Dynamics of plasma filling the gap of a module of the plasma opening switch of the MOL facility

The Baikal program aimed at the creation of a superhigh-power generator for experiments in the field of inertial thermonuclear fusion includes the development of a MOL facility for modeling the main principles of the generator structure. It is suggested to use a six-module plasma opening switch (POS) in an external magnetic field as the output power sharpener. The scheme of upgraded plasma guns of the module of the MOL facility’s POS and the results of studying the dynamics of filling the POS gap with plasma transversely to an external magnetic field are presented. It is shown that, in the presence of a long (～40 μs) prepulse in this module, a POS cathode-anode closure with plasma along the force lines of the external magnetic field occurs within a time interval between the first turning-on of the plasma guns and the POS operation, which is hazardous for its operation. To eliminate this threat, it is proposed to increase the POS length to ～70 cm. On the basis of the results obtained, a simpler and more reliable solution of the prepulse problem is proposed.     

Electric strength of a vacuum gap during breakdown in an inhomogeneous electric field for switching high-power pulses

A system for matching the output of a multimodular plasma opening switch (POS) to a “liner”-type load having a small initial impedance is described. The main element of this system is an isolating spark gap that allows synchronization of the POS modules and prevents a repeated POS closure cutting off the inductive storage from the load. A spark gap based on a multigap explosive-emission diode, which was proposed and tested earlier, is insufficiently strong electrically to be used in the Baikal project aimed at the development of a superpowerful pulsed generator. An urgent problem of increasing the electrical strength of a 1-mm-wide vacuum gap, which is a basic element of the aforementioned spark gap, is being solved. It is proposed to use electrodes of the point (anode)-plane (cathode) type that allow the electric field to be concentrated at the point-type anode, thereby increasing the gap’s electrical strength. The results of experiments aimed at the study of vacuum breakdown in this system of electrodes are presented. It is shown that this design allows the gap’s strength to be raised by several times, a long service life of electrodes to be ensured thanks to the low energy-deposition density, and a multichannel (i.e., low-inductance) breakdown to be stably attained. A diagram of the spark-gap prototype is presented, its breakdown characteristics are obtained, and a full-scale scheme of the spark gap is presented.     

A scheme of a modular plasma-opening-switch-based generator of megaampere currents for experiments with Z-pinches

A modular scheme of a megaampere current pulse generator on the plasma opening switch (POS) basis with insulation of the POS interelectrode gap by a magnetic field created by an external source is proposed for experiments with wire liners and other applications. The POS is fed from independent modules that are switched on in parallel. On the basis of an НК-50-3 (50 kV, 3 μF) capacitor, a supply module with a current through the equivalent part of the POS of up to 200 kA is designed and tested. The limiting POS parameters and the maximum permissible number of modules are determined, and circuit designs of the basic units of the generator are presented.     

Studying the effect of adsorbed molecules on the operation of a diode with an explosive-emission cathode

The results of an experimental study of the effect of an adsorbed gas and surface contaminations of an explosive-emission cathode on the operation of a diode during generation of a high-current electron beam of nanosecond duration are presented. The effect of contaminations was revealed from the change in the rate of expansion of the planar-diode cathode plasma for cathodes of different designs manufactured from different materials and different initial anode-cathode gaps. The plasma velocity was calculated from the experimental perveance of the diode with a resolution of 0.2 ns. Experiments were performed on a ТЭУ-500 pulsed electron accelerator (350–450 kV, 100 ns, and 250 J/pulse) in a mode of matching the diode impedance to the output impedance of the nanosecond generator. It has been found that the velocity of cathode plasma is constant for 70–90 ns after applying voltages to different cathodes at different anode-cathode gaps. The velocities were 2.0 ± 0.5 cm/ μs for carbon cathodes (of different diameters), 3 ± 0.5 cm/μs for multispike tungsten cathodes, and 4.0 ± 0.5 cm/μs for copper (solid or multispike) cathodes. An appreciable dependence of the plasma velocity on the cathode material shows an insignificant influence of the adsorbed gas and cathode surface contaminations on the expansion velocity of the explosive-emission plasma in a planar diode during generation of the electron beam (10–15 ns after a voltage is applied).     

