Tungsten Ablation Research Articles

Development of a high-speed small-angle infrared thermography system in EAST.

A high-speed infrared small-angle infrared thermography system (SATS) has been developed and installed on the Experimental Advanced Superconducting Tokamak (EAST) for measuring the surface temperature of the divertor target to calculate the high heat flux induced by Edge Localized Modes (ELMs) and providing observation means for the further physical study of some key parameters, such as power decay length λq and the characteristic time of different types of ELMs. An endoscopic optical system is applied to realize the SATS for clear imaging of the divertor plate area and protection from the harm of impurity deposition and latent tungsten ablation during discharge. The field of view (FOV) of the endoscopic optical system is designed to be 13° in the horizontal direction and 9° in the vertical direction. As a consequence, ∼35° of coverage of the lower-outer divertor and a small part of the lower-inner divertor in toroidal are covered by the FOV with a spatial resolution of around 2 mm/pixel. This paper presents a detailed description of the new SATS and the preliminary experimental diagnostic results. The radial distribution of heat flux induced by an ELM crash was demonstrated.

Ablation of tungsten surfaces in collisions with Ar+, He+ and N2+ cation projectiles in the presence of D2

Surface ablation of tungsten, known as a plasma-facing material in fusion devices, was investigated mass-spectrometrically in ion beam-type experiments by exposing a tungsten surface in the presence of H2/D2 to fast atomic and molecular projectile ions with high electron-recombination energies (Ar+, He+ and N2+). The sputtered product ions were monitored by high resolution mass spectrometry, in the case of Ar+, at various beam energies, surface temperatures and partial pressures of D2 at beam and product-ion angles of 45°. Bare monatomic and diatomic tungsten ions were seen to be sputtered, more so at elevated ion impact energies and increasing surface temperatures. The relative yield of deuterated WD+ ions was enhanced at higher impact energies and increased partial pressures of D2, but diminished with increasing tungsten temperature. Overall, the observations under our experimental conditions suggest the occurrence of Physical Sputtering (PS) of bare tungsten ions, W+ and W2+, as well as Chemically Assisted Physical Sputtering (CAPS) of WD+ with concomitant dissociation of D2.

Estimation of Tungsten Ablation Test Conditions Using 150 Kilowatt Arc Heater

To investigate the tungsten ablation rate and secure the data needed to develop a numerical model on high-temperature environment supposed hypersonic flight condition, in which oxidation mainly causes the ablation, an ablation test of pure tungsten specimen was conducted in a 150 kW arc heater. Under three different flow conditions, the changes in shape of the specimen due to ablation were captured by a high-speed camera, from which the recession rate over time was calculated. A numerical simulation of the test conditions was performed on an axisymmetric nonequilibrium solver to calculate heat transfer rate and mass fraction, which could not be measured by the experiment because of the limitations of the arc-heated facility. In addition, the numerical simulation was used to investigate the effects of shape changes, particularly the position and radius of the specimen, on heat transfer rate during the test. The study confirmed that the change in shape of the specimen alone could reduce the stagnation-point heat transfer rate by approximately 35% during the test.

Vapor Shielding Effect on DEMO Divertor

The impact of the edge-localized modes (ELMs) on the tungsten divertor erosion by taking into account the screening effect of vapor shielding is analyzed for DEMO steady-state operation condition. The evaluation of tungsten ablation, energy radiation, and absorption by divertor plate due to a single ELM impact is calculated by using a model of vapor shielding inserted in the MEMOS code. The effect of repetitive ELM impact and the tungsten melt layer formation is described by using the model of W monoblock with a compliance layer of Cu alloy between the W and EUROFER water cooling tube.It is shown that the vapor plasma shielding results in saturation of the single ELM energy accumulated by the divertor plate and that the saturation level depends on the ELM duration. The ablation thickness can reach about 0.01 µm. The total number of ablated particles is rather critical for the shielding formation, and the lifetime of the divertor plate depends strongly on this effect.

