Tungsten Melting Research Articles

Experimental and theoretical investigation of droplet emission from tungsten melt layer

Tungsten in form of macrobrush structure is foreseen as one of candidate materials for the ITER divertor and the dome. Melting of tungsten and the following melt motion and melt splashing are expected to be the main mechanisms of damage which determine the lifetime of plasma facing components. New experimental investigations of droplet emission from the W melt layer for the Edge Localised Mode (ELM)-like heat loads have been carried out at the plasma gun facility quasistationary plasma accelerators (QSPA-T). In these experiments the threshold for droplet emission and the distributions of velocity on emission angles and amplitude of the ejected droplets were determined. In the paper the main physical mechanism (the Kelvin–Helmholtz instability) of the melt splashing under the heat loads being applied at QSPA-T and those anticipated after the ITER transients is analyzed. These numerical simulations demonstrated a reasonable agreement with the experimental data on the droplet sizes and droplet velocities and allowed the projections upon the W melt splashing at ITER conditions.

Melting tungsten nanoparticles: a molecular dynamics study

We report a molecular dynamics simulation of melting of tungsten (W) nanoparticles. The modified embedded atom method interatomic potentials are used to describe the interaction between tungsten atoms. The melting temperature of unsupported tungsten nanoparticles of different sizes are found to decrease as the size of the particles decreases. The melting temperature obtained in this study is approximately a decreasing function of inverse radius, in good agreement with the predictions of thermodynamic models. We also observed that the melting of a W nanoparticle is preceded by the premelting of its outer skin at a temperature lower than its melting temperature.

Effects of arc current and electrode size on electrode erosion in ac plasma torches

Heavy erosion of the electrode is a problem in using ac plasma torches. In a series of our studies, the effects of electrode stem size (diameter and length) and tip shape on the electrode erosion were clarified at a low current of 200 A. In this paper, on the basis of the 200-A data, in order to reduce the electrode erosion with the current increased, the effects of current and electrode size on electrode erosion were clarified experimentally and analytically between 100 and 550 A. The electrode used was of solid type and the material was tungsten containing 2 wt% of lanthanum oxide. The plasma gas was argon. Main results are as follows. (1) As the current increased, the electrode erosion increased linearly. In the case of thin electrodes (8 mm ø), between 450 and 550 A, the electrode tip was molten partially and the electrode erosion increased rapidly over 5 mg/h. However, for thick electrodes (13 mm ø), the electrode erosion did not increase rapidly. (2) The temperature distribution along the electrode axis was calculated. As a result, a good correlation was obtained between the calculated temperature of electrode tip and the measured electrode erosion. As the tip temperature decreased to 3000 K (cf. tungsten melting point: 3660 K), the electrode erosion decreased below 5 mg/h of the linearly increasing range.

Experience with bulk tungsten test-limiters under high heat loads: melting and melt layer propagation

The paper provides an overview of processes and underlying physics governing tungsten melt erosion in the fusion plasma environment. Experiments with three different bulk tungsten test-limiters were performed in TEXTOR: (i) thermally insulated solid plate fixed on a graphite roof-like limiter heated up by the plasma to the melting point, (ii) macro-brush of the ITER-relevant castellated structure and (iii) lamellae structure developed for the JET divertor. The main objectives were to determine the metal surface damage, the formation of the melt layer and its motion in the magnetic field. PHEMOBRID-3D and MEMOS-1.5D numerical codes were used to simulate the experiment with the roof-like test-limiter. Both experiments and simulation showed that the melting of tungsten can lead to a large material redistribution due to thermo-electron emission currents without ejection of molten material to the plasma.

Simulation of tokamak armour erosion and plasma contamination at intense transient heat fluxes in ITER

For ITER, the potential material damage of plasma facing tungsten-, CFC-, or beryllium components during transient processes such as ELMs or mitigated disruptions are simulated numerically using the MHD code FOREV-2D and the melt motion code MEMOS-1.5D for a heat deposition in the range of 0.5–3 MJ/m2 on the time scale of 0.1–1 ms. Such loads can cause significant evaporation at the target surface and a contamination of the SOL by the ions of evaporated material. Results are presented on carbon plasma dynamics in toroidal geometry and on radiation fluxes from the SOL carbon ions obtained with FOREV-2D. The validation of MEMOS-1.5D against the plasma gun tokamak simulators MK-200UG and QSPA-Kh50, based on the tungsten melting threshold, is described. Simulations with MEMOS-1.5D for a beryllium first wall that provide important details about the melt motion dynamics and typical features of the damage are reported.

