High-current Vacuum Arc Discharge Research Articles

Thermal vacuum evaporation peculiarities of perovskite-type barium-strontium hafnate

Experimental results are presented on the thermal vacuum evaporation of barium hafnate BaHfO3 and double barium-strontium hafnate Ba0.5Sr0.5HfO3, which are included together with finely dispersed tungsten as components in sintered composites. It is found that at T = 1900 K only barium and barium oxide are the products of BaHfO3 evaporation, i.e., congruent evaporation of BaHfO3 does not occur. Investigations of Ba0.5Sr0.5HfO3 evaporation at T = 1600 K showed that it also does not evaporate congruently. At the same time, the nucleation of a new phase of BaO·SrO in the form of structures with D6 symmetry was detected. Using a high-current vacuum arc discharge, congruent evaporation of Ba0.5Sr0.5HfO3 takes place, forming various crystalline formations of double barium-strontium hafnate on the substrate.

Скоростная видеоспектроскопия сильноточного вакуумно-дугового разряда с медь-хромовыми электродами

The processes of channel contraction of a high-current vacuum-arc discharge and plasma formation in the anode region are of high actuality in terms of improving the switching characteristics of vacuum circuit breakers. In present paper the spatial-spectral structure of the discharge and its dynamics were studied with the videospectroscopic apparatus complex. The obtained data clearly show changes in the charge composition of the anode and cathode macrospots plasma in the process of changing discharge forms.

Measurement of the expansion velocity of the plasma high-current vacuum arc discharge

In this work, we present experimental results on measuring the velocity of vacuum arc discharge plasma expansion. In the experiments, two designs of plasma guns were used. In the first version, the end of the arc discharge cathode was located below the plane of the anode, and the surface of the insulator separating them was parallel to the axis of symmetry of the plasma gun. In this design, the arc discharge plasma escapes the anode through a hole, the diameter of which coincides with the diameter of the cathode. In the second variant, the plane of the end face of the arc discharge cathode coincided with the plane of the anode, and the surface of the insulator separating them was located perpendicular to the axis of symmetry of the plasma gun. To obtain an image of plasma in the optical range, an FER-7 optical streak camera was used. Based on the results obtained, it can be concluded that the expansion velocity of the plasma of a high-current vacuum arc discharge does not depend on the design of the guns considered in this experiment.

Determination of the voltage drop on a high-current vacuum arc discharge under conditions of a limited cross-section of the plasma flow

The work is devoted to the study of the high-current vacuum arc discharge characteristics under conditions of a limited cross-section of the plasma flow. The experiments were carried out on the IMRI-5 setup with a sinusoidal arc current amplitude of 300–350 kA and a rise time of 500 ns. Aluminum rods with diameters from 3 to 7 mm were used as a cathode. The plasma flow was formed in a channel whose diameter was equal to that of the cathode. The features of the formation of a plasma jet with various configurations of the used plasma gun are described. The electrophysical parameters of the arc discharge are presented. Theoretical estimates of the voltage drop across the high-current arc during the outflow of a plasma flow through holes with a limited diameter are provided.

Investigation of the aluminum electrodes erosion of a plasma gun during the operation of a high-current vacuum arc discharge

The aim of this work was to obtain magnitude quantitative estimates of the “closed-type” plasma gun aluminum electrodes erosion that occurs during the course of a high-current vacuum arc discharge. The experimental setup consisted of two current generators. The first generator capable of generating a current with an amplitude of up to 450 kA and a rise time of 500 ns was used as a current source for a plasma gun. The second one was used as an X-ray radiograph to visualize the object under study in the soft X-ray range (hv ≈ 0.5–3 keV). Quantitative distributions of the plasma linear mass are obtained both along the radius and along the length of the jet at different times. It was shown that the erosion properties of the electrode material are related to the current characteristics of the arc discharge current.

