V. N. Pimenov

Damage of structural materials for fusion devices under pulsed ion and high temperature plasma beams

Results of experiments are presented on the influence of high-energy pulses on austenitic and ferritic steels carried out using dense plasma foci devices PF-1000 and PF-60 with hydrogen and deuterium as working gas, respectively. Pulsed irradiation of specimens was performed in two regimes: (1) under microsecond hydrogen plasma pulses with power density q=107–109 W/cm2, and (2) under 100-ns deuterium plasma pulses with q=109–1011 W/cm2. Features of damage, phase-structural transformations and compositional changes in these materials under these conditions were investigated.

Generation of shock waves in materials science experiments with dense plasma focus device

The paper presents a comparison of the results of numerical simulations of the shock wave (SW) produced inside a stainless steel plate by a powerful pulsed stream of fast ions generated in a dense plasma focus device with its experimental observations. A SW was detected for the first time in a materials science experiment directly by means of multiframe nanosecond laser interferometry. This was visualized in experiments with the PF-1000 facility after the SW went out at the rear side of the thin stainless steel plate into the residual gas atmosphere. In particular, the pressure amplitude of the SW measured by the interferometric method was 16 GPa. The observed value is in good agreement with the results of numerical simulations of the SW pressure amplitude.

Physical processes of the interaction of ion and plasma streams with a target surface in a dense plasma focus device

Dynamics of the interaction of powerful streams of high-temperature plasma and fast ions generated in a device of the “Dense Plasma Focus” (DPF) type has been studied for a special case. In these experiments solid conductive targets with the shape of a plate and a tube, respectively, were placed normally and axially with respect to the Z axis of the DPF chamber on its cathode side. The secondary plasma spread out from the target surface has been examined. The shock-wave action upon the flat targets produced by the ion beam has been revealed.

Damage of chrome-manganese steels by pulsed fluxes of ions and dense plasma under their separate action on the material in the plasma focus setup

Two methods of temporal and spatial separation of the fluxes of hot plasma and fast ions in the Plasma Focus setup are experimentally investigated. In the first case, in a traditional geometry of the experiment, different distances from the focus to the target are used. Ion and plasma fluxes are generated at different instants, move with different velocities, and fall normally to the surface of the target (a chromium-manganese specimen). In this case, the processes of melting and evaporation are accompanied by implantation of plasma-forming gas ions and redistribution of elements in the surface layer. In the second case, separation of plasma and ion fluxes takes place inside the cavity of an extended steep pipe owing to different transport mechanisms of the quasi-neutral plasma and the beam of fast ions. This method allows distinguishing an ion flux of a higher power density (∼109–1010 W/cm2) in the pulse discharge. The damage to the outer pipe surface is found to monotonically decrease with an increase in the distance from the focus, while that along the inner surface is nonmonotonic. The reasons for the abnormally high damage to the ring domain of the inner surface of the pipe by a high-energy ion flux are discussed.

Armor materials’ behavior under repetitive dense plasma shots

Experiments on the plasma focus device PF-12 have been carried out to investigate changes in the structure of the surface and bulk of tungsten and tungsten doped with 1% lanthanum oxide after repeated powerful deuterium plasma shots (8, 25 and 100). The surface morphology of the targets exposed to plasma streams is analyzed by electron and optical microscopy. Due to the plasma effect, different surface structures, such as wave-like structures, a melted layer, a mesh of microcracks, droplets, craters, crevices and holes, appear. The change of cross-section hardness after a number of shots in different materials is investigated.

Features of the Damage and the Structural Changes in the Tungsten Surface Layer under the Pulsed Action of Laser Radiation,Ion and Plasma Fluxes

The degradation of a bilateral pressed tungsten surface layer by pulsed laser irradiation in the free-running mode (power density q = 105–5 × 106 W/cm2, pulse duration τ = 0.7 ms) and the Q-switched mode (q = 109–1010 W/cm2, τ = 80 ns) as well as under plasma beam irradiation in a plasma focus (PF) device (q = 108–1012 W/cm2, τ = 10–100 ns) has been investigated. The features of the degradation, erosion, and structural changes in the tungsten surface layer under different irradiation conditions have been determined. It has been shown that the use of PF devices in combination with laser equipment is promising for the simulation of the extreme radiation–thermal effects in materials that are typical of thermonuclear fusion devices with magnetic and inertial plasma confinement.

