Hollow Cathode Effect Research Articles

Ion-diffusion losses and j/p2-scaling of hollow-cathode effect in DC discharges with parallel-plate cathode configuration

Ion-diffusion losses and j/p2-scaling of hollow-cathode effect in DC discharges with parallel-plate cathode configuration

Study on the structure, growth pattern and corrosion behavior of CoCrFeNiAl high entropy alloy coatings - base on hollow cathode effect

The surface of TC18 titanium alloy was coated with a CoCrFeNiAl HEA coating using double-glow plasma surface metallurgy. The structure, phase, and growth mode of the coating were investigated, along with its electrochemical corrosion behavior. The prepared HEA coating, under the influence of the hollow cathode effect and non-equilibrium diffusion, exhibits a composite structure consisting of a deposited layer and an interdiffused layer. It demonstrates robust metallurgical bonding with the substrate, and it exhibits a hardness of 12.66 GPa and an elastic modulus of 158.52 GPa, along with exceptional hardness and high elastic modulus. The primary phase of the coating consists of an FCC solid solution, with a minor presence of Ni3Al and Al8Cr5 phases. TEM results demonstrate that hollow cathode enhanced sputtering effectively refines the grain structure on the substrate surface. The high entropy alloy coating, characterized by a significant abundance of nanocrystalline and amorphous structures, is obtained under the bombardment of high-energy particles. Gradually increasing from the interface to the deposited layer, there is an augmentation in the content of nanocrystals. The transition from the substrate to the sedimentary layer comprises three distinct regions: a surface fine crystal region, a nanocrystalline structure region, and a nanocrystalline and amorphous precipitated phase-rich region. In 3.5 wt% NaCl solution, the electrochemical corrosion rate of the coating is 1.36 × 10−1 μm·year−1, which is about ten times lower than that of the substrate. Moreover, pre-soaking forms a stable oxide film on the coating's surface, effectively preventing corrosion and demonstrating its remarkable corrosion resistance.

Plasma density enhancement in radio-frequency hollow electrode discharge

The plasma density enhancement outside hollow electrodes in capacitively coupled radio-frequency (RF) discharges is investigated by a two-dimensional (2D) particle-in-cell/Monte-Carlo collision (PIC/MCC) model. Results show that plasma exists inside the cavity when the sheath inside the hollow electrode hole is fully collapsed, which is an essential condition for the plasma density enhancement outside hollow electrodes. In addition, the existence of the electron density peak at the orifice is generated via the hollow cathode effect (HCE), which plays an important role in the density enhancement. It is also found that the radial width of bulk plasma at the orifice affects the magnitude of the density enhancement, and narrow radial plasma bulk width at the orifice is not beneficial to obtain high-density plasma outside hollow electrodes. Higher electron density at the orifice, combined with larger radial plasma bulk width at the orifice, causes higher electron density outside hollow electrodes. The results also imply that the HCE strength inside the cavity cannot be determined by the magnitude of the electron density outside hollow electrodes.

Hollow cathode effect in radio frequency hollow electrode discharge in argon

Radio frequency capacitively coupled plasma source (RF-CCP) with a hollow electrode can increase the electron density through the hollow cathode effect (HCE), which offers a method to modify the spatial profiles of the plasma density. In this work, the variations of the HCE in one RF period are investigated by using a two-dimensional particle-in-cell/Monte-Carlo collision (PIC/MCC) model. The results show that the sheath electric field, the sheath potential drop, the sheath thickness, the radial plasma bulk width, the electron energy distribution function (EEDF), and the average electron energy in the cavity vary in one RF period. During the hollow electrode sheath’s expansion phase, the secondary electron heating and sheath oscillation heating in the cavity are gradually enhanced, and the frequency of the electron pendular motion in the cavity gradually increases, hence the HCE is gradually enhanced. However, during the hollow electrode sheath’s collapse phase, the secondary electron heating is gradually attenuated. In addition, when interacting with the gradually collapsed hollow electrode sheaths, high-energy plasma bulk electrons in the cavity will lose some energy. Furthermore, the frequency of the electron pendular motion in the cavity gradually decreases. Therefore, during the hollow electrode sheath’s collapse phase, the HCE is gradually attenuated.

Study of plasma characteristics of hollow cathode in glow discharge

In order to further investigate the conditions and the coupling mechanisms for generating the hollow cathode effect (HCE) for the process optimization in the field of surface modification, the plasma distribution and density in single glow and double glow hollow cathode discharge (HCD) were explored by experiments and simulations based on the two-dimensional fluid model. Additionally, we investigated the effects of the key parameters on the plasma density, the electron temperature and the potential field during the HCD. The experimental and simulation results showed that the high plasma density is owing to the HCE in the hollow cathode. It is found that there is a critical pore diameter for generating HCE, which is approximately the sum of the thickness of the plasma sheath and four times the electron-neutral mean free path in the single glow discharge while twice thickness of the plasma sheath and four times the electron-neutral mean free path in the double glow discharge. The HCE in the double glow discharge can be observed when the ignition voltage that the source cathode voltage be 250 V greater than the workpiece cathode voltage can be met while the PD value is about 1.5 ∼ 3.8 Torr mm. Consequently, we believe that our investigation on the plasma characteristics of hollow cathode in glow discharge will contribute to the development of the more optimal processing parameters for the application of the glow plasma discharge phenomena.