Исследование переходных процессов анодного пятна сильноточной вакуумной дуги методом скоростной спектроскопии и видеорегистрации

The processes of plasma formation in the anode region of a high-current vacuum arc discharge are of high actuality in terms of improving the switching characteristics of vacuum circuit breaker. In present paper the dynamics of the radiation spectrum of anode region was recorded with the system that allows time scanning of a spectrum. In parallel, there was high-speed videorecording. The obtained data clearly show changes in the charge composition of the anode spot plasma in the process of changing discharge forms.

The Concept of a Hybrid Pulsed Plasma Thruster for Small-Size Space Satellites

A concept of a hybrid plasma source to be used as a micro satellite thruster is proposed, where the propellant is a liquid gallium–indium alloy. The plasma source operates in two modes: 1) generation of laser ablation plasma and 2) generation of plasma in a laser-triggered, low-voltage vacuum-arc discharge. The cathode in the discharge cell of the source represents a stainless-steel capillary filled with the working liquid. The laser beam target is the working liquid meniscus. Without any voltage in the discharge gap, only laser-ablated plasma is generated. Upon application of even a relatively low voltage (<1 kV), the ablation initiates a high-current vacuum-arc discharge. In the experiments, the discharge current and voltage waveforms were recorded and so were the images of plasma glow obtained with a HSFC-Pro 4-channel high-speed camera. Regularities of current flow in the high-current vacuum-arc discharge, initiated by the cathode laser ablation, are investigated.

Radiographic research of the Bi plasma jet formed by the vacuum arc discharge

The results of experiments on a soft x-ray radiography (≈ 1-2 keV) of a bismuth plasma formed by the high-current vacuum arc discharge are represented. The plasma gun with the arc current ≈ 60 kA and the current rise time ≈ 7 μs was used to produce the Bi plasma jet. The compact pulsed radiograph XPG-1 (250 kA, 220 ns) with an X-pinch load consisting of four Mo wires with a diameter 25 μm was used as a source of the soft X-ray radiation. The X-ray backlighting images of the researched plasma jet and the Bi step-wedge with a step thickness of ≈ 100 nm were recorded simultaneously in the course of the experiment. A comparison of the plasma jet x-ray image with the current trace has enabled to estimate dependencies of the linear mass on the arc current. The experiments have shown that when the arc current density reaches ≈ 3·105 A/cm2, the evaporation rate of the electrode material reaches ≈ 100 μg/μs, that under the plasma velocity ≈ 0.5 cm/μs, provides a plasma jet linear mass ≈ 200 μg/cm. At a distance of ≈ 1-2 mm from the arc cathode surface, the sharp increase of the jet linear mass (up to ≈ 500 μg/cm) occurred.

Surface modification of compressor steels using thermally assisted ionic diffusion in the titanium plasma of a vacuum arc discharge

The thermally stimulated ionic diffusion (ionic modification) of titanium ions at the surfaces of EP866 and EI961 compressor steels is considered in the plasma of the high-current vacuum-arc discharge (VAD) in an ion-plasma MAP-3 plant. The dependences of the sample temperature in the sputtering chamber of the ion-plasma MAP-3 plant and the rate of specific change of the sample mass on the bias voltage at a VAD current of 300 A are obtained. The elemental composition of the surface layers of the samples subjected to ion treatment is studied. It is shown that, at a VAD current of 300 A and a bias voltage up to 400 V, the compressor steel sample temperature does not exceed 440°C and the inversion voltage, which determines the transition from coating condensation to ion etching of a substrate, is ~360 V for EP866 steel and ~390 V for EI961 steel. The corrosion resistance of the compressor steels modified at a VAD current of 300 A is investigated.

