Experimental study of the near-electrode plasma-tungsten cathode system in high-current atmospheric-pressure nitrogen arcs

Results of spectroscopic and pyrometric studies of the near-electrode plasma-tungsten cathode system are presented. The 2D-spectra of radiation of the multiply ionized nitrogen plasma of electric-arc discharge under atmospheric pressure are obtained at different current values. The axial electron temperature in this plasma is determined depending on the distance from the cathode tip, and the temperature distribution over the current-collecting surface of the cathode is found. The high-speed visualization of the cathode-near-cathode plasma region made it possible to observe phase transitions on the cathode surface and processes of crack formation and detachment of its fragments. The power released at the cathode is analyzed including the contribution of the arc column resonance emission, and the peculiarities of phase transitions on the cathode surface are revealed. The densities of the electron emission current at the cathode are estimated theoretically and compared with the data obtained in the experiment. Possible causes of the difference between the theoretical values of the emission current density and those found experimentally are discussed.     

Ion sputtering of cathode surface in a hollow cathode discharge

In this paper, ion sputtering of cathode material in a specific type of glow discharge—hollow cathode discharge (HCD)—is analyzed. To estimate both real sputtering yield and screening effect of the buffer gas, two different methods—combination of experimental and analytical approach (applicable for Ar buffer gas only) and use of Monte Carlo simulations—are used. The latter, which is introduced for the first time here, can be used for any buffer gas. Real sputtering yield S_k is estimated by Monte Carlo simulations for several commercial HCD lamps with Ne buffer gas: Ne-Li (0.046), Ne-As (0.862), Ne-Ca (0.337) and Ne-Cd (1.069).     

Formation of thick titanium carbide films by the hollow cathode discharge reactive deposition process

Thick titanium carbide films were successfully deposited on mild steel sheets using a hollow cathode discharge reactive deposition process. Methane, ethylene and acetylene were used as reactant gases during the titanium evaporation and the substrates were either negatively biased or grounded. The deposited films were characterized by microhardness measurements, X-ray diffraction, Auger electron spectroscopy and secondary ion mass spectrometry. The results are discussed with regard to deposition conditions and chemical composition.     

Generation of a homogeneous plasma in a glow discharge with a hollow anode and a wide-aperture hollow cathode

The principles for designing electrode systems for sources of ion beams of large cross section on the basis of a glow discharge are considered, and a system with combined magnetic and electrostatic confinement of the fast electrons in a wide-aperture hollow cathode and the generation of an ion-emitting plasma in the anode cavity is proposed. It is shown that the system investigated generates a plasma with a nearly homogeneous distribution of the ion emission current density at low gas pressures and can be effectively used to obtain ion beams over a broad range of energies. Pis’ma Zh. Tekh. Fiz. 25, 83–88 (June 26, 1999)     

Axial evolution of the negative glow in a hollow cathode discharge.

The purpose of these studies was to examine the axial evolution of the negative glow in a hollow cathode discharge. The time-average negative glow profiles along the central axis of a hollow cathode were recorded for 81 pulse widths over the range of 5-25 microseconds in increments of 0.25 microseconds at a constant interpulse delay of 206.4 microseconds. Subsequent numerical processing yielded the instantaneous negative glow profile. The negative glow was viewed through a mesh-covered slot along the length of the hollow cathode. The negative flow profiles were imaged using a vidicon video camera. A PC-based frame grabber digitized the video images and stored them for subsequent processing.     

Specific features of wide-aperture hollow cathode discharge in helium

The characteristics of wide-aperture discharge with a hollow cathode in helium have been studied under the conditions of predominating electron generation via photoemission from the cold cathode. In this regime, currents exceeding by an order of magnitude those for the conventional abnormal discharge have been obtained. The current-voltage characteristics are independent of the helium pressures in a range of 5–15 Torr, which is a distinctive feature of the photoemission dominated discharge with electron beam in the system under consideration.     

Effect of aging the hollow cathode by sputtering on the analytical precision of the hollow cathode discharge emission source.

Sputtering changes the analytical performance of the hollow cathode as an emission source. Proper conditioning (aging) of the hollow with sputtering greatly improves precision. Conditioning decreases the blank emission signal and changes the shape of the hollow. Micrographs which show the evolution of the bottom of the hollow into a stable bulb shape are presented. The blank emission signal decreases with aging of the hollow and approaches a constant value when the hollow is properly conditioned. A short-term stability (n = 5) of approximately 1.8% RSD has been observed frequently. An average long-term stability over several days (n = 45) of 4.5% for Li and 3.5% for Na in the emission signal from microsamples (less than 50 nL) deposited in the hollow cathode discharge source is reported here. Instrumentation, operation, and sample preparation procedures are described.     