Spatial distributions of the ion flux in a capacitive hydrogen RF discharge using a hollow cathode with double toroidal grooves enclosed by magnets

A hydrogen high-density capacitively coupled plasma has been developed using a hollow cathode with double toroidal grooves enclosed by magnets and without an iron yoke disk. It is found that this plasma source allows generating higher plasma densities compared to the conventional RF magnetron plasma sources. Spatial distributions of the ion flux have been measured at various H2 gas pressures, p, of 1–20 Pa. It is found that the hybrid combination of a hollow cathode effect and magnetic confinement of electrons is attained for p ≥ 5 Pa, while for p ≤ 3 Pa, a conventional capacitive discharge is generated. The radial profile of the ion flux becomes uniform with increasing axial distance from the target for the hollow cathode discharge. The characteristic decay length of the roughness of the ion flux profile increases with increasing the gas pressure.

Two-dimensional particle-in-cell simulation of electron heating effect in multiple hollow cathode trenches with curved boundary

A particle-in-cell simulation is suitable for simulating capacitively coupled plasma devices because it calculates the trajectory of particles with arbitrary energy distributions and has the advantage of analyzing sheath dynamics precisely. Implementing the ghost grid method in the Poisson solver makes it possible to simulate a curved structure in the structured grids. The electron transport and heating mechanism are determined according to the two-dimensional (2D) sheath dynamics. The hollow cathode effect on electron transport was analyzed by varying the hole shape and gas pressure. The spatial distributions of electron density and temperature are influenced by the energy relaxation length of electrons, which is affected by the energy distribution function and gas pressure. As a result, a new electron heating mode appears in the 2D structure.

Insights into the effect of hollow cathode with external injection of fast ions in a sputter-type negative ion source

In sputter-type negative ion sources, the negative ion currents and ionization efficiencies are very dependent upon the creation of a sputter crater and formation of an intense plasma glow. This effect has many similarities with the well-known phenomena of hollow cathode discharges (HCDs). This paper discusses the phenomenological model related to conventional HCDs, including modes with injection of external energetic particles. We propose that HCDs play a major role in sputter-type negative ion sources. In this study, we evaluate the phenomenology of a discharge with a hollow cathode and derive an analytical formula that defines the neutral particle density inside the cathode cavity and substantiates how the classic HCD occurs in sputter-type negative ion sources. The cathode pit geometry was revealed by optical microscopy. Volt–ampere characteristics were investigated in order to compare it to typical discharges with hollow cathodes. In sputter-type negative ion sources, known as SNICS, discharge characteristics and observed light glow inside the cathode pit after its formation were very similar to data published on HCDs. The HCD phenomenon explains how the cathode current increases by orders of magnitude after creation of the cathode pit. The HCD effect in SNICS and similar sputter sources compliments existing mechanisms of negative ion production, especially ion pair production close to the cathode surface due to low energy ionization based on the collision–radiation model. The wealth of information accumulated over the last century on HCDs can be used to describe and explain the phenomenology of physics and operation of sputter negative ion sources.

Hollow Cathode Glow Discharge Initiation in a Fore-Vacuum Plasma–Cathode Electron Source

In this article, we present the results of a study on the initiation of a hollow cathode discharge in the electrode system of a fore-vacuum plasma–cathode electron source. It is shown that the development of the hollow cathode effect is facilitated by the back-streaming ion flux of the high-voltage glow discharge (HGD) that arises in the accelerating gap of the electron source. The effect of the type of operating gas and the cathode hollow geometry on the value of the initiating accelerating voltage, which ensures the HGD back-streaming ion flux, have been studied. It is proposed to use an auxiliary pulsed surface flashover discharge to trigger the hollow cathode discharge. The two methods used jointly provide a stable discharge initiation in the entire operating pressure range of the source, including the ultimately low-pressure range where the operation of the hollow cathode discharge in its low-voltage form is still stable.

