氦气空心阴极放电动力学过程的模拟研究

利用流体模型模拟研究了氦气空心阴极放电的时空动力学过程,计算得到了不同放电时刻电子和亚稳态氦原子密度、电势、电场、基态电离速率和分步电离速率等的时空分布特性。特别是讨论了亚稳态原子和分步电离对于放电的影响。结果表明,随着电流的增长,放电处于五个不同的放电模式:第一阶段电流上升非常缓慢,为汤生放电模式,带电粒子密度、亚稳态原子密度和径向电场均很弱;第二阶段电流迅速上升,放电模式由汤生放电向空心阴极放电过渡,带电粒子密度、亚稳态原子密度和径向电场迅速增强;第三阶段达到准稳态阶段,放电电流增长速度变缓,形成了明显的阴极鞘层结构;第四阶段为空心阴极效应形成阶段,向稳态阶段过渡;第五阶段为稳态放电阶段。研究结果同时表明,亚稳态氦原子和分步电离在放电的初始阶段对于放电的发展作用较弱,在前三阶段中,电子的产生以基态电离为主。随着放电的发展,由亚稳态原子引起的分步电离对新的电子产生的作用逐渐接近并超过基态电离,对总电离的贡献率越来越高。     

流体-亚稳态原子传输混合模型模拟空心阴极放电特性

利用流体-亚稳态原子传输混合模型研究了氩气矩形空心阴极放电稳态时的参数. 数值计算得到了压强为10 Torr时的电势、电子、离子和亚稳态氩原子密度以及电子平均能量的分布. 结果表明电子和离子密度峰值为4.7×10¹² cm^-3, 亚稳态原子密度峰值为2.1×10¹³ cm^-3. 本文同时对流体-亚稳态原子传输混合模型和单一流体模型模拟得到的放电参数进行了比较. 结果表明, 分步电离是新电子产生的重要来源, 亚稳态原子对空心阴极放电特性有重要影响. 与单一流体模型相比, 混合模型计算得到的电子密度升高, 阴极鞘层宽度和电子平均能量降低. 关键词： 空心阴极放电 流体-亚稳态原子传输模型 电子密度 分步电离     

Untersuchung der Prozesse in der aktiven Zone der Hohlkatodenbogenentladung. II. Die Bilanzgleichungen,Ergebnisse

In [1] we studied the electron distribution function and the processes of excitation and ionization inside the cathode of hollow cathode arc discharge. In this paper we solve the equations of energy and particle balances using the results of [1]. Adding the minimum principle of energy it is possible to calculate the concentration and the temperature of electrons, the cathode drop, the temperature of the cathode wall and the current density in the active zone. Some of calculated values will be compared with experimental ones. We found satisfactory agreement.     

Study of Intense Electron Beams Produced by High-Voltage Pulsed Glow Discharges

We report the generation of high-current-density (20 A/cm2) pulsed electron beams from high-voltage (48-100 kV) glow discharges using cathodes 7.5 cm in diameter. The pulse duration was determined by the energy of the pulse generator and varied between 0.2 ?s and several microseconds, depending on the discharge current. The largest electron beam current (900 A) was obtained with an oxidized aluminum cathode in a helium-oxygen atmosphere. An oxidized magnesium cathode produced similar results, and a molybdenum cathode operated at considerably lower currents. A small-diameter (<1 mm) well-collimated beam of energetic electrons of very high current density (>1 kA/cm2) was also observed to develop in the center of the discharge. Electrostatic probe measurements show that the negative glow plasma density and the electron beam current have a similar spatial distribution. Electron temperatures of 1-1.5 eV were measured at 7 cm from the cathode. The plasma density (8.5 · 1011 cm-3 at 450 A) was found to depend linearly on the discharge current. In discharges at high currents a denser and higher temperature plasma region was observed to develop at approximately 20 cm from the cathode. We have modeled the process of electron beam generation and predicted the energy distribution of the electron beam. More than 95 percent of the electron beam energy is calculated to be within 10 percent of that corresponding to the discharge voltage.     