Numerical simulation of the anode region of a high-current vacuum-arc discharge and the Bohm criterion

The anode region of the vacuum-arc discharge in an external magnetic field is studied with allowance for the dependence of the negative anode drop on the ratio of the directed velocity of electrons v0 to their thermal velocity vT. The Poisson equation is solved in a space charge layer near the anode at various values of the electric field E(0), ion velocity vi(0), and parameter v0/vT at the layer-plasma boundary. It is shown that there exists a minimum velocity vi* (0)of ions incoming to the anode layer at which the electric field vanishes at a single point inside the layer. The velocity vi* (0) determines the boundary of the existence of the stationary anode layer and depends on the ratio v0/vT. As v0/vT → 0, the value vi* (0) asymptotically tends to the ionsound speed, which agrees with the well-known Bohm criterion. The velocity vi* (0)increases with an increase in v0/vT. For vt (0) < vi*(0), there are no stationary solutions in the anode layer. The domain of existence of stationary solutions in the anode layer is determined for different values of the parameters E(0) and v0/vT.

New process of ion surface modification of compressor steel in the vacuum arc plasma of titanium

A new process of ion modification of the surfaces of EI961 and EP866 compressor steels by titanium in the plasma of high-current vacuum arc discharge in an ion-plasma MAP-3 device is studied and compared to the thermodiffusion solid-phase saturation of these steels by titanium under temperature-time conditions that are identical to the ion modification conditions. The phase and elemental compositions of the surfaces of the samples modified in titanium plasma at various bias voltages and the samples with a titanium coating after vacuum thermodiffusion annealing are analyzed. The phase state of the surfaces of the EI961 and EP866 steel samples is shown to begin to change during ion treatment in titanium plasma at a bias voltage of 150 V and an ion heating temperature of 470–480°C. No changes are detected in the phase state of the surfaces of the samples coated with titanium after vacuum diffusion annealing at a temperature of 1050°C.

Generalized Bohm’s criterion and negative anode voltage fall in electric discharges

The value of the voltage fall across the anode sheath is found as a function of the current density. Analytic solutions are obtained in a wide range of the ratio of the directed velocity of plasma electrons v 0 to their thermal velocity v T . It is shown that the voltage fall in a one-dimensional collisionless anode sheath is always negative. At the small values of v 0/v T , the obtained expression asymptotically transforms into the Langmuir formula. Generalized Bohm’s criterion for an electric discharge with allowance for the space charge density ρ(0), electric field E(0), ion velocity v i (0), and ratio v 0/v T at the plasma-sheath interface is formulated. It is shown that the minimum value of the ion velocity v * (0) corresponds to the vanishing of the electric field at one point inside the sheath. The dependence of v * (0) on ρ(0), E(0), and v 0/v T determines the boundary of the existence domain of stationary solutions in the sheath. Using this criterion, the maximum possible degree of contraction of the electron current at the anode is determined for a short high-current vacuum arc discharge.

Theory of the negative anode drop in low-pressure discharges

A kinetic model of the anode region with a negative anode potential drop is proposed, which explicitly takes into account the ratio of the directed velocity of electrons in plasma v0 to their thermal speed vT as a parameter of the electron velocity distribution function. A transcendent equation is derived for determination of the negative anode drop as a function of the ratio v0/vT. It is shown that, unlike the known Langmuir formula, the anode drop remains negative for any value of v0/vT. In the case of small values (v0/vT ≪ 1), the derived expression asymptotically reduces to the Langmuir formula. It is shown that the dependence of electron concentration on the potential differs from the Boltzmann law. The anode regions of the short high-current vacuum-arc discharge and the classical low-pressure discharge are considered as examples. For the vacuum-arc discharge, the current density distribution and the anode drop distribution over the anode surface under strong current contraction are calculated. For the low-pressure discharge, potential drops are calculated across the region of the inhomogeneous near-anode plasma and the space charge layer.

Two-dimensional kinetic model of a short high-current vacuum arc discharge

A two-dimensional kinetic model of a short high-current vacuum arc discharge has been developed in which magnetized electrons move in the hydrodynamic regime and fast cathode ions move in the free-flight regime in a two-dimensional electric field. The model includes the distribution of ions over the emission angles at the cathode boundary of the plasma. A method has been proposed for calculating the distribution of the plasma density in the gap. The two-dimensional distributions of the plasma density, electric field, discharge current density, and trajectories of ions in an external longitudinal magnetic field have been calculated. It has been shown that ion trajectories intersect each other and some ion trajectories return to the cathode. The number of ion trajectories reaching the anode decreases with a decrease in the external magnetic field. The ion “starvation” effect appears near the anode and can lead to the violation of steady-state current transfer and to the formation of an anode spot. It has been found that the kinetic description of the motion of ions does not indicate the appearance of a shock wave.