Dynamics of prompt electrons, ions, and neutrals of nanosecond laser ablation of tungsten investigated using optical emission

A Nd:YAG laser with a power density of 15 GW/cm2, operating at a wavelength of 1064 nm, was used to ablate a tungsten (W) target placed in an ambient atmosphere of air at a pressure of 10 mbar. A systematic study of plasma properties employing fast photography and time and space resolved optical emission spectroscopy was carried out. The results showed that there were continuum, neutral, and singly ionized W atom optical emission in the laser-induced plasma; the spectra of ambient gas N2 and N2+ were aslo observed which appeared at very early times (<30 ns). The spectra of N2+ presented a temporal profile with a narrow width (∼20 ns) and a steep rising edge (∼10 ns) that was very close to the laser pulse duration, which revealed that the prompt electrons ejected from the W target collisionally excited and ionized ambient N2 gas molecules. The approximate timescales from nanoseconds to microseconds of four laser-produced W plasma processes including prompt electrons, continuum radiation, ions, and neutral emission were given according to time-resolved optical emission spectroscopy. Fast imaging showed that the prompt electrons were expanding hemispherically. The optical time of flight dynamics of N2+ corresponding to the prompt electrons at different positions showed that the kinetic energy of prompt electrons was up to 22 eV. These prompt electrons with high energy result in the separation from the core plasma which indicated the existence of the transient dynamic plasma sheath during the laser ablation process and interpreted the acceleration of ions.

Morphology of Tungsten Nanooxides, Synthesized by Laser Ablation of Metal in Water

Colloidal solution precipitates obtained during laser ablation of tungsten in water and containing nanostructured metal oxides are studied using X-ray diffraction and scanning electron microscopy. The nanostructure composition and morphology are analyzed. It is shown that the material composing nanostructures is X-ray amorphous, i.e., the particle size does not exceed 1–2 nm. The high degree of the structure surface development implies prospects of their use as substrata when analyzing the composition of various materials by surface-enhanced Raman scattering.

Evaluation of explosive sublimation as the mechanism of nanosecond laser ablation of tungsten under vacuum conditions

A non-equilibrium mechanism for nanosecond laser ablation is suggested herein, and its predictions are compared to the results of W experiments performed under vacuum conditions. A mechanism of particle formation is explained via this model, with partial sublimation of the superheated irradiated zone of the target considered to be the mechanism of laser ablation. In this study, a mixture of vapor and particles was explosively generated and subsequently prevented the rest of a laser pulse from reaching its intended target. This mechanism was found to play an essential role in the ablation of W under vacuum conditions, and it provides a theoretical justification for particle formation. Moreover, special considerations were taken into account for the expansion of plasma into a vacuum. The model was evaluated by measuring the mass of ablated particles using a quartz crystal deposition monitor and time-resolved optical emission spectroscopy. The results of this model were found to be in good agreement with experimental values.

Fabrication of protective-coated SiC reinforced tungsten matrix composites with reduced reaction phases by spark plasma sintering

SiC reinforced tungsten matrix composites were fabricated via the spark plasma sintering process. In order to prevent an interfacial reaction between the SiC and tungsten during sintering, TiOx coated SiC particles were synthesized by a solution-based process. TiOx layer coated SiC particles were treated in high temperature nitriding conditions or annealed in a high temperature vacuum to form TiN or TiC coated SiC particles, respectively. The TiC layers coated on SiC particles successfully prevented tungsten from reacting with SiC; hence the proposed process resulted in successful fabrication of the SiC/W composites. The mechanical properties such as compressive strength and flexural strength of the composites were measured. Additionally, the effect of SiC on the high temperature oxidative ablation of tungsten was also investigated. The addition of SiC resulted in an improved oxidative ablation resistance of the tungsten-based composites.

Investigation and analysis of arc ablation on WCu electrical contact materials

In this paper, arc ablation behavior of tungsten–copper (WCu) alloy contacts was investigated by the measurement of weight-loss, or erosion rate, and microstructures of tungsten–copper alloy contacts were examined by using scanning electron microscopy after electrical breakdown process. Experimental results showed that the arc erosion process started at the tiny sharp edges, then the arc spreaded on the second phases, or copper phase on the surface. Because of electrons/ions bombardment, the rapid evaporation and sputtering of liquid copper droplets were serious. As a result, an even contact surface became rugged, and tungsten skeleton remained because of being less eroded. With the increase of applied breakdown voltage, the ablation of tungsten–copper alloy became more significant. The WCu alloys with a higher content of copper showed more severe erosion rate as compared with that with a higher tungsten content under the same erosion conditions. Furthermore, when the number of breakdown is less, WCu alloys with a high copper content are easier for erosion for the conditions of fewer numbers of breakdowns. Whereas, WCu alloys with a high content of tungsten are easier for ablation for the conditions with many times of arc breakdown.

