Method for producing nanomaterials in the plasma of a low-pressure pulsed arc discharge

A new method for the production of nanomaterials in the plasma of a low-pressure arc discharge is developed and experimentally studied. This method can be used to synthesize nanoparticles 5–10 nm in size with a narrow size distribution. In this method, a low-pressure arc discharge is used to melt a material, to disperse the molten material, to deliver liquid material droplets to the plasma, to cool the liquid nanoparticles forming in the plasma up to their solidification, and to deposit the solidified nanoparticles onto a substrate.     

Thermal regime of self-heated hollow cathode in a low-pressure high-current pulsed-periodic discharge

Technical Physics - We have studied the thermal regime of a self-heated hollow cathode in combined low-current (1–5 A) dc discharge and high-current (up to 100 A) pulsed-periodic discharge...     

Ion flux from the cathode region of a vacuum arc

The properties of the ion flux generated in a vacuum arc are reviewed. The structure and distribution of mass erosion from individual cathode spots and the characteristics of current carriers from the cathode region at moderate arc currents are described. An appreciable ion flux (~10% of the total arc current) is emitted from the cathode of a vacuum arc. This ion flux is strongly peaked in the direction of the anode, although some ion flux may be seen even at angles below the plane of the cathode surface. The observed spatial distribution of the ion flux is expressed quite well as an exponential function of the solid angle. The ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply charged ions. The average ion potential and ion multiplicity increase significantly for cathode materials with higher arc voltages but decrease with increasing arc current for a particular material. The main theories concerning ion acceleration in cathode spots are the potential hump theory and the gas dynamic theory. Experimental data indicate that these theories serve reasonably well when used to predict the mean values of the charge state, ion potential, and ion energies for the ion flux, but are quite insufficient when compared with the results for the potentials and energies of individual ions     

Model of liquid-metal splashing in the cathode spot of a vacuum arc discharge

The formation of microjets is studied during the extrusion of a melted metal by the plasma pressure from craters formed on a cathode in a burning vacuum arc. An analytic model of liquid-metal splashing that includes two stages is proposed. At the first stage, the liquid motion has the axial symmetry and a liquid-metal wall surrounding the crater is formed. At the second stage, the axial symmetry is broken due to the development of the Plateau–Rayleigh instability in the upper part of the wall. The wall breakup process is shown to have a threshold. The minimal plasma pressure and the minimal electric current flowing through the crater required for obtaining the liquid-metal splashing regime are found. The basic spatial and temporal characteristics of the jet formation process are found using the analytic model.     

Anomalous plasma phenomena of hollow cathode arc discharge

Magnetically confined argon plasma produced by hollow cathode arc discharge has been studied in different experimental conditions, with discharge current from 10–50 A, vessel argon pressure between 10^–3 and 10^–4 torr (1 torr=133·32 Pa) and axial magnetic field up to 0·12 T. The plasma density measured by a cylindrical Langmuir probe is found to be 10¹⁹ to 4 × 10¹⁹ m^–3 and the electron temperatureT _e varies between 2·5 and 4·8 eV. When an external axial magnetic field is applied the plasma temperature decreases with the increase in the magnetic field intensity until it reaches a minimum value at 0·075T and then increases with the same rate. This has been interpreted as high frequency waves excitation due to electron beam-plasma interaction, which explains the electron density jumps with the magnetic field intensity. Enhanced plasma transport across the magnetic field is studied and classified as anomalous diffusion.     

Acceleration of a multicomponent plasma in the cathode region of a vacuum arc

A general solution to the problem of the steady-state spherical expansion of a current-carrying multicomponent plasma into a vacuum is derived. It is shown that, in vacuum arc discharges, the main force accelerating the cathode material, which becomes a plasma at distances of 1 to 300 μm from the cathode surface, is the electron pressure gradient force maintained by Joule heating. It is established that ions of different charges move with the same hydrodynamic velocity, which is uniquely determined by the mass and mean charge of the ions and the maximum electron temperature in the cathode region.     

Plasma-chemical reactor based on a low-pressure pulsed arc discharge for synthesis of nanopowders

A reactor for producing nanopowders in the plasma of a low-pressure arc discharge has been developed. As a plasma source, a pulsed cold-cathode arc evaporator has been applied. The design and operating principle of the reactor have been described. Experimental data on how the movement of a gaseous mixture in the reactor influences the properties of nanopowders have been presented.

     

Extension of the perturbed plasma region in a low-pressure discharge near the negative probe

It is suggested to consider the region in which the electric field intensity changes its sign as a perturbed plasma region near the negative probe. It is demonstrated that the size of this perturbed region can considerably exceed the probe radius and depends not only on the probe radius but also on the dimensions of the discharge chamber. Within the framework of the considered problem, the chosen criterion is equivalent to another condition that the number of particles in the perturbed zone is equal to the number of particles striking the probe. This contradicts the approach conventionally used in the probe theory according to which particles are accelerated rather than generated in the perturbed region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 10–12, June, 2007.     