Self-sustained self-sputtering: a possible mechanism for thesuperdense glow phase of a pseudospark

Self-sustained self-sputtering occurring during high current pseudospark operation (≈10⁴ A/cm², I>10³ A) is shown to be a possible mechanism for the superdense glow. The mean-free-path for ionization of cathode material sputtered in the low-current hollow-cathode phase can be shorter than the cathode-anode gap distance, and ionized atoms can return to the cathode surface, self-sputtering with a yield greater than one. The self-sputtered cathode atoms become ionized in the beam of electrons accelerated in the cathode sheath. A large fraction of the discharge current at the cathode surface can be carried uniformly over the surface by ions and a very high electron emission density is not required to maintain the high current     

Magnetron source of accelerated plasma flow

A new source of an accelerated plasma flow intended for depositing high-quality coatings is described. In this source, a magnetron discharge for cathode target sputtering is combined with a high-voltage discharge with longitudinal oscillation of electrons for ionization of the accrued vapor in which the plasma density is distributed uniformly owing to the application of three-phase ionizer.     

Gas-filled electron diode based on a glow discharge

Stable ignition and sustention of a pulsed discharge with a current of up to 180 A and duration of 12 μs at a pressure of 10⁻¹–10⁻² Pa are achieved in a glow-discharge plasma cathode with the help of an auxiliary initiating discharge. An electron emission current density of up to 100 A/cm² and accelerating voltageof 15 kV are obtained in a gas-filled diode based on this type of a plasma cathode. An electron beam witha neutralized space charge can be transported almost without losses in a weak axial magnetic field alonga plasma channel formed due to the gas ionization by the accelerated electrons over a distance of up to 30 cm.     

空心阴极直流放电的二维自洽模型描述和阴极溅射分析

建立了空心阴极放电的二维自洽理论模型,理论研究了气压为50—120Pa,电压为150—300V的范围内Ar空心阴极放电特性、粒子密度和电离速率空间分布,特别考察了影响阴极溅射分布有关因素:阴极面上的电场、离子流和离子密度的沿阴极截面的空间分布.研究结果不仅证实了在所讨论的范围内,空心阴极效应明显存在而且发现归一化电离速率的空间分布形状强烈依赖于气压.通过研究电场、离子流和离子密度的空间分布解释了空心阴极溅射型离子激光器中不均匀阴极溅射的现象来源于阴极面附近的电场、离子流和离子密度的不均匀分布 关键词： 空心阴极放电 自洽模型 气体激光 阴极溅射     

具有空心阴极放电特征的射频放电的两电子组模型

 运用两电子组模型，考虑了射频放电中的α过程和γ过程两种电离机制，并结合流体模型，研究了中等气压下窄电极间隙容性耦合射频放电在运行模式转变区的等离子体密度以及电离速率分布等特性。理论研究表明，γ电离过程在高电流模式运行中起主要作用，并证实了此类放电中存在显著的电子摆钟效应，具有类似于空心阴极放电的特征。     

A physical model of the low-current-density vacuum arc

A one-dimensional (1-D) physical model of the low-current-density steady-state vacuum arc is proposed. The model is based on the continuity equations for ions and electrons and the energy balance for the discharge system; the electric potential distribution in the discharge gap is assumed to be nonmonotonic. It is supposed that the ion current at the cathode is generated within the cathode potential fall region due to the ionization of the evaporated atoms by the plasma thermal electrons having Boltzmann's energy distribution. The model offers a satisfactory explanation for the principal regularities of a hot-cathode vacuum arc with diffuse attachment of the current. The applicability of the model proposed to the explanation of some processes occurring in a vacuum arc, such as the flow of fast ions toward the anode, the current cutoffs and voltage bursts, and the backward motion of a cathode spot in a transverse magnetic field is discussed     

