Elena V. Demina

The experimental and theoretical investigations of damage development and distribution in double-forged tungsten under plasma irradiation-initiated extreme heat loads

Abstract The influence of extreme heat loads, as produced by a multiple pulses of non-homogeneous fl ow of slow plasma (0.1-1 keV) and fast ions (100 keV), on double-forged tungsten (DFW) was investigated. For generation of deuterium plasma and fast deuterons, plasma-focus devices PF-12 and PF-1000 are used. Depending on devices and conditions, the power flux density of plasma varied in a range of 107-1010 W/cm2 with pulse duration of 50-100 ns. Power flux density of fast ions was 1010-1012 W/cm2 at the pulse duration of 10-50 ns. To achieve the combined effect of different kind of plasmas, the samples were later irradiated with hydrogen plasma (105 W/cm2, 0.25 ms) by a QSPA Kh-50 plasma generator. Surface modification was analysed by scanning electron microscopy (SEM) and microroughness measurements. For estimation of damages in the bulk of material, an electrical conductivity method was used. Investigations showed that irradiation of DFW with multiple plasma pulses generated a mesh of micro- and macrocracks due to high heat load. A comparison with single forged tungsten (W) and tungsten doped with 1% lanthanum-oxide (WL10) reveals the better crack-resistance of DFW. Also, sizes of cells formed between the cracks on the DFW’s surface were larger than in cases of W or WL10. Measurements of electrical conductivity indicated a layer of decreased conductivity, which reached up to 500 μm. It depended mainly on values of power flux density of fast ions, but not on the number of pulses. Thus, it may be concluded that bulk defects (weakening bonds between grains and crystals, dislocations, point-defects) were generated due to mechanical shock wave, which was generated by the fast ions flux. Damages and erosion of materials under different combined radiation conditions have also been 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.

Studying tungsten under the cyclic action of a plasma gun jet

We have studied the cyclic action of a plasma gun jet, which modeled nonstationary thermal load related to the edge localized modes (ELMs), on the surface of tungsten elements intended for the ITER divertor. The results revealed significant changes in both the structure and morphology of a surface layer. After 100 irradiation cycles, a columnar or “ridged” structure of elements perpendicular to the initial irradiated surface is formed. Exposure to 1000 cycles changes the mechanism of material degradation and radically modifies the character of the surface topography, since the ridges are melted with the formation of droplet structures. The surface becomes rough, a loose material layer appears under the irradiated surface, and a 100-μm-thick recrystallized layer is formed.

Parallel and sequential tests of radiation resistance of double forged tungsten in various plasma devices

Identical tungsten specimens perspective for their use in modern nuclear fusion reactors have been tested under powerful plasma streams and beams of fast ions in various radiation devices (plasma accelerator, plasma gun and dense plasma foci) in wide ranges of radiation pulse durations (from ~ 10-8 s to ~ 10-4 s), power flux densities of the above streams (from ~ 106 W/cm2 to ~ 1012 W/cm2), plasma densities (from 1015 cm-3 to ~ 1019 cm-3), temperatures (from ~ few eV to 1 keV) and fast ions energies (from ~ 10 keV to ~ a few MeV). Metallographic indexes of damageability examined in irradiated samples of tungsten demonstrate in our case that in the conditions of high-energy action with the damage factor similar to those in the regimes of disruption instability, VDE and ELMs effects in tokamaks the nature (character) of the materials degradation is practically the same in all cases and it does not depend on a source of energy load and a type of plasma used (hydrogen, deuterium). Within the interval of the high power flux densities used in present researches with Dense Plasma Foci (q ~ 109-1012 W/cm2) the damage character depends not only and not so much from the energy of single pulses produced by an irradiation facility as from the number of pulses retransmitted to the material. Depth of noticeable damage of a layer where the breakdown of the materials integrity takes place is equal to 200 μm and more. The nature of the damage is of the shock-wave and fatigue-thermal character. In the layer the discontinuity flaws (micro-cracks including V-shaped ones) appear and spread as in parallel to the irradiated surface so at an angle to it - very likely along the borders of blocks. An increase of the number of irradiation pulses results in spalling of material from the surface in early stages and in intermixing of material at prolonged (~1000 shots) irradiation campaign. Recrystallization processes manifest themselves faintly and they occupy a very thin sub-layer of melting and multiple re-melting appeared at the prolonged tests and large number of pulses. Depending on the irradiation conditions this formed and crystalized thin (of the order of a few μm) sub-layer contains a net of micro-cracks, spool-like figures of crystallization, and tungsten droplets. Character of the surface after multiple cycles of irradiation may be changed by height from the “flat” ridge-wave shape to the large-scale pot-hole one.