Reactive sintering of porous tungsten for higher homogeneity

One of the most important applications of tungsten is its use as a high current density cathode in the form of porous tungsten impregnated with electron emissive salts. Powder metallurgy is the usual processing route to make these parts since melting and casting of tungsten is very tedious due to its high melting point (3410±20°C). The overall porosity and porosity distribution are crucial parameters for the performance and lifetime of porous tungsten cathodes. Although there is an active debate over what constitutes the optimal porosity distribution, lack of homogeneity is observed in conventional parts. This may contribute to a shorter cathode and hence lamp lifetime. These parts are conventionally sintered at temperatures in excess of 2000°C. Furthermore, due to the high temperatures involved, the conventional sintering is very costly and energy consuming. This paper briefly looks into an alternative sintering process being developed for porous tungsten technology and its outcomes in terms of porosity distribution across the parts. As a result, the sintering temperature is dropped down to 1150°C and more homogeneous porous structures have been obtained. A characterisation method previously developed by the authors based on microhardness measurements is proved to be a good measure of the homogeneity of porous tungsten parts.

Metastable States of Liquid Tungsten Under Subsecond Wire Explosion

A numerical simulation of the initial stage of tungsten wire self-heating by a high-power microsecond current pulse was carried out. A wide-range semiempirical equation of state to account for the effects of melting and evaporation of tungsten at high temperatures was used. The metastable states were included in the process model, and the results of the simulation are in good agreement with experimental data.

Emission spectroscopy and size distribution of gas phase nanoparticles generated by laser-based methods

Tungsten and iron/carbon composite nanoparticles were generated via ArF excimer laser-assisted CVD from WF 6/H 2/Ar gas mixture and photolytic decomposition of ferrocene (Fe(C 5H 5) 2), respectively. Time-resolved spectroscopic measurements were performed and the emitted black-body-like radiation from these particles, due to the laser heating, was analyzed determining the temperature of the particles. Dominant cooling processes as evaporation and heat transfer by the ambient gases were identified; thermionic emission and black-body-like radiation were found to be negligible. Melting of tungsten and boiling of iron nanoparticles were observed. The size distributions of the particles were found lognormal and it was explained by the residence time approach model, functioning at the limit with no coagulation of the particles. Amorphous carbon nanoparticles were formed via laser ablation of a highly oriented pyrolytic graphite target at 193 nm excimer laser wavelength in N 2 atmosphere at normal pressure. The size distribution of the particles was in situ monitored by a differential mobility analyzer in the size-window of 7–130 nm. Change of size distributions was monitored as the laser fluence was varied. The observed effects are discussed and related to the ablation dynamics. Compositional and structural examinations of deposited particle-films were performed by Raman spectroscopy, X-ray and electron diffraction techniques.

Effects of arc current and electrode size on electrode erosion in ac plasma torches

Heavy erosion of the electrode is a problem in using ac plasma torches. In a series of our studies, the effects of electrode stem size (diameter and length) and tip shape on the electrode erosion were clarified at a low current of 200 A. In this paper, on the basis of the 200-A data, in order to reduce the electrode erosion with the current increased, the effects of current and electrode size on electrode erosion were clarified experimentally and analytically between 100 and 550 A. The electrode used was of solid type and the material was tungsten containing 2 wt% of lanthanum oxide. The plasma gas was argon. Main results are as follows. (1) As the current increased, the electrode erosion increased linearly. In the case of thin electrodes (8 mm o), between 450 and 550 A, the electrode tip was molten partially and the electrode erosion increased rapidly over 5 mg/h. However, for thick electrodes (13 mm o), the electrode erosion did not increase rapidly. (2) The temperature distribution along the electrode axis was calculated. As a result, a good correlation was obtained between the calculated temperature of electrode tip and the measured electrode erosion. As the tip temperature decreased to 3000 K (cf. tungsten melting point: 3660 K), the electrode erosion decreased below 5 mg/h of the linearly increasing range.

