电场和带电粒子密度的分布均有重要影响,并且对维持辉光放电区域的影响更加明显.第二阳极是形成维持辉光放电的必要条件.     

微空心阴极放电时空特性

为了揭示微空心阴极放电的放电机理,利用流体模型研究了矩形微空心阴极放电的时间和空间分布特性。在氩气环境下计算得到了压强为1.3×10~4Pa时电流、电势、电场、电子和离子密度等随时间的发展变化。结果表明,整个放电过程分为四个阶段,即预放电阶段、电场由轴向向径向转换阶段、电流缓慢增长向空心阴极效应过渡阶段和稳态放电阶段。稳态放电时出现明显的空心阴极效应,阴极位降区存在很高的径向电场和较高的电子平均能量,而负辉区径向电场很弱,电子平均能量较低,电子和离子密度峰值出现在负辉区,二者数值基本相等,而在阴极位降区离子密度远高于电子密度。     

PICPMCC 数值模拟四阳极直流辉光放电

利用OOPIC 代码并结合MCC 方法考察四阳极低气压直流辉光放电中粒子间的碰撞, 并对其进行了数值模拟。首先分析OOPIC 代码的计算流程, 给出模拟的边界条件和时间、空间步进; 然后对所得到的等离子体密度、温度的分布情况进行了分析。结果表明, 放电气压、电流以及环电极的有效宽度均影响等离子体密度和温度的径向分布。当气压为300Pa 时, 电子密度在径向较大范围内是均匀分布的, 而当气压升高到800Pa 时, 其径向不均匀增大, 且总的密度和电子的平均温度下降。放电电流的增大使得等离子体密度和温度均相应升高。但环电极有效宽度的减小使得等离子体密度的分布更不均匀, 并使总的密度和平均温度均下降。     

赝火花开关放电的蒙特卡罗粒子模拟

采用粒子模拟和蒙特卡罗相结合的方法,应用静电求解模型,对赝火花开关初始放电过程进行了模拟。赝火花开关初始放电过程主要由汤森放电过程、等离子体形成、空心阴极效应和场致发射引发主放电组成;等离子体形成和空心阴极效应对赝火花开关的发展导通具有至关重要的作用。改变赝火花开关工作参数,如气压、电极孔径、阳极电压和阴极腔中初始粒子密度,研究其对赝火花开关电子峰值电流形成时间的影响。结果表明:随着气压、电极孔径、阳极电压和初始粒子密度的增大,赝火花开关电子峰值电流形成时间减小。     

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介绍了一种对等离子体边缘进行径向扫描的往复静电探针系统,该系统由高压气源、传输杆、光栅尺、探针组件构成。它在一次放电中能测量主等离子体边缘的温度、密度、悬浮电位、空间电位、离子饱和电流、极向电场、粒子通量等参数的径向分布以及电子温度和密度的衰减长度。测量结果表明,利用该系统测量的主等离子体边缘参数分布与JT-60U、TEXT、HT-7等装置上测量的结果一致。     

13.56 MHz/2 MHz柱状感性耦合等离子体参数的对比研究

通过Langmuir双探针和发射光谱诊断方法,对比研究了驱动频率为13.56 MHz和2 MHz柱状感性耦合等离子体中电子密度和电子温度的径向分布规律.结果表明:在高频和低频放电中,输入功率的增加对等离子体参数产生了不同的影响,高频放电中主要提升了电子密度,低频放电中则主要提升了电子温度.固定气压为10 Pa,分别由高频和低频驱动时,电子密度的径向分布均为"凸型".而电子温度的分布差异比较明显,高频驱动时,电子温度在腔室中心较为平坦,在边缘略有上升;低频驱动时,电子温度随径向距离的增加而逐渐下降.为了进一步分析造成这种差异的原因,在相同放电条件下采集了氩等离子体的发射光谱图,利用分支比法计算了亚稳态粒子的数密度,发现电子温度的径向分布始终与亚稳态粒子的径向分布相反.继续升高气压到100 Pa,发现不论高频还是低频放电,电子密度的径向分布均从"凸型"转变为"马鞍形",较低气压时电子密度的均匀性有了一定的提升,但低频的均匀性更好.     

