Cathode Cavity Research Articles

Low-energy, high-current, ion source with cold electron emitter

An ion source based on a two-stage discharge with electron injection from a cold emitter is presented. The first stage is the emitter itself, and the second stage provides acceleration of injected electrons for gas ionization and formation of ion flow (<20 eV, 5 A dc). The ion accelerating system is gridless; acceleration is accomplished by an electric field in the discharge plasma within an axially symmetric, diverging, magnetic field. The hollow cathode electron emitter utilizes an arc discharge with cathode spots hidden inside the cathode cavity. Selection of the appropriate emitter material provides a very low erosion rate and long lifetime.

Energy Fluctuations in a Direct Current Plasma Torch with Inter-Electrode Inserts Operated at Reduced Pressure

Direct current (dc) plasma torch with inter-electrode inserts has the merits of fixed arc length, relative high enthalpy and may show advantages in future plasma processes where stability and controllability are must-have. Energy fluctuations in the plasma may result from power supply ripple, arc length variation, and/or acoustic oscillation. Using an improved power supply with a flat waveform, the characteristics of an argon plasma energy instabilities under reduced pressure were studied by means of simultaneously monitoring the arc voltage and arc current spectrum. Dependence of the arc fluctuation behavior on the plasma generating parameters, such as the current intensity, the plasma gas flow rates and the vacuum chamber pressure were investigated and discussed. Results show that the plasma torch has a typical U-shaped voltage-ampere characteristic (VAC). The correlation between the VAC and the probability of energy distributions was studied. Through pressure measurements at the cathode cavity and the vacuum chamber, the existence of sonic flow in the inter-electrode insert channel was confirmed.

Study of the rectangular hollow cathode discharge

The rectangular hollow cathode discharge is studied with respect to the applicability of similarity mechanisms analogously to other gas discharge types. Probe characteristics of the discharge are measured, the conditions of steady and continuous discharge glow in the cathode cavity are determined.

Improvement of Plasma Spray Torch Stability by Controlling Pressure and Voltage Dynamic Coupling

The development of coating formation processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coating properties. This is particularly important when considering suspension plasma spraying or solution precursor plasma spraying. Submicron particles closely follow plasma instabilities and have nonhomogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in direct-current (dc) plasma torches, showing dominant fluctuation frequencies between 4 and 6 kHz, are linked to pressure oscillations in the cathode cavity of the plasma torch. In this study, first, a method to isolate the different oscillation modes in arc voltage and pressure signals using signal processing methods is presented. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters. Lastly, it is shown that the use of an acoustic stub, mounted on the torch body, decreases the amplitude of arc voltage fluctuations and slightly increases the mean voltage.

A source of gas, vapor, metal, and carbon ions based on a low-pressure hollow-cathode discharge

A compact source of gas, vapor, metal, and carbon ions based on a cold-hollow-cathode reflective discharge has been developed, in which a 6-mm-diameter flat target (Cu, Mo, W, C) is installed on the bottom of the cold cathode insulated from it. The density of the ion flow from cathode plasma reaches 100 mA/cm2 at an accelerating voltage of up to 10 kV and a discharge current of 0.2-0.5 A. Vapors produced during ion sputtering of the target are ionized in the cathode and anode cavities. A beam containing ions of the plasma-producing gas and vapor is extracted throug h the channel in the reflector cathode. A fraction of the vapor of the sputtered target, the flow of which is sufficient for growing layers at a rate of ∼0.03 nm/s at a distance of 10 cm from the emission channel under the action of an ion beam, is extracted together with ions. The fraction of metal ions in the extracted beam is 0.05-0.10. The total current of the ion beam is 20-30 mA.

Acoustic stabilization of electric arc instabilities in nontransferred plasma torches

Electric arc instabilities in dc plasma torches lead to nonhomogeneous treatments of nanosized solid particles or liquids injected within thermal plasma jets. This paper shows that an additional acoustic resonator mounted on the cathode cavity allows reaching a significant damping of these instabilities, particularly the Helmholtz mode of arc oscillations. The acoustic resonator is coupled with the Helmholtz resonator of the plasma torch limiting the amplitude of arc voltage variations. It is also highlighted that this damping is dependent on friction effects in the acoustic resonator.