Generation of nitrogen beams with very high N/N2 ratio using hollow cathode discharge

Nitrogen beams with very high N⁺/N₂⁺ ratio were generated using hollow-cathode discharge. The dependence of N⁺/N₂⁺ ratio in the extracted beams on the discharge parameters was studied by mass spectroscopy. Very high N⁺ fractions (up to 90% and the maximum is about 98%) can be easily achieved without mass separation under typical operating conditions. The extracted beams with discharges using mixture precursors, such as N₂/NH₃, or N₂/Ar were also investigated.     

An analog of the hollow cathode effect in low-pressure high-frequency capacitive discharge

We have studied the process of ion flux formation in a low-pressure high-frequency capacitive discharge in the presence of a cavity on the electrode surface. If the cavity size is on the order of an electrode space charge sheath thickness, the spectrum of ions arriving at the cavity bottom surface contains a low-energy peak. The mechanism responsible for this peak is analogous to the hollow cathode effect in a dc discharge.     

Increased analytical precision in the hollow cathode discharge emission source by improved discharge current control.

A significant improvement in the precision of the hollow cathode as an emission source is reported. Precision of 1% or less has been observed several times over periods of several hours. An average long-term stability of 4.3% for Ll and 6.0% for Na in the emission signal from microsamples (less than 50 nL) deposited in the hollow cathode discharge source is reported. The improved precision is attributed primarily to the introduction of electronics that hold the discharge current more nearly constant and to the shielding of all wiring to the source from the power supply. A current-controlled switch that is capable of driving a hollow cathode discharge in either dc or pulsed mode is described. This switch is capable of generating current pulses as short as 2 microseconds through a resistive load and greatly improves the discharge stability and repeatability at turn-on in both the dc and pulsed modes. The characteristics of pulses produced by this switch are presented; however, analytical performance is reported only for the dc mode. Temporal current plots are presented for the new instrumentation and compared to plots taken with commonly used current-controlled power supplies. Instrumentation, operation, and sample preparation procedures are described. Typical temporal profiles of the emission signal from microsamples deposited in AI and stainless steel hollow cathodes are given.     

A compact ion source using a hollow cathode discharge and its application to thin film formation

A compact and simple ion beam source was developed and its application to ion beam sputter deposition of thin films was studied. The ion source consisted of a hollow cathode, an anode and an extraction electrode. Two acceleration electrodes were mounted at the edge of the cathode. Since the gas pressure in the cathode should be kept at a relatively high value (more than 0.05 Torr) to maintain the discharge, the ion source employed a differential pumping system. The fundamental discharge characteristics of the ion source were studied using argon or helium as the working gas.The ion source was applied to the ion beam sputter deposition of copper and ruthenium oxide films. The crystallinity of the copper films increased with an increase in the acceleration voltage, while all the ruthenium oxide films appeared to be amorphous or of very small grain size, regardless of the acceleration voltage. The effect of the acceleration voltage on the film resistivity was also studied.     

Visualization of emitting sites on carbon fiber cathode by surface treatment in high-current vacuum discharge process

In this paper, a carbon fiber cathode, having robust, easily shaped, and epoxy-free properties, is constructed by squeeze casting technique that can overcome some disadvantages of conventional methods. Carbon fiber emitters on the cathode surface had a high distribution density, thus ensuring sufficient emission centers or emission uniformity. The fabricated cathode was tested in a diode powered by a 350 kV, 40 Ω, 400 ns high-voltage pulse generator. The turn-on electric field was estimated to be 50 kV/cm, and the field enhancement factor was (1.2–2.0) × 10³. It was found that the electron emission of carbon fiber cathode is initiated from the individual bright spots at a current density of up to 400 A/cm². Most notably, the X-ray images of electron beam on anode foil demonstrate the development of bright spots on the cathode surface. As a whole, this class of cathodes can endure high-current pulsed emission, and has a positive application prospect.     

Field electron emission from cathode as a possible factor in the transition from a streamer to spark discharge channel

A possible scenario of the transition from a streamer to spark discharge channel during the breakdown of narrow discharge gaps between metal electrodes is considered. It is suggested that the discharge gap bridging by a streamer may create an electric field at the cathode, which may be strong enough to induce field electron emissions that, in turn, will cause the development of processes that lead to the transition from a streamer to spark discharge channel.     