Cavitation erosion characteristics of nickel-based alloy-composite coatings obtained by plasma spraying

Nickel-based alloy and composite coatings were obtained on an 18-8 stainless steel substrate by a plasma spraying technique. They were sintered in a vacuum furnace (10^?2 Pa) to improve toughness and to reduce hardness. The cavitation erosion resistance of these coatings was evaluated using a rotating disc apparatus. The results of the investigation reveal that sintered coatings are several times better than “as-sprayed” plasma coatings. The sintered coatings can help in increasing the length of the incubation period.     

Subsurface damage during dry sliding wear of Al-Al3Ni eutectic alloy

Cylindrical Al-Al₃Ni eutectic alloy wear pins (10 mm in diameter) were slid against a polished steel surface in a pin-on-disc rotating machine under unlubricated conditions with bearing pressures of 6–60 kPa and a constant sliding speed of 70 m min^?1. Metallographic changes in the subsurface region of contact were examined by optical microscopy and microhardness measurements. In the bearing pressure range investigated the alloy exhibited “mild” wear in two linear regions identified as pure “oxidative” wear at low bearing pressures and oxidative with superimposed “metallic” wear at higher bearing pressures. Plastic deformation and fragmentation of the Al₃Ni phase occurred under all bearing pressures. However, in composites prepared by unidirectional solidification containing large Al₃Ni particles fragmentation was insignificant. In all other specimens the size of the fragmented particles in the subsurface region of contact was about 5 μm irrespective of the bearing pressure.     

A sclerometric study of unidirectionally solidified Cr-Mo white cast irons

Low speed scratch experiments were carried out on a unidirectionally solidified bar of a eutectic Cr-Mo white iron. By means of a piezoelectric sensor the normal and tangential forces acting on the indenter faces were measured. For each phase the “scratch hardness”, the “scratch toughness” and the “specific grooving energy” were calculated. These values make it possible to draw conclusions about the contribution of each phase to the abrasive wear process. The effect of imposed conditions such as the normal force (0.15–2.0 N) and the scratch speed (125–520 μm s⁻¹) was analysed. The influence of some metallurgical variables such as the intercarbide spacing on the abrasion resistance was indicated.     

Measurements of the profile of an intense electron beam

Methods for measuring the profiles of high-power electron beams by using a thin tungsten wire moved transversely to the beam have been developed. In one method, the electron current intercepted by the wire is measured and the beam profile is determined from a solution to the Abel equation under the assumption of axial beam symmetry. The second method is based on the detection of the local radiation emitted by the wire being heated by the beam to 1700–2200 K. The wire is additionally heated by an electric current in order to improve the sensitivity and spatial resolution. The measured beam current density is ～5–50 A/cm², and the resolution is ～0.1 cm.     

A facility for permeation measurements under plasma irradiation

An PIM (magnetic plasma source) experimental facility for investigation of permeation of hydrogen isotopes through the structural and plasma-facing materials, which are used in fusion devices, is described. The facility allows investigation of the hydrogen permeation through metal and porous (carbon and carbon composites) membranes in a temperature range of 290–1000 K during interaction of membranes with gaseous hydrogen (the pressure difference between the membrane sides is up to 0.1 atm) and also under irradiation with microwave plasma with a flux density of up to 3 × 10²⁰ atoms/(m² s). The permeability of ChS-68 austenitic steel was investigated. The experimental data confirmed the correctness of the operation of the facility and the reliability of the results.     

High-voltage high-current air-filled spark gaps of an artificial-lightning-current generator

The designs of two high-voltage high-current air-filled spark gaps of an artificial-lightning-current generator-a two-electrode ДMBP-5 gap for a voltage of up to ±5 kV with graphitized-carbon electrodes and a three-electrode TKBP-50 gap for a voltage of up to ±50 kV with steel electrodes—are described, and their main switching characteristics and the results of tests are presented. Tests were performed with a ДMBP-5 spark gap that switched the normalized long C lightning-current component with an amplitude of up to 0.85 kA, a rise time of up to 0.77 × 10⁵ A/s, a maximum duration of 1000 ms, and a transferred electric charge of up to 210 C. When a TKBP-50 spark gap was used, the normalized pulse A lightning-current component with an amplitude of up to 212 kA, a rise time of up to 7 × 10⁹ A/s, a maximum duration of 500 μs, and an action integral of up to 2.07 × 10⁶ A² s was switched.     