Planar hollow cathode sputtering with asymmetrical voltage supply

This article studies a planar hollow cathode discharge (HCD) with an asymmetric negative potential applied to parallel cathodes. The hollow cathode effect is preserved when an additional high voltage bias is applied to one of cathodes. For the argon pressures of 15, 20 and 60 Pa, the current-voltage characteristics of the HCD were obtained with an extra bias voltage applied on the iron cathode/target. The paper describes the sputtering system based on a HCD, consisting of two cathodes: a flat target and a tungsten mesh located at a distance of 7 mm, through which the deposited material passes. At the argon pressure of 40 Pa, the dynamics of the local plasma parameters and the iron deposition rate on a substrate under floating potential located at a distance of 40 mm from the grid were measured. The asymmetrical voltage supply in hollow cathode sputtering lead to a significant increase in the iron sputtering rate (from 2 to 16 nm/min), which is associated with both an increase in the plasma density and the sputtering coefficient of the target material. Sputtering system based on a planar HCD is advantageous due to the absence of magnetic field, which facilitates deposition of soft magnetic materials.

Peculiarities of Simulation of the Hollow-Cathode Effect in a Glow-Discharge Electrode System with an Extended Cathode Slit

Peculiarities of Simulation of the Hollow-Cathode Effect in a Glow-Discharge Electrode System with an Extended Cathode Slit

Study of structure, phase composition and mechanical properties of hardened layer of austenitic steel during local ion nitriding with hollow cathode effect

The effect of local ion nitriding in a glow discharge with a hollow cathode effect on the structure, phase composition, and microhardness of 38KhMYuA structural steel has been studied.

Recent Advances in Hollow Cathode Technology for Plasma-Enhanced ALD—Plasma Surface Modifications for Aluminum and Stainless-Steel Cathodes

Recent designs have allowed hollow cathode gas plasma sources to be adopted for use in plasma-enhanced atomic layer deposition with the benefit of lower oxygen contamination for non-oxide films (a brief review of this is provided). From a design perspective, the cathode metal is of particular interest since—for a given set of conditions—the metal work function should determine the density of electron emission that drives the hollow cathode effect. However, we found that relatively rapid surface modification of the metal cathodes in the first hour or more of operation has a stronger influence. Langmuir probe measurements and hollow cathode electrical characteristics were used to study nitrogen and oxygen plasma surface modification of aluminum and stainless-steel hollow cathodes. It was found that the nitridation and oxidation of these metal cathodes resulted in higher plasma densities, in some cases by more than an order of magnitude, and a wider range of pressure operation. Moreover, it was initially thought that the use of aluminum cathodes would not be practical for gas plasma applications, as aluminum is extremely soft and susceptible to sputtering; however, it was found that oxide and nitride modification of the surface could protect the cathodes from such problems, possibly making them viable.

Plasma-assisted rapid sintering of nanotitania powders

In this work, Plasma-Assisted Rapid Sintering (PARS), a pulsed DC plasma system in a hollow cathode regime, is presented as a novel technology to sinter nanoceramics. Nano-TiO2 powders are used as proof of concept and submitted to thermal treatment using several PARS conditions and sintering schedules. PARS heating process induced solely by the hollow cathode effect is consistent and affordable, providing a homogeneous temperature distribution to the compact. Furthermore, the heating rate and the maximum temperature are easily tunable by the discharge current applied in the plasma source and can go safely from room to maximum temperature in a matter of seconds with heating speed comparable with other reported rapid sintering techniques. Using 1-min of non-isothermal PARS cycles up to 1000 °C, porous nanostructured samples were obtained; reaching 80 % relative density while the grains remained at the nanoscale. Adding dwell times, the relative density was increased up to 96 % using a temperature plateau for up to 10 min, but the TiO2 grains grow intensively when temperatures exceeded 1000 °C. The results indicate that the developed process is a promising new alternative technology for sintering nanoceramics.

The dynamics of the formation of initial stages of a transverse nanosecond discharge with an extended slot cathode in argon

The dynamics of the main characteristics of a limited nanosecond discharge in an extended slot cathode in argon at the values of the applied voltage to the electrodes close to the values of the voltages of the formation of a volume discharge are studied by numerical simulation. It is shown that this type of discharge can be used to create an extended dense plasma column with a high density of charged and excited particles. The analysis of the spatiotemporal dynamics of development of the electron density and the electron energy distribution function was carried out. It is shown that the high-energy electrons are formed at the front of the ionization wave due to the hollow-cathode effect.

Duplex treatment of active screen plasma nitriding and amorphous hydrogenated carbon coating

Active screen plasma nitriding (ASPN) has recently emerged as an alternative nitriding technology with the potential to eliminate problems such as edge effect, arcing, and hollow cathode effects, associated with conventional direct current plasma nitriding technology. Amorphous hydrogenated carbon (a-C:H) is a thin film characterized by high hardness, a low friction coefficient, and superior wear resistance. However, the high residual stress of the a-C:H leads to poor adhesion between the a-C:H film and substrate. This study's objective was to investigate the properties of samples obtained by a duplex treatment (i.e., ASPN and a-C:H coating) of SUS304 austenitic stainless steel. The ASPN treatment was performed in 25% N2 + 75% H2 atmosphere for 5 h at 673 K under 600 Pa. An a-C:H film was coated using RF plasma-enhanced chemical vapor deposition in the presence of methane for 90 min. The results indicated that an a-C:H film with an S-phase interlayer formed after the duplex treatment of ASPN and a-C:H coating. Moreover, this treatment improved the adhesion and wear properties of the resulting sample.