Phenomenological model of the unstable stage of a vacuum spark discharge

A model of the unstable stage of a spark discharge in vacuum is proposed, which describes all typical manifestations of this stage, including current spikes in the diode, an increase in the potential at the cathode flame front, collective acceleration of ions in vacuum and plasma diodes, change in the cathode erosion mechanism, and the emergence of electron microbeams with a high current density at the anode. It is shown that these processes are associated with the formation of a charged electron layer of a spatially inhomogeneous plasma at the cathode flame boundary at the unstable stage of the spark discharge in vacuum. The emergence of this layer is associated with a limited emissive ability of the plasma at the cathode flame front during its expansion in vacuum. This leads to disruption of the plasma (field-induced emission of electron from the boundary region of the flame) and the formation of a short-lived charged plasma, viz., high-density ion cluster at the cathode flame boundary. The estimates obtained using this model are in good agreement with the experimental data on physical processes at the unstable stage of a vacuum spark discharge.     

Operational characteristics of a plasma cathode with a grid stabilization in a two-stage ion source

The discharge characteristics and the parameters of the cathode plasma in a two-stage ion source with a grid plasma cathode and a magnetic trap in the anode region are investigated. It is shown that an increase in the gas pressure and the accompanying increase in the reverse ion current in the bipolar diode between the cathode and anode plasmas lead to an increase in the cathode plasma potential and a transition of the cathode into the regime of electron emission from the open plasma boundary. The dependence of the ion current extracted from the anode plasma on the area of the exit aperture of the hollow cathode and the mesh size of the grid plasma cathode is explained. The conditions at which the ion emission current from the anode plasma is maximum are determined. The potential difference at the bipolar diode is measured by using the probe method. It is shown that, when the gas pressures reaches a critical value determined by the mesh size of the grid plasma cathode, the discharge passes into a contracted operating mode, in which the ion current extracted from the anode plasma decreases severalfold.     

Numerical simulation of super-short pulsed discharge in Helium with particle-in-cell Monte--Carlo collisions technique

This paper reports that a simulation of glow discharge in pure helium gas at the pressure of 1.333×10³ Pa under a high-voltage nanosecond pulse is performed by using a one-dimensional particle-in-cell Monte Carlo collisions (PIC--MCC) model. Numerical modelling results show that the cathode sheath is much thicker than that of anode during the pulse discharge, and that there exists the phenomenon of field reversal at relative high pressures near the end of the pulse, which results from the cumulative positive charges due to their finite mobility during the cathode sheath expansion. Moreover, electron energy distribution function (EEDF) and ion energy distribution function (IEDF) have been also observed. In the early stage of the pulse, a large amount of electrons can be accelerated above the ionization threshold energy. However, in the second half of the pulse, as the field in bulk plasma decreases and thereafter the reverse field forms due to the excessive charges in cathode sheath, although the plasma density grows, the high energy part of EEDF decreases. It concludes that the large volume non-equilibrium plasmas can be obtained with high-voltage nanosecond pulse discharges.     

Generation of a submillisecond electron beam with a high-density current in a plasma-emitter diode under the conditions of open plasma boundary emission

The generation of a 250-μs-wide electron beam in a plasma-emitter diode is studied experimentally. A plasma was produced by a pulsed arc discharge in hydrogen. The electron beam is extracted from a circular emission hole 3.8 mm in diameter under open plasma boundary conditions. The beam accelerated in the diode gap enters into a drift space in the absence of an external magnetic field through a hole 4.1 mm in diameter made in the anode. The influence of electron current deposition at the edge of the anode hole on the beam’s maximum attainable current, above which the diode gap breaks down, is studied for different accelerating voltages and diode gaps. The role of processes occurring on the surface of the electrodes is shown. For an accelerating voltage of 32 kV, a mean emission current density of 130 A/cm² is achieved. The respective mean strength of the electric field in the acceleration gap is 140 kV/cm. Using the POISSON-2 software package, the numerical simulation of the diode performance is carried out and the shape of steady plasma emission boundaries in the cathode and anode holes is calculated. The influence of the density of the ion current from the anode plasma surface on the maximum attainable current of the electron beam is demonstrated.     