Research of micro EDM/ECM method in same electrolyte with running wire tool electrode

A micro rod machining method which can switch between electrical discharge machining (EDM) and electrochemical machining (ECM) by attaching/detaching a diode to/from a bipolar pulse generator in parallel to the working gap was newly developed using a wire electrode made of tungsten. The problem of the wire electrode wear was eliminated by the use of the wire electrochemical turning (WECT) method in which the tungsten wire electrode is continuously running. The ultra-short bipolar pulse current was generated by the electrostatic induction feeding method where a pulse voltage is coupled to the working gap through a feeding capacitance. The machining characteristics of three types of wire guide; disk-shaped WC guide, laminated wire guide and cylindrical acrylic guide, were studied. The experimental results showed that the cylindrical acrylic guide has the best machining characteristics without the influence of guide wear and with less stray current flowing through the working gap. Using the cylindrical acrylic guide, the influences of the feeding capacitance C₁, and the total amplitude of the pulse voltage on the machining characteristics were studied. Finally, a stainless steel SUS 304 micro-rod with a high aspect ratio of 14 was fabricated efficiently by using the EDM and ECM modes for rough and finish machining in sequence with the same setup, pulse generator, and neutral electrolyte.     

An arc generator of a broad plasma stream

The design and basic parameters of an arc plasma generator based on a combined cathode are described. The cathode consists of a hot tungsten filament located in the hollow cathode. A plasma stream with a cross section of 150×10 cm² and a density of ∼10¹⁰ cm⁻³ at a pressure of 0.1–1 Pa is generated at a discharge current of up to 60 A without a cathode spot. The plasma generator can be utilized for final cleaning and activation of surfaces of materials and articles before depositing functional coatings on them and in plasma-assisted deposition by using either vacuum arc or magnetron discharges.     

An impact and erosion study of polyetheretherketone

Single-particle impacts by 4 and 5 mm diameter steel spheres on polyetheretherketone (PEEK) at various angles and speeds were studied using high speed photography and scanning electron microscopy. The following two material removal mechanisms were identified:

• 1.(1) the drawing out of filaments in oblique impact;
• 2.(2) material jetting in normal impact for speeds of approximately 480 m s⁻¹. A “deformation map” was constructed giving the types of crater formed by these spheres at given angles and speeds. Low speed (about 30 m s⁻¹) damage by quartz sand grains, sieved to give the size range 300–600 μm, was also studied. Various types of damage site were identified and the proportion of each for angles of impact ranging from 20° to 90° determined. The compressive stress-strain curve at a strain rate of 10⁴ s⁻¹ has been measured and a high speed photographic sequence of the deformation of a PEEK disc at a strain rate of approximately 2 × 10³ s⁻¹ is presented.

     

The role of cementite in the formation of magnetic hysteresis properties of plastically deformed high-carbon steels: I. Magnetic properties and structural state of cementite

Magnetic properties of cementite after strong plastic deformations and subsequent annealing in a broad range of temperatures are studied. The plastically deformed cementite is shown to exist in a soft (H _c ≈ 80 A/cm) state; the annealed cementite, in a hard (H _c ≈ 240 A/cm) state. The nature of the cementite’s soft and hard states is discussed. The field dependence of the cementite’s magnetostriction is measured. The longitudinal magnetostriction of the polycrystalline cementite’s saturated state is shown to be negative and approximately four times smaller than iron’s magnetostriction in the saturated state.     