交流プラズマトーチにおける電極損耗に与えるアーク電流および電極形状の影響

Heavy erosion of the electrode is a problem in using ac plasma torches. In a series of our studies, the effects of electrode stem size (diameter and length) and tip shape on the electrode erosion were clarified at a low current of 200 A. In this paper, on the basis of the 200-A data, in order to reduce the electrode erosion with the current increased, the effects of current and electrode size on electrode erosion were clarified experimentally and analytically between 100 and 550 A. The electrode used was of solid type and the material was tungsten containing 2 wt% of lanthanum oxide. The plasma gas was argon. Main results are as follows. (1) As the current increased, the electrode erosion increased linearly. In the case of thin electrodes (8 mm o), between 450 and 550 A, the electrode tip was molten partially and the electrode erosion increased rapidly over 5 mg/h. However, for thick electrodes (13 mm o), the electrode erosion did not increase rapidly. (2) The temperature distribution along the electrode axis was calculated. As a result, a good correlation was obtained between the calculated temperature of electrode tip and the measured electrode erosion. As the tip temperature decreased to 3000 K (cf. tungsten melting point: 3660 K), the electrode erosion decreased below 5 mg/h of the linearly increasing range.

The melting of tungsten during plasma disruptions

Plasma facing components in tokamak type fusion reactors are subjected to intense heat loads during plasma disruptions. The influence of high heat fluxes on the depth of heat affected zones on pure tungsten metal, has been studied by using a two-dimensional transient computer model which solves the equations of motion and energy. Results are presented for a range of energy densities and disruption times. It is demonstrated that the presence of surface tension creates convective flows, hence exercising an influence on the flow intensities and resulting depths of tungsten molten layers during plasma disruptions. The calculated depths of molten layers are also compared with other mathematical models that have been based on the assumption that heat is transported through the material by conduction only.

Laser-heated diamond anvil cell experiments at high pressure: Melting curve of nickel up to 700 kbar

Laser-heated experiments in a diamond anvil cell have been performed on high pressure melting of nickel up to 700 kbar. The laser heating system consists of diamond anvil cell, Nd:YAG and argon lasers, spectrograph with diode array, computer with software, CCD camera with monitor and optics. Experiments on melting of tungsten, nickel and platinum at 1 bar outside the diamond anvil cell and melting of nickel below 80 kbar in the cell were carried out to check the system for pressure and temperature measurements. The results show that for solid pressure medium the uncertainties in measurements of pressure at the experimental spot vary between ±5 kbar at 100 kbar and ±25 kbar at 660 kbar. Spectroradiometrically determined temperature is reliable within ±70 K. Melting was detected in situ by visual observation. The melting point of nickel at 660 kbar has been found to be 2557 ±66 K.

Alloys of the Mo?W System

1. Melting of Mo−W alloys with >20% W requires more power than melting of unalloyed molybdenum or tungsten. The power required is highest for melting alloys with 80–85% W. 2. Alloys with 17–85% W have considerably finer grains as-cast than unalloyed molybdenum or tungsten; the grains are finest for the most highly alloyed alloy (Mo+56%W). 3. With increasing tungsten concentrations the density of Mo−W alloys increases, reaches a peak for the alloy with 60% W, and then declines. 4. At low (20°) and high (1800°) temperatures the strength characteristics of Mo−W alloys increase with the tungsten content; at moderate temperatures (400–1600°) the strength is highest for the alloys most highly alloyed. Alloying of molybdenum with tungsten reises the temperature range in which molybdenum exhibits hot shortness. The ductile-britle transition temperature increases most when over 85% W is added.

The “Quality” of an X-ray Tube and How to Measure It

IN earlier papers the writer has described some investigations on the temperature rise of the anode of an X-ray tube and its load-carrying capacity and on the size and shape of the focal spot.1 Some of the conclusions are: 1. An anode of copper provided with a thin tungsten target may carry a load of 200 watts per sq. mm. for one second. A much larger load would result in too high a temperature and consequently in evaporation or melting of tungsten or copper. 2. The best type of focus is the line focus, with the X-ray beam at an angle of about 20° to the surface. Such a line focus carries a load which is one and a half to two times larger than that carried by a circular focus of equal area. 3. The size of the focal area is far less important than the type of focus and its load-carrying capacity per unit area or “specific capacity.” For a given sharpness of definition, a large focus requires a longer distance, and, as the energy required for a given exposure time and the area of the focal spot are both pro...