Structural Changes in the Vanadium Sample Surface Induced by Pulsed High-Temperature Deuterium Plasma and Deuterium Ion Fluxes

The structural changes in the vanadium sample surface are studied as functions of the conditions of irradiation by pulsed high-temperature deuterium plasma and deuterium ion fluxes in the Plasma Focus installation. It is found that processes of partial evaporation, melting, and crystallization of the surface layer of vanadium samples take place in the plasma flux power density range q = 108–1010 W/cm2 and the ion flux density range q = 1010–1012 W/cm2. The surface relief is wavelike. There are microcracks, gas-filled bubbles (blisters), and traces of fracture on the surface. The blisters are failed in the solid state. The character of blister fracture is similar to that observed during usual ion irradiation in accelerators. The samples irradiated at relatively low power density (q = 107–108 W/cm2) demonstrate the ejection of microparticles (surface fragments) on the side facing plasma. This process is assumed to be due to the fact that the unloading wave formed in the sample–target volume reaches its irradiated surface. Under certain irradiation conditions (sample–anode distance, the number of plasma pulses), a block microstructure with block sizes of several tens of microns forms on the sample surfaces. This structure is likely to form via directional crack propagation upon cooling of a thin melted surface layer.

Electrical and elastic properties of an array of carbon nanotubes after irradiation by high-energy electrons

The effect of the irradiation of arrays of annealed and unannealed multiwalled carbon nanotubes (CNTs) by electrons with an energy of 21 MeV and a dose of 2.2 × 1017 el/cm2 on their electrical conductivity, thermal electromotive force, and elasticity coefficient is studied. It is established that the irradiation of annealed nanotubes considerably decreases (by nearly a factor of two) their electrical conductivity and coefficient of elasticity in the material pressing direction and increases their thermal electromotive force by nearly 1.5 times. The effect of the same dose of electron irradiation on an array of unannealed CNTs also leads to an appreciable decrease in its coefficient of elasticity, but favors a considerable increase in the electrical conductivity and thermal electromotive force of the material. The observed effects are discussed in relation to the specifics of the formation of radiation defects in CNTs and their interaction with primary defects in unannealed specimens.

Effect of irradiation with pulsed fluxes of high-temperature deuterium plasma and fast ions on the structure of the surface layer of W-1% La2O3 alloy

Scanning electron microscopy, X-ray electron probe analysis, and X-ray phase analysis are used to investigate the surface structure of W-1% La2O3 alloy after irradiation with submicrosecond pulses of deuterium ions and high-temperature deuterium plasma in a Plasma Focus installation for simulation of a plasma disruption in the ITER tokamak. It is established that melting of the tungsten surface and formation of micro-cracks take place under irradiation with a power flux density up to 1012 W/cm2. Introduction of La2O3 particles in the alloy leads to the formation of complex La-W-O oxides with lower melting points, which results in the formation of a nonuniform structure affecting the high-temperature response of the material.

Surface effects under the action of pulsed beams of nitrogen ions and nitrogen plasma on Ti-Al-V alloy

We present results of experiments on the action of nanosecond pulsed beams of nitrogen ions and nitrogen plasma on specimens of Ti-4 Al-3 V (wt %) alloy that were obtained using a plasma focus device. Two regimes of material treatment were studied: a high-energy irradiation (with radiation power density of 108 W/cm2), where the surface layer is melted, and a softer irradiation regime (with radiation power density of 107 W/cm2), where a liquid phase on the surface is not formed. The mechanisms of surface modification and hardening of Ti-4 Al-3 V alloy corresponding to different irradiation regimes are discussed.