Differences in Nanosecond Laser Ablation and Deposition of Tungsten, Boron, and WB2/B Composite due to Optical Properties

The first attempt to the deposition of WB3films using nanosecond Nd:YAG laser demonstrated that deposited coatings are superhard. However, they have very high roughness. The deposited films consisted mainly of droplets. Therefore, in the present work, the explanation of this phenomenon is conducted. The interaction of Nd:YAG nanosecond laser pulse with tungsten, boron, and WB2/B target during ablation is investigated. The studies show the fundamental differences in ablation of those materials. The ablation of tungsten is thermal and occurs due to only evaporation. In the same conditions, during ablation of boron, the phase explosion and/or fragmentation due to recoil pressure is observed. The deposited films have a significant contribution of big debris with irregular shape. In the case of WB2/B composite, ablation is significantly different. The ablation seems to be the detonation in the liquid phase. The deposition mechanism is related mainly to the mechanical transport of the target material in the form of droplets, while the gaseous phase plays marginal role. The main origin of differences is optical properties of studied materials. A method estimating phase explosion occurrence based on material data such as critical temperature, thermal diffusivity, and optical properties is shown. Moreover, the effect of laser wavelength on the ablation process and the quality of the deposited films is discussed.

Soft X-ray laser observation of femtosecond-laser-driven ablation of tungsten

Ablation dynamics of tungsten irradiated with a 70 fs laser pulse is investigated with X-ray interferometry and X-ray imaging using a 13.9 nm soft X-ray laser of 7 ps pulse duration. The evolution of high-density ablation front of tungsten (i.e., W) is presented. The ablation front expands to ~120 nm above the original target surface at 160 ps after femtosecond-laser irradiation with an expansion speed of approximately 750 m/s. These results will provide important data for understanding ablation properties of W, which is a candidate material of the first wall of magnetic confinement fusion reactors.

Molecular dynamic simulation of tungsten ablation under transient high heat flux

Molecular dynamic (MD) method is used to simulation the tungsten ablation under transient high heat flux generated by energetic ions. A model including 363,600W atoms was built based on Finnis–Sinclair potential. The results show that the ablation threshold is much lower than the one of boiling. So the ablation effects might be underestimated if using energy threshold of boiling instead of that of ablation. Particle size distribution of ablation products follows a power decay law with an exponent around −2.5, which does not affect by the incident heat flux. The transverse velocities of particles obey normal distribution, and a stream speed is added to the random movement for the longitudinal velocity. As the ablation start up, the recoiled impulse can induce shock wave in remained target, which is supported by experimental pressure wave measurements.

Experimental characterization of the excitation state of picosecond laser-induced Tungsten plasmas

In order to quantitatively determine the retention of light atoms by plasma facing components (mainly tungsten) used in fusion reactors, the Laser-Induced Breakdown Spectroscopy (LIBS) technique can be used. The nanosecond laser regimes classically used for LIBS present major limitations mainly due to the conditions of the laser-matter interaction process itself and cannot be used in the present context. Picosecond laser pulses are more appropriate. To validate the diagnostic based on picosecond laser pulses, preliminary characterizations of the produced plasmas have to be performed. Some related results are reported in the present communication. The study is focused on (1) the ablation of tungsten under two laser (low and high) fluence conditions and (2) the excitation equilibrium of the plasma from the nanosecond to microsecond time scales.

Super-stable ultrafine beta-tungsten nanocrystals with metastable phase and related magnetism

Ultrafine tungsten nanocrystals (average size of 3 nm) with a metastable phase (beta-tungsten with A15 structure, β-W) have been prepared by laser ablation of tungsten in liquid nitrogen. The as-prepared metastable nanocrystals exhibited super-stablity, and can keep the same metastable structure over a period of 6 months at room temperature. This super-stability is attributed to the nanosized confinement effect of ultrafine nanocrystals. The magnetism measurements showed that the β-W nanocrystals have weak ferromagnetic properties at 2 K, which may arise from surface defects and unpaired electrons on the surface of the ultrafine nanocrystals. These findings provided useful information for the application of ultrafine β-W nanocrystals in microelectronics and spintronics.