Two-dimensional kinetic model of short high-current vacuum-arc discharge

A two-dimensional mathematical model of a short high-current vacuum-arc discharge is developed, according to which magnetized electrons move in a hydrodynamic regime and fast cathode ions propagate in a free flight regime in a two-dimensional electric field. The proposed model takes into account the distribution of ions with respect to their escape angles from the cathode plasma boundary. A method for calculation of the plasma density distribution in the interelectrode gap is proposed. Two-dimensional distributions of the plasma density, electric field, and discharge current density in an external magnetic field are calculated. It is shown that ion trajectories exhibit mutual intersections, partly return to the cathode, and partly rotate in the oppositely oriented electric field at the side boundary of plasma. A decrease in the applied magnetic field intensity leads to a decrease in the number of ion trajectories reaching the anode (ion starvation), which can result in the violation of a stationary current transfer.

The Effect of Boundary Conditions on the Distribution of Physical Parameters in a High-Current Vacuum Arc Discharge

Through the use of a 2-D model of a high-current vacuum arc discharge (VAD) in an axial magnetic field, the distribution of high-current VAD parameters in a plasma area was calculated for different boundary conditions on the cathode plasma boundary, the anode, and the side boundary of plasma. Calculations were made for different current and external magnetic field values. The cathode boundary of plasma was assumed as either equipotential or having specified current density distribution. The side boundary of plasma coincided with either the external ion trajectory or the boundary of the calculated area. In the case with a specified current density, potential distribution on the cathode plasma boundary was calculated. There is a significant difference in the distribution of high-current VAD parameters when calculations are made with the use of models having different boundary conditions. A possibility of different-boundary-condition application for the adequate description of a high-current VAD is discussed. A simple mathematical model has been used for a vivid interpretation of the obtained results.

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.

A theoretical study of the impact of magnetic field of Hall current on the parameters of a high-current vacuum-arc discharge

It is demonstrated that the magnetic field of Hall current in a short high-current vacuum-arc discharge significantly distorts the external axial magnetic field and affects the distribution of current density in the discharge gap. This effect decreases with increasing external magnetic field, with decreasing arc current, and with decreasing ratio of the length of discharge gap to its transverse dimension. A 2D magnetohydrodynamic mathematical model is used to calculate the discharge parameters for different values of induction of external magnetic field. The calculation results are compared with similar results obtained using a 1.5D model in which the impact made by magnetic field of Hall current is ignored. It is inferred that the simpler 1.5D model may be employed in calculations of parameters of vacuum arc in a wide range of variation of arc current and of induction of external magnetic field.

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 B0. Calculations are performed of the dependence of critical current on B0 for discharge gaps of different geometries. The calculation results agree with experiment.

Laser measurement of copper vapor density after a high-current vacuum arc discharge in an axial magnetic field

The copper vapor density after the occurrence of a magnetically stabilized high-current vacuum arc was measured by the laser absorption method. Measurements were performed in a vacuum triggered gap with oxygen-free, axial magnetic field (AMF) Cu electrodes (40-mm-diameter cathode and 60-mm-diameter anode), separated by a gap of 30 mm. The arc current was a sinusoidal wave having a half-period of approximately 10 ms and a peak value ranging from 3 to 10 kA. The arc was stabilized by a magnetic field produced by the AMF electrodes. The copper vapor density at current zero increased with peak arc current and did not depend on the current zero slope. After current zero, the copper vapor density decayed with a time constant of approximately 400 /spl mu/s. These results can be explained by a model based on vapor expansion from cathode spot craters.