Minimum voltage principle in the theory of a high-current vacuum-arc discharge

The effect of an inhomogeneous magnetic field with axial and radial components on the structure of a high-current vacuum-arc discharge (VAD) has been theoretically studied. The characteristic features of current passage in a short VAD are considered using analytical expressions for the slope of the total current lines (TCLs) relative to the discharge axis, the axial components of the electric and magnetic fields, and the effective conductivity of discharge plasma. A two-dimensional mathematical model has been used to calculate the TCL shapes and the discharge voltage for various dimensions of the region of cathode spots. Calculations showed that the voltage drop on the discharge gap as a function of the arc discharge radius on the cathode has a minimum. The shape of TCLs and the arc radius on the cathode at this minimum agree with the available experimental data.     

Peculiarities of volt-ampere characteristics of a tungsten cathode in a high-current arc

A model making it possible to calculate the emission currents in a cathode and its volt-ampere characteristic (VAC) taking the phenomena in the near-cathode plasma layer into account is proposed. The effect of plasma electrons and ions, thermal field-emission processes, and radiation from the plasma and evaporation of the cathode material are taken into account. The fields of the electrical potential and temperature in the cathode body, with the Joule heat release taken into account, are calculated. The numerical simulation yielded the conditions causing the volt-ampere characteristic ambiguity in the near-cathode layer in the absence of a liquid cathode spot. The obtained characteristics make it possible to estimate the lifetime and efficiency of a plasmotron with a tungsten cathode and argon serving as an actuating medium used in a number of plasma technologies.     

The preparation of permalloy 80/20 thin films using a pulsed DC discharge in a hollow cathode

Permalloy thin films have many applications as sensors and actuators but the preparation of magnetic films by magnetron sputtering is problematic since the target material reduces or changes the magnetic configuration of the magnetron. Hollow cathode discharges can produce similar or greater plasma densities to that found in magnetron sputtering and can therefore be operated over a similar pressure range. Pulsed DC sputtering has been seen to have some advantages compared to DC or RF sputtering. In this paper we report the use of a combination of pulsed DC sputtering with a hollow cathode system to prepare thin films of Permalloy. The deposition rate was found to strongly depend on the gas flow used to prepare the thin films. Combinations of the experimental conditions were found to produce films with a (111) preferential crystal orientation and that the grain size of the crystals was mainly determined by the deposition rate. Furthermore, changes in the degree of ion bombardment did not appear to have any significant affect on the structure of the deposit.     

Critical flow of high-speed ions in a high-current vacuum-arc discharge

A physical model of vacuum arc is developed, which is used to construct a quasi-one-dimensional model and a two-dimensional two-fluid mathematical model. The two-dimensional magnetohydrodynamic model is based on the method of trajectories, according to which a set of partial equations is reduced to a set of ordinary differential equations written for derivatives along the lines of current. The quasi-one-dimensional model is based on the principle of compensation of radial magnetic forces, which is valid for a short high-current vacuum arc in axial magnetic field. A criterion is obtained, which defines the range of validity of the quasi-one-dimensional model in external axial magnetic field B ₀. Calculations are performed of the dependence of critical current on B ₀ for discharge gaps of different geometries. The calculation results agree with experiment.     

Experimental study of the converging density wave excitation in a cylindrical anode of a high-current diode

Density waves excited in a cylindrical anode irradiated by a short pulsed high-current electron beam in a relativistic vacuum diode have been experimentally studied. The electron trajectories are almost perpendicular to the side surface of a cylindrical anode coaxial with the cathode. As a result of the discharge, the surface layer of the continuous anode exhibits no visible changes, whereas an inner region of the anode is evaporated. The mechanism of the isentropic wave excitation is elucidated by experiments with a coaxial anode comprising a tube tightly fit to the central rod. According to the most probable explanation, the electron beam induces collapse in the subsurface layer of the anode.     

Periodic plasma structures in nanosecond discharge with a slit cathode

The spatial structure of optical emission from a transverse nanosecond pulsed electric discharge with a hollow (slit) cathode has been experimentally studied. A regular periodic plasma structure has been observed during nanosecond pulsed discharge in helium at medium pressures. Conditions of the plasma structure formation with respect to the discharge voltage and current are determined. It is shown that the observed plasma structure differs from the known types of striations.