An application of bubble collapse pulse height spectra to venturi cavitation erosion of 1100-o aluminum

Venturi cavitation erosion tests were performed and correlated with bubble collapse pulse height spectra measured by a microtransducer. The effects of the throat velocity and the cavitation number σ (referred to the downstream pressure and throat velocity) on the erosion rate (MDPR) were studied. The velocity damage exponent was 4.11 for 0.62 ? σ ? 0.80, while the MDPR is almost independent of velocity for σ = 0.85. The MDPR decreases with increased σ for 0.62 ? σ ? 0.85. The data were reduced to “acoustic power” (from pulse height spectra) and “erosion power” (the ultimate resilience multiplied by the MDPR). A near-linear relationship was found between these. Their reciprocal ratio η_cav ≈ 7 × 10^?11. For σ = 0.62, the data deviated from the others, possibly because of the work hardening of the eroded surface.     

Thermal imaging system based on a high-temperature superconductor

A laboratory prototype of an infrared imaging system is described. Its operating principle is based on the formation of an area on the surface of a patterned high-temperature superconducting (HTSC) film, which is sensitive to external radiation and its displacement by local heating. The basic parameters of the activated pixel zone were measured: dimensions A ≈ (95 × 95) μm², detectivity D* = 1.3 × 10⁸ cm Hz^1/2/W, and the time constant τ = 6 ms. The considered pixel-by-pixel data readout procedure and the bolometric nature of the detector sensitivity open the possibility of utilizing the imaging system over a broad spectral range.     

Lithium plasma emitter for collisionless magnetized plasma experiment

This paper presents a newly developed lithium plasma emitter, which can provide quiescent and low-temperature collisionless conditions for magnetized plasma experiments. This plasma emitter generates thermal emissions of lithium ions and electrons to produce a lithium plasma. Lithium type beta-eucryptite and lanthanum-hexaboride (LaB(6)) powders were mixed and directly heated with a tungsten heater to synthesize ion and electron emissions. As a result, a plasma with a diameter of ~15 cm was obtained in a magnetic mirror configuration. The typical range of electron density was 10(12)-10(13) m(-3) and that of electron temperature was 0.1-0.8 eV with the emitter operation temperature of about 1500 K. The amplitude fluctuations for the plasma density were lower than 1%.     

Arc welding robot system with seam tracking and weld pool control based on passive vision

The automatic control in square-wave alternating current (AC) gas tungsten arc welding (GTAW) is significant for a “teach and playback” robot to overcome the variation of the seam trajectory and the seam gap in the welding process. This paper presents a welding robot system based on the real-time visual measurement in the different levels of the welding current. The primary objective is to measure the offset of the torch to the seam center and the size of seam gap by passive vision, track the seam and control the weld pool in real time. A novel visual image analysis algorithm was developed for seam tracking and seam gap measuring, free from robot calibration. The control algorithm based on the knowledge base was established to control the weld formation by regulating the welding current and wire feed rate. The welding practice for the rocket storage tank demonstrates the efficiency of the proposed system.     

A microwave detector based on an MCT photodiode for subthermonuclear plasma research

A microwave detector including a single-element infrared (IR) photodiode mounted in a cooled Dewar and a preamplifier is developed. An IR photodiode of the n ⁺-p type is created on the basis of mercury-cadmium telluride (HgCdTe or MCT) heteroepitaxial structures grown by means of molecular beam epitaxy. A special profile of MCT structures throughout the MCT layer thickness ensures a low series resistance of less than 10 Ω and a high quantum efficiency at a level of 0.65 without any antireflection coating. For the spectral range of 8 to 12 μm, IR photodiodes are characterized by a low threshold power of 3.8·10^?13 W·Hz^?1/2 and 10^?19 W·Hz^?1 in the direct and heterodyne regimes, respectively. The cooled Dewar maintains the operating temperature of the IR photodiode in the interval of 77 to 80 K for 8. The preamplifier operates in a frequency range up to 1 GHz. With the use of this microwave detector, a method is developed for detection of intensity of a CO₂ laser beam on plasma fluctuations caused by heating by a relativistic electron beam (REB). At high REB intensities, dips of the turbulence signal with a duration of several nanoseconds are observed, which is interpreted as a collapse of Langmuir waves.