直流电源耦合高功率脉冲非平衡磁控溅射电离特性

采用直流电源放电形成高功率脉冲非平衡磁控溅射(dc-high power impulse unbalanced magnetron sputtering,dc-HPPUMS 或dc-HiPiUMS),利用雪崩放电的击穿机理形成深度自触发放电,同轴线圈和空心阴极控制放电特性和提高功率密度.磁阱俘获雪崩放电形成的二次电子和形成漂移电流,形成了大电流脉冲放电,放电脉冲电流密度峰值超过100 A/cm²,脉冲频率小于40 Hz.由于放电等离子体远没有达到平衡状态,放电电流主要受到空间电荷效应 关键词： 放电 脉冲技术     

筒形溅射阴极的磁场优化及其高功率放电特性研究

基于高功率脉冲磁控溅射(HiPIMS)技术开发的筒形溅射阴极,配合电磁系统可有效地提升等离子体的输运效率.然而电磁系统的引入反作用于筒内放电特性,从而使靶面放电面积和放电强度无法同时维持.鉴于此,本文通过调整磁场布局,研究了靶面切向(横向)磁场和法向(纵向)磁场对靶面放电的作用规律,优化后靶面切向磁场分布更加均匀,磁场强度高于40 mT的靶面区域占比由51%增至67%,同时法向峰值强度外移,强度由73 mT增至96 mT.采用Ar/Cr体系放电发现:相同工艺条件下,优化后的溅射阴极辉光变亮,靶电流增大,放电面积变宽,放电特性得到显著提升.利用等离子体整体模型仿真和发射光谱仪检测发现优化后离子电流和光谱强度得到明显提升,Cr粒子密度提高一倍,增至2.6×10^20 m^–3,且离化率上升至92.1%,同时输出离子通量提高近一倍,实现了靶面放电与离子输出的双促进.     

Untersuchung der Prozesse in der aktiven Zone der Hohlkatodenbogenentladung I. Die Energieverteilungsfunktion der Elektronen,die Anregungs- und Ionisationsgeschwindigkeit

In this paper the processes of excitation and ionization inside the cathode of a hollow cathode are discharge will be studied. The electron energy distribution function is calculated from the kinetic equation. For this we take into account the following processes: elastic and inelastic collisions of electrons with atoms, Coulomb interaction between emitted fast electrons and electrons of the plasma.     

Importance of high local cathode spot pressure on the attachment ofthermal arcs on cold cathodes

The importance of having high local cathode spot pressures for the self-sustaining operation of a thermal arc plasma on a cold cathode is theoretically investigated. Applying a cathode sheath model to a Cu cathode, it is shown that cathode spot plasma pressures ranging 7.4-9.2 atm and 34.2-50 atm for electron temperatures of ~1 eV are needed to account for current densities of 10⁹ and 10¹⁰ A·m^-2, respectively. The study of the different contributions from the ions, the emission electrons, and the back-diffusing plasma electrons to the total current and heat transfer to the cathode spot has allowed us to show the following. 1) Due to the high metallic plasma densities, a strong heating of the cathode occurs and an important surface electric field is established at the cathode surface causing strong thermo-field emission of electrons. 2) Due to the presence of a high density of ions in the cathode vicinity, an important fraction of the total current is carried by the ions and the electron emission is enhanced. 3) The total current is only slightly reduced by the presence of back-diffusing plasma electrons in the cathode sheath. For a current density j_tot=10⁹ A·m^-2 , the current to the cathode surface is mainly transported by the ions (76-91% of j_tot while for a current density j_tot = 10¹⁰ A·m^-2, the thermo-field electrons become the main current carriers (61-72% of j_tot). It is shown that the cathode spot plasma parameters are those of a high pressure metallic gas where deviations from the ideal gas law and important lowering of the ionization potentials are observed     

Simulation of radio-frequency atmospheric pressure glow discharge in γ mode

The existence of two diffe1：ent discharge modes has been verified in an rf （radio-frequency） atmospheric pressure glow discharge （APGD） by Shi [J. Appl. Phys. 97, 023306 （2005）]. In the first mode, referred to as a mode, the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion. In the second mode, termed γ mode, the discharge current density is relatively high, the secondary electrons emitted by cathodc under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. In this paper, a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in the mode in helium discharge between two parallel metallic planar electrodes. The results show that as the applied voltage increases, the discharge current becomes greater and the plasma density correspondingly increases, consequentially the discharge transits from the a mode into the γ mode. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region, and the electron Joule heating and the electron collisional energy loss reach their maxima in the region. The validity of the simulation is checked with the available experimental and numerical data.     