Broad-beam high-current dc ion source based on a two-stage glow discharge plasma

We have designed, made, and demonstrated a broad-beam, dc, ion source based on a two-stage, hollow-cathode, and glow discharges plasma. The first-stage discharge (auxiliary discharge) produces electrons that are injected into the cathode cavity of a second-stage discharge (main discharge). The electron injection causes a decrease in the required operating pressure of the main discharge down to 0.05 mTorr and a decrease in required operating voltage down to about 50 V. The decrease in operating voltage of the main discharge leads to a decrease in the fraction of impurity ions in the ion beam extracted from the main gas discharge plasma to less than 0.2%. Another feature of the source is a single-grid accelerating system in which the ion accelerating voltage is applied between the plasma itself and the grid electrode. The source has produced steady-state Ar, O, and N ion beams of about 14 cm diameter and current of more than 2 A at an accelerating voltage of up to 2 kV.

Influence of the Injection Speeds on the Discharge Characteristics and Minimization of Delay Time in a Pseudospark Discharge

The study put forth demonstrates that the seed electrons average kinetic energy influences the discharge characteristics making it possible to maximize the rate of development of the virtual anode in a pseudospark, with a suitable choice of the neutral gas pressure as determined by the seed's average injection speed. This investigation also brings to light two distinct operating regimes; (1) mid-energy, where electron-impact ionization energy losses result in a decrease in the cross-section as the electrons travel downstream and (2) high-energy, where, in contrast, the ionization cross-section increases. In the latter case, both the fastest delay time and the neutral gas pressure producing this value have linear dependencies on the seed electrons energy resulting in a constant value of their product over the different injection speeds. The discharge is seeded by injecting a current pulse for a period of one nanosecond along the axis from the hollow cathode cavity back wall over a range of mean speeds corresponding to 100 to 900 V accelerations; the initial electric field is insufficient to enhance ionization throughout most of the hollow cathode backspace. Data is obtained through computer simulation using the two-dimensional kinetic plasma code OOPIC Pro. 51.50.+v, 52.75.Kq, 52.80.Tn.

Modes of generation of the optical properties of a moderate-pressure nanosecond discharge with a slit cathode in argon

The spatial distribution of the optical radiation of the plasma in a discharge gap and inside the cathode cavity was experimentally investigated for a nanosecond transverse electrical discharge with a slit cathode in argon at moderate pressures. For the conditions investigated, estimates of the factor γc have been obtained and the free paths of accelerated electrons in the plasma have been determined. It has been revealed that the structure of the optical radiation of a nanosecond discharge with a slit cathode is related to the features of the fast electron energy relaxation.

Modernization of the Element 2 mass spectrometer

To perform direct elemental analysis of solids, it is proposed to complement an Element 2 ICP mass spectrometer commercially produced by Thermo Electron Corp. with a glow-discharge ion source based on a hollow cathode. The analyzed sample, in the form of a rod 1.0–2.5 mm in diameter and 15–20 mm in length, is set along the axis of the cathode cavity with an inner diameter of 15–16 mm and a depth of 15 mm. The cathode is placed in a discharge chamber, which, using a viton seal, is substituted for the ICP-source sampler. The use of a plasma mirror and getter evacuation of the source chamber allows a decrease in the source’s hydrocarbon background by a factor of 103–104. The ion source is evacuated by a mechanical pump of the mass spectrometer and an additional turbomolecular pump. Ion sources in a mass spectrometer are replaced (a change from one analytical method to another) within 5 min. The ion current extracted from the IS allows analysis of conducting solids with a sensitivity at a level of several ppb (10−7%) at a resolution of the mass spectrometer of 4000. Combining two easily replaceable ICP and GD ion sources in a single high-resolution analyzer significantly extends the analytical capabilities of the Element 2 mass spectrometer.

Inhomogeneous extended hollow cathode discharge for raising the current density in a forevacuum plasma source of a ribbon electron beam

The plasma parameters and the emissivity of a ribbon electron beam source based on a discharge with an inhomogeneous extended hollow cathode are measured. A constriction in the cathode cavity increases the plasma density near the emitting area boundary, which adds to the electron current density in the beam. The reason for the above effect is the formation of the plasma density distribution nonuniform across the cavity with a maximum in the middle. This maximum is caused primarily by a plasma electron flow from the constriction, which is generated by the electric field and is directed toward a slit emission-extracting aperture.

