A. Suslova

Material ejection and surface morphology changes during transient heat loading of tungsten as plasma-facing component in fusion devices

We investigated the effect of edge-localized mode like transient heat events on pristine samples for two different grades of deformed tungsten with ultrafine and nanocrystalline grains as potential candidates for plasma-facing components. Pulses from a laser beam with durations ?1?ms and operating in the near infrared wavelength were used for simulating transient heat loading in fusion devices. We specifically focused on investigating and analysis of different mechanisms for material removal from the sample surface under repetitive transient heat loads. Several techniques were applied for analysing different mechanisms leading to material removal from the W surface under repetitive transient heat loads which include witness plates for collected ejected material, and subsequent analysis using x-ray photoelectron spectroscopy and scanning electron microscopy, visible imaging using fast-gated camera, and evaluating thermal emission from the particles using optical emission spectroscopy. Our results show a significantly improved performance of polycrystalline cold-rolled tungsten compared to tungsten produced using an orthogonal machining process under repetitive transient loads for a wide range of the power densities.

Recrystallization and grain growth induced by ELMs-like transient heat loads in deformed tungsten samples

Tungsten has been chosen as the main candidate for plasma facing components (PFCs) due to its superior properties under extreme operating conditions in future nuclear fusion reactors such as ITER. One of the serious issues for PFCs is the high heat load during transient events such as ELMs and disruption in the reactor. Recrystallization and grain size growth in PFC materials caused by transients are undesirable changes in the material, since the isotropic microstructure developed after recrystallization exhibits a higher ductile-to-brittle transition temperature which increases with the grain size, a lower thermal shock fatigue resistance, a lower mechanical strength, and an increased surface roughening. The current work was focused on careful determination of the threshold parameters for surface recrystallization, grain growth rate, and thermal shock fatigue resistance under ELM-like transient heat events. Transient heat loads were simulated using long pulse laser beams for two different grades of ultrafine-grained tungsten. It was observed that cold rolled tungsten demonstrated better power handling capabilities and higher thermal stress fatigue resistance compared to severely deformed tungsten. Higher recrystallization threshold, slower grain growth, and lower degree of surface roughening were observed in the cold rolled tungsten.

The significance of in situ conditions in the characterization of GaSb nanopatterned surfaces via ion beam sputtering

A systematic study is conducted in order to elucidate the underlying mechanism(s) for nanopatterning with low-energy irradiation of GaSb (100) under normal incidence. Ion energies between 50 and 1000 eV of Ar+ and ion fluences of up to 1018 cm−2 were employed. Characterization of the shallow (e.g., 1 to 6 nm) amorphous phase region induced by irradiation and the sub-surface crystalline phase region is accomplished with low-energy ion scattering spectroscopy and x-ray photoelectron spectroscopy, respectively. In situ studies are conducted due to the strong chemical affinity for oxygen of GaSb. The studies conclude that at energies below 200 eV, the native oxide layer hampers nanopatterning until it becomes removed at a fluence of approximately 5 × 1016 cm−2. At this energy and threshold fluence, the surface is enriched with Ga atoms during irradiation. At energies above 200 eV, the native oxide layer is efficiently removed in the early irradiation stages, and thus the detrimental effects from the oxide on ...

Nanopatterning of metal-coated silicon surfaces via ion beam irradiation: Real time x-ray studies reveal the effect of silicide bonding

We investigated the effect of silicide formation on ion-induced nanopatterning of silicon with various ultrathin metal coatings. Silicon substrates coated with 10 nm Ni, Fe, and Cu were irradiated with 200 eV argon ions at normal incidence. Real time grazing incidence small angle x-ray scattering (GISAXS) and x-ray fluorescence (XRF) were performed during the irradiation process and real time measurements revealed threshold conditions for nanopatterning of silicon at normal incidence irradiation. Three main stages of the nanopatterning process were identified. The real time GISAXS intensity of the correlated peaks in conjunction with XRF revealed that the nanostructures remain for a time period after the removal of the all the metal atoms from the sample depending on the binding energy of the metal silicides formed. Ex-situ XPS confirmed the removal of all metal impurities. In-situ XPS during the irradiation of Ni, Fe, and Cu coated silicon substrates at normal incidence demonstrated phase separation and the formation of different silicide phases that occur upon metal-silicon mixing. Silicide formation leads to nanostructure formation due the preferential erosion of the non-silicide regions and the weakening of the ion induced mass redistribution.