Laser-induced breakdown spectroscopic characterization of tungsten plasma using the first, second, and third harmonics of an Nd:YAG laser

Tungsten has become an important material in fusion reactors and plasma-facing components. Laser ablation of tungsten has been carried out at 1064nm, 532nm, and 355nm at atmospheric pressure to determine the capability of laser-induced breakdown spectroscopy (LIBS) for in situ diagnosis of plasma-facing components. Stark broadening and Boltzmann plots were used for the measurement of electron density and temperature, respectively of laser produced tungsten plasma. At a constant laser irradiance (1.05×1015Wm−2) the maximum temperature (16,304K) was reached at 1064nm. Maximum electron density (1.12×1018cm−3) was observed at 355nm. Variations in electron temperature and density as a function of laser irradiance and time are also discussed. This work could provide important reference data for the design and optimization of LIBS systems involved in plasma-facing components diagnostics.

Modern numerical methods for plasma tomography optimisation

Tomography is a technique which is widely applied to fusion plasmas as it can provide improved understanding of plasma emissivity distributions. It is challenging because of the sparse nature of data available from the measured plasma projections. An optimised version of robust and fast tomographic algorithm based on the Tikhonov regularisation constrained to Minimum Fisher Information is presented in this contribution. A new regularisation matrix enforcing preferential emissivity smoothness along magnetic flux surfaces is introduced. The paper also details application of advanced numerical methods which lead to a substantial decrease in computation time. Subsequent implementation of fast presolvers of the inverse problem further contributes to the algorithm's efficiency and also an improved stability of the tomography reconstruction. Finally, reliability and performance of the tomography algorithm is exemplified by the reconstruction of soft X-ray data evolution following tungsten ablation into a JET plasma. The resulting speed of reconstruction is compared to other referenced tomographic algorithms.

Development of novel tungsten processing technologies for electro-chemical machining (ECM) of plasma facing components

Plasma facing components for fusion applications must exhibit long-term stability under extreme conditions, and therefore material imperfections cannot be tolerated due to a high risk of technical failures. To prevent or abolish defects in refractory metals components during the manufacturing process, some methods of electro-chemical machining as S-ECM and C-ECM were developed, enabling both the processing of smooth plain defect-free surfaces of different geometry and the removal of bulk material for the shaping of three-dimensional structures, also without cracks. It is discussed, that tungsten ablation with accurate electro-chemical molding is very sensitive to the kind of electric current, and therefore current investigations focused also on the effects of frequency profiles on the sharpness of edge rounding.

Cluster growth in an ablation plume propagating through a buffer gas

A phenomenological mixed-propagation model that describes the expansion of an ablation plume through a buffer gas is introduced. Selected experiments including LaMnO3 and tin ablation in oxygen, as well as tungsten ablation in argon, are analysed. For given ablation conditions the expansion parameters required to model the growth of clusters in the expanding plasma plume are deduced and the average asymptotic size of the clusters is calculated and compared (for tungsten) with the size of clusters measured by transmission electron microscopy.

Plasma-assisted laser ablation of tungsten: Reduction in ablation power threshold due to bursting of holes/bubbles

Nanosecond laser ablation of tungsten (W) exposed to helium plasma is investigated using optical emission spectroscopy. Submicrometer-sized holes/bubbles are formed on the surface of W when it was exposed to the helium plasma at a sufficiently high temperature (≳1500–1600K). The emissions from a virgin W (before the helium plasma irradiation) cannot be detected when the fluence is <1J∕cm2; however, the threshold fluence for the detection of neutral W emission after it was exposed to the helium plasma is ∼0.2J∕cm2. The physical mechanism of laser-induced bursting of holes/bubbles is proposed for achieving a significant reduction in ablation power threshold.

Measurements of nanoparticle size distribution produced by laser ablation of tungsten and boron-carbide in N2 ambient

Abstract Nanoparticles (NPs) were produced by ablating tungsten and boron-carbide (B 4 C) target materials in atmospheric pressure nitrogen ambient using ArF excimer laser pulses. The size distributions of the NPs formed during the ablation were monitored—within a 7–133 nm size window—by a condensation particle counter connected to a differential mobility analyzer. The laser repetition rate was varied between 1–50 Hz, and the fluence was systematically changed in the range of 0.5–15 J/cm 2 , for both materials, allowing a comparative study in an extended laser parameter regime. The multishot ablation threshold ( Φ th ) of B 4 C was determined to be ∼1.9 J/cm 2 for the laser used (ArF excimer, λ = 193 nm). Similarly to earlier studies, it was shown that the size distributions consist of mainly small nanoparticles ( Φ th . An additional broad peak appears (between 20 and 40 nm) above Φ th as a consequence of the thermally induced macroscopic ablation. Chemical composition of deposited polydisperse nanoparticles was studied by X-ray photoelectron spectroscopy showing nitrogen incorporation into the boron-carbide.