Hollow cathode arc discharge as an effective energy source for welding processes in vacuum

This paper presents the results of an investigation of thermal and physical properties of the hollow cathode arc discharge (HCAD) with respect to its application for welding processes in vacuum. The following main parameters of the arc discharge were studied: the external voltage–current (V–I) characteristics; plasma parameters inside the cathode cavity and in the arc external column and the radial heat flux density distribution into the anode. Langmuir electrical probes have been utilized to investigate plasma parameters. Electron energy distribution function was determined from the probe V–I characteristics by the computation of an inverse ill-posed problem. It was shown that, depending on welding parameters, HCAD can exist in two different forms: diffusive or constricted. At currents below 60 A, HCAD has the diffusive form, and with the increase in the current it changes to the constricted form. The discharge constriction phenomenon, we believe, could be explained by the appearance in the external plasma of high velocity electrons with energies from 12 to 22 eV. Parameters of the heat flux into the anode were investigated with spot and split-anode calorimeters. The heat flux density on the anode of the diffusive form of the discharge has a Gaussian distribution. The heat flux of the constricted form is significantly different from the diffusive one and can be approximated by the sum of two combined normal-circular heat sources with different power concentration coefficients. It was also found that the efficiency parameter of the discharge energy transfer to the anode can reach 0.7–0.86 of the discharge voltage, which confirmed that HCAD is a highly effective energy source for welding processes in vacuum. Examples of industrial applications of HCAD for welding, brazing and alloying in vacuum are presented.

The formation of accelerated electrons and their effect on the structure of nanosecond discharge with slot cathode

Experimental investigations are performed of the current-voltage characteristics and spatial distribution of optical radiation of plasma within the cathode cavity and in the discharge gap under conditions of transverse nanosecond electric discharge with a slot cathode. The investigations are performed in helium in the gas pressure range from 1 to 100 torr. It is demonstrated that the cathode layer is formed at the initial stage of discharge and that the electrons accelerated in the narrow cathode region pick up energy of the order of 1 keV. The efficiency of electron beam generation is estimated, and the mechanisms of generation of fast electrons are identified. The free paths of accelerated electrons in plasma under different initial conditions are determined, and the mechanisms of relaxation of energy of this group of electrons are analyzed. The correlation is established between the special features of relaxation of energy of fast electrons and the formation of the structure of beam-plasma discharge with a slot cathode.

Characteristics of a reflex discharge with a box-shaped cathode cavity and emission properties of the discharge plasma

A new geometrical relationship between the shape and dimensions of the cathode cavity of a reflex discharge with a cold hollow cathode is determined. The discharge characteristics and the emission properties of the discharge plasma are investigated experimentally. The conditions are determined for generating a cathode plasma that is highly inhomogeneous in the radial direction and the possibility of creating such a plasma is demonstrated. The existence of a stationary double electric layer with a potential drop of 14–16 V in the aperture of the cathode cavity is revealed. Specific features of the longitudinal extraction of plasma ions through the wall layer of the reflex and hollow cathodes are considered.

Mechanism for efficient electron beam generation in a pixel of an open-discharge plasma display

A concept is proposed of a plasma pixel based on an open-discharge microstructure. The concept employs the capability of an open discharge to generate an electron beam at moderate (1–3 kV) discharge voltages with an efficiency close to 100%. To determine the possible application of this type of discharge, the parameters of the electron beams generated in open discharges operating in different working gases at various geometries of the discharge cell and various dimensions of the discharge channel were investigated. The electric potential distributions in the dielectric plate channel and in the cathode cavity were measured. The effect of additional illumination by radiation generated in the drift space on the current-voltage characteristic of the discharge is studied. Based on the results obtained, a noncontradictory model of a discharge capable of very efficiently generating an electron beam is proposed. According to this model, the main contribution to the electron beam comes from the photoelectron emission from the cathode under the action of radiation from the working-gas atoms excited by fast heavy particles in a highly nonuniform electric field in the cathode cavity. Such a field also scatters ions and fast atoms, thus reducing their fluxes toward the cathode. The results obtained indicate that highly efficient light sources and plasma panels can be created on the basis of open-discharge microstructures with a cathode cavity. Such microstructures allow very efficient conversion of electric energy into light.