Real time x-ray studies during nanostructure formation on silicon via low energy ion beam irradiation using ultrathin iron films

Real time grazing incidence small angle x-ray scattering and x-ray fluorescence (XRF) are used to elucidate nanodot formation on silicon surfaces during low energy ion beam irradiation of ultrathin iron-coated silicon substrates. Four surface modification stages were identified: (1) surface roughening due to film erosion, (2) surface smoothing and silicon-iron mixing, (3) structure formation, and (4) structure smoothing. The results conclude that 2.5 × 1015 iron atoms in a 50 nm depth triggers surface nanopatterning with a correlated nanodots distance of 25 nm. Moreover, there is a wide window in time where the surface can have correlated nanostructures even after the removal of all the iron atoms from the sample as confirmed by XRF and ex-situ x-ray photoelectron spectroscopy (XPS). In addition, in-situ XPS results indicated silicide formation, which plays a role in the structure formation mechanism.

The effect of native oxide on ion-sputtering-induced nanostructure formation on GaSb surfaces

We have investigated the influence of native oxides on ion-sputtering-induced nanostructure formation on GaSb using in situ low energy ion scattering spectroscopy (LEISS) and X-ray photoelectron spectroscopy (XPS). Comparing an oxygen-free sample with a native oxide sample, LEISS and XPS reveal the effect of oxygen in generating higher surface Ga fractions during early stages (fluences of 1 × 1015–1 × 1016 cm−2) of low energy (<100 eV) Ar+ irradiation. Enhanced surface Ga and Ga2O3 fractions were also observed on “oxide free” samples exposed to air following irradiation. The results suggest preferential Ga oxidation and segregation on the top of the amorphous layer if oxygen is present on the surface. In addition, the native oxide also increases the fluence threshold for nanopatterning of GaSb surfaces by almost a factor of four during low energy irradiation.

Numerical simulation of ballistic electron dynamics and heat transport in metallic targets exposed to ultrashort laser pulse

The role of ballistic electrons generated during ultrashort pulse laser (USPL) absorption in metallic targets was investigated in a wide range of laser intensities using our developed simulation package FEMTO-2D. The simulation package is based on the numerical solution of the two-temperature model with the assumption of local thermal equilibrium for electron and lattice subsystems within the simulation cell at any time step. Electron thermodynamic parameters were calculated through the processes of material transition from the cold solid state into the dense plasma state during and after the pulse based on the collision theory. The appropriate model for temperature dependent thermodynamic parameters allows defining the heat transport during an early stage of the USPL-matter interaction directly, without relying on the effective absorption depth model. The study investigated, for the first time, using integrated computer simulation the role of ballistic electrons in energy transfer and heat conduction during USPL deposition. The simulation predictions of the electron heat transport dynamics during and shortly after the laser pulse were benchmarked for the gold target against available experimental data and were able to confirm the dominant role of the ballistic electrons in the initial heat propagation within 100–120 nm of the target at laser intensities below 1013 W/cm2.The role of ballistic electrons generated during ultrashort pulse laser (USPL) absorption in metallic targets was investigated in a wide range of laser intensities using our developed simulation package FEMTO-2D. The simulation package is based on the numerical solution of the two-temperature model with the assumption of local thermal equilibrium for electron and lattice subsystems within the simulation cell at any time step. Electron thermodynamic parameters were calculated through the processes of material transition from the cold solid state into the dense plasma state during and after the pulse based on the collision theory. The appropriate model for temperature dependent thermodynamic parameters allows defining the heat transport during an early stage of the USPL-matter interaction directly, without relying on the effective absorption depth model. The study investigated, for the first time, using integrated computer simulation the role of ballistic electrons in energy transfer and heat conduction dur...

Tungsten response to transient heat loads generated by laser pulses

Tungsten (W) has been selected as a plasma-facing component (PFC) material in the activated phase of ITER. High melting point and thermal conductivity and low erosion rate and low tritium inventory are the major advantages of W material, which makes it suitable for plasma fusion technology. However, ductile-to-brittle transition is also a major drawback of W, which could produce large macroscopic particles as well as small dust particles. The emission of dust particles from W are found to persist for longer duration compared to plasma lifetime with velocities of several tens m/s. The lifetime of the PFCs also depends on ELMs, VDEs, disruptions and runaway electrons.