真空弧放电TiH合金阴极表面形貌分析

TiH合金电极是一种含氢量非常高的金属材料,用它作真空弧离子源的电极,可在真空环境下产生强度非常高的氢离子流。相比纯金属材料电极,TiH电极除了出现真空弧放电特有的融蚀现象外,还存在气体释放过程,所以它的表面形貌具有一定的独特性。利用扫描电子显微镜观察了单次放电和多次放电后阴极表面形貌,发现阴极斑在阴极表面微裂纹附近连续分布,气体释放生成很多小孔,使阴极斑区域呈絮状结构;弧流越大,阴极斑数量越多;多次放电后,阴极斑朝含氢量多的地方移动。该结果有助于了解含氢电极的真空弧放电过程,对该类放电的应用具有一定参考意义。     

On the problem of electron-emission mechanisms in the cathode spot of a vacuum arc discharge

A criterion for the F-type mechanism of electron emission in a vacuum-arc cathode spot is developed. It is shown that the implementation of F-electron emission in the cathode spot necessitates the fulfillment of three conditions: first, generation of the optimal electric field E _opt, second, implementation of atomic-ionic balance at which the E _opt field is generated and, third, retention of the cathode temperature lower than the inversion temperature of Nottingham’s effect. The calculations show that the heat of the evaporation of cathode atoms which meets these requirements does not exceed λ_a ～ 1.1–1.2 eV. Taking the possibility of F-T-emission into account, the evaporation heat can be slightly higher than λ_a ～ 1.5–1.6 eV. However, in this case, it turns out to be fairly small. Note that the number of such metals is not very large.     

Excitation of He+ in a hollow cathode arc discharge

Population densities of the excited He(II) levels i=4 to i=10 in hollow cathode arc discharge have been determined and compared with those calculated from a collisional-radiative model. In the experiment the electron temperature was about 2x10⁵K and the electron density about 3x10¹⁹ m^-3. The observed population densities agree within 50% with those calculated by Drawin and Emard, but differ by a factor 4–6 from the ones found by McWhirter and Hearn. The results of the present experiment suggest that near the threshold energies the cross sections for electronic excitation of He⁺ levels from the He⁺ ground state are half those expected from the calculations of Burgess. We conclude that Drawin and Emard's calculations should preferably be employed for the determination of the electron density and the electron temperature from observed population densities.     

Resonance radiation in the cold cathode region of a glow discharge in argon

The imprisonment of resonance radiation in the cold cathode region of a glow discharge in argon is calculated. It is shown that the number of resonance photons reaching the cathode surface is substantially smaller than the number of positive ions.     

Interaction of plasma jets and droplets in the cathode region of a vacuum arc

The interaction of liquid-metal droplets with plasma jets in the cathode region of a vacuum arc is considered in the context of an ecton model. It is shown that heating of a droplet in the cathode spot region can initiate the droplet transition to the plasma state.     

氘化物阴极真空弧放电光斑分布

氘化物真空弧放电在许多领域均有应用,如无损检测、石油探井、中子活化分析等。和金属阴极不同,氘化物阴极放电时会释放大量的气体分子,表现出许多不同性质。采用放大镜头和ICCD相机观察了氘化物阴极真空弧放电光斑分布。测量系统的空间分辨率约为5 μm,时间分辨率最小2 ns。放电脉冲半高全宽（FWHM）0.9 μs,弧流波形为半周期正弦波。实验结果表明,氘化物真空弧放电时,所有阴极斑聚集为一个群落,表现为一个大光斑;在液滴作用下,阴极斑群落偶尔也会分裂为两个或多个群落;光斑形状不受弧流影响,但面积和亮度会随弧流增加而增大。氘化物阴极放电斑点聚集有利于产生高密度等离子体,提高放电效率。     

Beam-plasma mechanism for anode plasma generation in a low-pressure two-stage self-sustained discharge with a cold hollow cathode

Results are presented from experimental studies of the anode region of a low-pressure two-stage self-sustained discharge with a closed cold hollow cathode. It is shown that applying an external longitudinal magnetic field promotes the generation of a dense anode plasma, whereas the transverse field impedes this generation. It is established that the beam-plasma mechanism for plasma generation plays a dominant role in the anode region of the discharge. The geometry of the electrodes of the gas-discharge chamber is optimized.     

Distribution of plasma parameters in the cathode region of a glow discharge in nitrogen

The experiments with a low-pressure glow discharge in nitrogen indicate the formation of a potential well for thermal electrons and the sign reversal of the electric field near the concentration maximum of the near-cathode plasma. Depending on reduced discharge length pL, one or two field reversal points exist, which correlate with the sign of the anode fall (negative or positive, respectively).     

Analytical calculation of cathode spot parameters on the electrode surface in arc discharge

The model of a heat transfer in the cathode spot on an electrode in the arc discharge is developed and analytical expressions for the spot radius and temperature as the functions of basic characteristics of the discharge and the electrode material are obtained. The dependence of the spot radius on the voltage drop of the cathodic discharge, which is consistent with results of numerical simulation, is calculated.