Hollow cathode magnetron deposition of AlN thin films: crystalline structure and morphology

A new dc hollow cathode plasma source has been assembled whith a conventional planar magnetron cathode used together with another plane cathode plate to form a hollow cathode cavity. The system comprises two cathode plates of aluminium separated by a distance d, one of them acting as target of the magnetron cathode, the other being an ordinary plate. The discharge anode is a metallic flange of the vacuum chamber. This leads to enhanced ionization in the cathode cavity region and enables the discharge to operate at significantly lower pressures than for a typical planar magnetron configuration. As a consequence, sputtered atoms can reach a substrate with minimum energy loss due to collisions with filling gas atoms. The discharge gas was a mixture of argon and nitrogen. AlN thin films were grown on silicon substrates, at ambient temperature, and characterized with respect to the structure and morphology by XRD and AFM analyses respectively. The structure and roughness of the AlN films were studied as a function of the deposition parameters.

Plasma Cathode for a Broad-Beam Electron Accelerator

We propose a grid-stabilized plasma cathode based on a slit-contracted low-pressure glow discharge with hollow anode. The area of the plasma cathode is one order of magnitude higher than that in systems where electrons are extracted immediately from plasma in the cathode cavity. Conditions for the discharge initiation, the current switching to the hollow anode, and the obtaining of uniform emission from the plasma cathode are determined. At an accelerating voltage of 160 kV, an electron beam with a 1000 × 180 mm cross section, a total current of several amperes, and a current pulse duration of up to 10−3 s was obtained. The plasma cathode operates under technical vacuum conditions (air, 0.1 Pa) and ensures high stability and reproducibility of the beam current pulses.

Microdischarge devices with 10 or 30μm square silicon cathode cavities: pd scaling and production of the XeO excimer

Silicon microplasma devices with square trench cathode cavities having cross sections of (10μm)2 or (30μm)2 and a depth of 200μm have been fabricated and operated successfully in the rare gases and Xe∕O2 mixtures at pressures (300K) up to 1100Torr. The (10μm)2 structures exhibit electrical characteristics that contrast with the behavior of larger devices and may indicate the onset of the breakdown of pd scaling. Also, a distinct minimum in the ignition voltage of 10μm square devices is observed for pd≃0.9Torrcm (p and d are the Ne gas pressure and microcavity cross-sectional dimension, respectively). Strong emission on the 2∑+1→1∑+1 transition of XeO in the green (∼510–560nm) is observed in mixtures of Xe(300–700Torr) and O2(1–10mTorr).

Advanced structure of cathode for gas discharge lamp of super high pressure

This paper presents a new cathode structure for short length arc-plasma circular lamp DRKs2-1500 filled with mercury–xenon of super high pressure. A conventional cathode of thoriated tungsten was replaced by a new structure of tungsten–barium. This cathode has emitting surface of cavity form which enables to reduce active material evaporation from the surface and to increase emission stability. It was found that light parameters of the lamp depend on the geometry of its electrodes. Fusing and evaporation of the cathode material result in a cathode size change and lead to changes of arc-plasma parameters, blackening of the lamp envelope and decreasing luminous flux intensity. The tests showed that the lamp service life with the new cathode grows as much as 2–3 times. Optimization of the cathode cavity geometric size is expected to enlarge the lamp service life up to 3000–5000 h (guaranteed service life of a lamp with usual structure of cathode is 500 h).

Effect of the cavity structure on the discharge features of pseudospark switches

The cavity structure of the anode and the cathode of a pseudospark switch is an important factor for its performances. When the cathode has a cavity structure, the electric erosion on the cathode and anode is small and a concentric ring-shaped erosion pattern is observed at the central area around the apertures of both electrodes. Without the cathode cavity, the erosion becomes stronger, and the patterns become irregular and unstable. Several kinds of electrode structure have been designed and investigated. Among them, a unique cavity structure of intermediate floating electrode in pseudospark switch has a good performance. This configuration could reduce the erosion on the electrodes and increase the selfbreakdown voltage of the pseudospark switches up to 80 kV. In this paper, we have studied the influence of the electrode structure on the erosion of the electrode and the selfbreakdown characteristic of the switches.