Simulation of transition from Townsend mode to glow discharge mode in a helium dielectric barrier discharge at atmospheric pressure

The dielectric barrier discharge characteristics in helium at atmospheric pressure are simulated based on a one-dimensional fluid model. Under some discharge conditions, the results show that one discharge pulse per half voltage cycle usually appears when the amplitude of external voltage is low, while a glow-like discharge occurs at high voltage. For the one discharge pulse per half voltage cycle, the maximum of electron density appears near the anode at the beginning of the discharge, which corresponds to a Townsend discharge mode. The maxima of the electron density and the intensity of electric field appear in the vicinity of the cathode when the discharge current increases to some extent, which indicates the formation of a cathode-fall region. Therefore, the discharge has a transition from the Townsend mode to the glow discharge mode during one discharge pulse, which is consistent with previous experimental results.     

Characteristics of an ion source with a plasma cathode and a multipole magnetic system for confining fast electrons

Parameters and ion-emission characteristics of the plasma generated in the anode stage of an ion source with a hollow glow-discharge plasma cathode are studied. To decrease the minimum operating gas pressure to 5×10³ Pa, a multipole magnetic system was installed on the surface of the hollow cathode and the peripheral magnetic field was enhanced in the anode stage of the source. The effect of the gas pressure, the plasma-cathode current, and the voltage between the electrodes of the anode stage on the value of the ion current extracted from the plasma is investigated. It is found that the size of the exit aperture of the hollow cathode substantially affects the efficiency of ion extraction. The potential (1–5 V) and the electron temperature (1–8 eV) of the anode-stage plasma are measured by the probe method. The conditions are determined that ensure the maximum ion-emission current from the plasma at low gas pressures.     

介质阻挡放电点燃大气压直流均匀放电的光谱研究

由于大气压均匀放电等离子体在工业领域具有广泛的应用前景,为了获得大尺寸的大气压均匀等离子体,采用氩气作为工作气体,在大气压空气环境中利用同轴介质阻挡放电点燃了针-板电极间的大气隙(气隙宽度达到5 cm)直流均匀放电。研究发现,同轴介质阻挡放电能够有效降低针-板电极间的击穿电压。该均匀放电由等离子体柱、等离子体羽、阴极暗区和阴极辉区组成。其中等离子体柱和阴极辉区都是连续放电。而等离子体羽不同位置的放电是不同时的。事实上,等离子体羽放电是由从阴极向着等离子体柱移动的发光光层(即等离子体子弹)叠加而成。利用电学方法测量了放电的伏安特性曲线,发现其与低气压正常辉光放电类似,均具有负斜率。采集了放电的发射光谱,发现存在N₂第二正带系、氩原子和氧原子谱线。通过Boltzmann plot方法对放电等离子体电子激发温度进行了空间分辨测量,发现等离子体柱的电子激发温度比等离子体羽的电子激发温度低。通过分析放电机制,对以上现象进行了定性解释。这些研究结果对大气压均匀放电等离子体源的研制和工业应用具有重要意义。     

Modification of Graphene on a Copper Grid during Femtosecond Laser Irradiation: Electron Diffraction and Raman Spectroscopy Studies

The initial stage of a nanosecond discharge in gaps with a high electric field at a cathode is studied by laser methods (interferometric, shadow, schlieren methods). The studies are performed in air at atmospheric pressure. Prominence is given to studying the evolution (appearance and growth) of the plasma channels at an anode and to estimating their parameters.     

Time resolved spectroscopic studies of a FXR discharge

Time resolved spectroscopic studies of some electrode material lines and an impurity line have been performed in a flash X-ray (FXR) discharge. The results indicate that the breakdown takes place in impurities and adsorbed gases released from the electrodes. Thereafter, a front of anode material propagating with a velocity of (1–2)×10⁶ cm/sec moves towards the cathode. A cloud of cathode material is localized outside the cathode during an early stage of the discharge. The anode front and the cathode cloud penetrate each other after current maximum and a “metallic arc” is formed between the electrodes