Plasma nitrided compound layers in sintered parts: Microstructures and wear mechanisms

The shape and dimensions of the component play an essential role in plasma nitriding processes, a surface treatment crucial for the powder metallurgy industry since salt bath and gas nitriding poorly interact with porous structures. When the geometry of the components contains cavities, holes, or even parallel flat surfaces, the hollow cathode effect (HCE) can be activated, and, as a consequence, microstructural transformations that develop columnar nitride compound layers and the formation of a N-rich eutectoid, the braunite, may occur. The tribological properties of nitride compound layers with microstructural characteristics induced by the hollow cathode effect were evaluated in this work. Hollow cylinders of sintered iron were plasma nitrided and the tribological behaviour was assessed via reciprocating dry sliding tests. Unlike the friction coefficient, which is high (~0.4) regardless of the surface treatment, the wear rate is strongly increased by the presence of columnar morphology (2.4x) and braunite layer (2.1x), which are at the origin of brittle mechanisms such as crack propagation and interaction, resulting in the detachment of the compound layer.

Особенности инициирования эффекта полого катода в электродной системе тлеющего разряда с протяженной катодной щелью

The influence of the size of the cathode gap on the initiation of the hollow cathode effect in a glow discharge system with an extended rectangular hollow cathode is presented. It is established that the threshold current of the discharge transition to the mode with a hollow cathode is determined by the size of the cathode gap. With a decrease in the width of the gap, the threshold current increases disproportionately, with an increase in the longitudinal size of the gap, this current decreases sharply

Numerical simulation and optimization of cooling flow field of cylindrical cathode with annular magnetic field

In order to solve the problems of unstable discharge, low deposition rate and large difference in ionization rate between different targets in high power impulse magnetron sputtering, a novel cylindrical cathode with annular magnetic field based on hollow cathode effect is proposed, which can be used to produce ion beam with high ionization rate, high plasma density and no large particles. However, the traditional channel structure could not guarantee its high efficiency and uniform heat dissipation. The sealing ring may be damaged by ablation due to high power density, which restricts the further improvement of power density. Therefore, it is necessary to optimize the design of the channel structure. SolidWorks flow simulation software is used to simulate the cooling channel of plasma source. The influence of water hole structure parameters on cooling effect is analyzed, including distribution angle, hole number, diameter and inlet hole height. And the channel structure parameters are optimized. The results show that the increasing of the circumferential distribution range of the water hole is beneficial to the uniformity of heat dissipation, ensuring a large temperature difference between cooling water and copper sleeve, and strengthening heat exchange. The water inlet hole set in the upper layer of the structure is conducive to alleviating the temperature stratification phenomenon of the cooling water, so that the copper sleeve and sealing ring are in good cooling condition. Appropriately reducing the aperture is beneficial to increasing the cooling water jet velocity, enhancing the jet impact effect, and then increasing the turbulence degree, strengthening the heat transfer and improving the heat transfer efficiency. By systematically studying the influencing factors, the optimized cooling flow field structure of cylindrical cathode with an annular magnetic field is obtained. The distribution angle is 30°, the number of holes is 6, the aperture is 4 mm, and the height of water inlet hole is 36 mm. The optimized channel structure can improve the utilization rate of cooling water, obtaining better cooling effect at the same flow rate, and improving the discharge stability of the plasma source, which provides a basis for designing the cooling structure of the cylindrical cathode with annular magnetic field.

Effects of electrode gap on radio-frequency discharge characteristics with a hollow electrode

In this paper, the effects of electrode gap on the discharge characteristics in low-pressure argon radio frequency (RF) discharge with a hollow electrode were investigated by a two-dimensional particle-in-cell/MCC model. The results show that the time-averaged electric field, electron density, sheath potential drop, and sheath thickness change with the increasing electrode gap. It is also found that the electron density varies non-monotonically with the electrode gap increasing from 4 to 12 mm due to the different intensities of the hollow cathode effect (HCE) and electron heating at different electrode gaps. The axial sheath at the bottom of the hollow electrode is found to determine the depth of the plasma in the hole and can significantly modulate the intensities of the HCE and electron heating. With the reducing electrode gap, the hole of the hollow electrode is eventually filled with the sheath, and the bulk plasma region is compressed. The results indicate that a small electrode gap is not beneficial to obtain large area plasma with high density outside the hole. Altering the electrode gap can tailor the spatial distribution of the plasma in the RF discharge with a hollow electrode and obtain an optimal electron density.