Über den zeitlichen Verlauf des Stromes einer Gasentladung in Wasserstoff

The transient growth of currents in a Townsend gas discharge system under uniform dc field conditions in Hydrogen is examined. The discharge is started by 10³ to 10⁶ electrons released from the cathode by an UV light pulse within some 10^?7 seconds. Observed oscillations of the current are found to be due to the motion of the electrons through the gap creating new electrons by photoelectron emission at the cathode due to photons generated in the gap. At sparking threshold conditions (Μ ₀=1) the electron current becomes self-sustaining after a few electron transit times. The positive ion current soon exceeds the electron current and grows linearly with time (Μ ₀=1) until the positive ions of the first generation enter the cathode. For times greater than a positive-ion transit time the current becomes self-sustaining. Neglecting space-charge effects one would not expect a spark to occur. The space-charge of the positive ions, however, causes a distortion of the field changing the ionization efficiency of the electrons. It is shown that an observed rapid growth of current leading to breakdown after some positive-ion transit times is in agreement to this conception.     

Onset of breakdown and formation of cathode spots

When an increasing diode voltage is applied, enhanced field emission of electrons begins from a growing number of small spots or whiskers on the cathode surface. This stimulates desorption of weakly bound adsorbates from the surface of a whisker. As the diode voltage increases, the 100-V equipotential surface moving toward the cathode is met by the desorbed neutrals moving away from the cathode, resulting in sharp risetime for the onset of ionization of desorbed neutrals by field-emitted electrons. Positive ions produced in the ionization region a few microns from the electron emitting spot are accelerated back to it. This bombardment leads to surface heating of the spot. The onset of breakdown by this mechanism requires much less current than the Joule heating mechanism. The localized buildup of plasma above the electron emitting spot leads to pressure and electric field distributions that ignite unipolar arcs. The high current density of the unipolar arc and the associated surface heating by ions result in the explosive formation of cathode spot plasma     

介质阻挡均匀大气压辉光放电数值模拟研究

通过数值求解一维电子、离子连续性方程和动量方程，以及电流连续性方程，计算了氦气介 质阻挡大气压辉光放电电子、离子密度和电场在放电空间的时空分布，以及放电电流密度和 绝缘介质板充电电荷密度随时间的变化. 分析讨论所加电压频率、幅值及介质板性质等对均 匀大气压辉光放电性质的影响. 当外加电压频率足够高时，大量离子被俘获在放电空间，空 间电荷场又引起足够多的电子滞留在放电空间. 这些种子电子使得在大气压下发生汤森放电 ，放电空间结构类似于低气压辉光放电，即存在明显的阴极位降区、负辉区、法拉第暗区和 等离子体正柱 关键词： 大气压辉光放电 介质阻挡 数值模拟 等离子体     

Tonks-Langmuir problem for a bi-Maxwellian plasma

An analytical solution of the Tonks-Langmuir (TL) problem with a bi-Maxwellian electron energy distribution function (EEDF) is obtained for a plasma slab. The solution shows that the ambipolar potential, the plasma density distribution, and the ion flux to the wall are mainly governed by the cold electrons, while the ionization rate and voltage drop across the wall sheath are governed by the hot electrons. The ionization rate by direct electron impact is found to be spatially rather uniform, contrary to the T-L solution where it is proportional to the plasma density distribution. The temperature of hot electrons defined by the ionization balance is found to be close to that of the T-L solution for a mono-Maxwellian EEDF, and is in reasonable agreement with experiments carried out in a low pressure capacitance RF discharge. The energy balance for cold electrons in this discharge shows that their heating by hot electrons via Coulomb interaction is equalized by the cold electrons' escape to the RF electrodes during collapse of the RF sheath