Tungsten Melting Research Articles

Overview of the EUROfusion Tokamak Exploitation programme in support of ITER and DEMO

Abstract Within the 9th European Framework programme, since 2021 EUROfusion is operating five tokamaks under the auspices of a single Task Force called "Tokamak Exploitation”. The goal is to benefit from the complementary capabilities of each machine in a coordinated way and help in developing a scientific output scalable to future largre machines. The programme of this Task Force ensures that ASDEX Upgrade, MAST-U, TCV, WEST and JET (since 2022) work together to achieve the objectives of Missions 1 and 2 of the EUROfusion Roadmap: i) demonstrate plasma scenarios that increase the success margin of ITER and satisfy the requirements of DEMO and, ii) demonstrate an integrated approach that can handle the large power leaving ITER and DEMO plasmas. The Tokamak Exploitation task force has therefore organized experiments on these two missions with the goal to strengthen the physics and operational basis for the ITER baseline scenario and for exploiting the recent plasma exhaust enhancements in all four devices (PEX: Plasma EXhaust) for exploring the solution for handling heat and particle exhaust in ITER and develop the conceptual solutions for DEMO. The ITER Baseline scenario has been developed in a similar way in ASDEX Upgrade, TCV and JET. Key risks for ITER such as disruptions and run-aways have been also investigated in TCV, ASDEX Upgrade and JET. Experiments have explored successfully different divertor configurations (standard, super-X, snowflakes) in MAST-U and TCV and studied tungsten melting in WEST and ASDEX Upgrade. The input from the smaller devices to JET has also been proven successful to set-up novel control schemes on disruption avoidance and detachment.

Statistical moment method in studying the melting of tungsten: a comprehensive review

Statistical moment method in studying the melting of tungsten: a comprehensive review

Crack-free tungsten coating on ODS plate via a modified laser-based additive technique: Offset laser cladding

A modified laser-based additive manufacturing technique, offset laser cladding was introduced to achieve dense tungsten coating on an oxide dispersion-strengthened (ODS) substrate as a plasma facing material (PFM). Different from ordinary direct energy deposition (DED) which leads to incomplete melting of tungsten and excessive melting of ODS substrate, the offset laser cladding technique can achieve complete melting of tungsten while substrate melting is suppressed. Experimental conditions (i.e., laser power, scanning speed, powder size, offset distance and injection powder nozzle setting) were carefully calculated using two validated simulation models to minimize the consumption of valuable experimental resources. Tungsten coating using the offset laser cladding in an open chamber condition showed that dense coating was achieved with a pore size of 1–2 μm. The result is comparable to that of vacuum plasma spray (VPS) tungsten coating which requires a high-level vacuum chamber. A thin interface layer (2–5 μm) was formed between the tungsten coating and ODS substrate that can alleviate thermal damage due to a large thermal expansion coefficient difference. While a small amount of surface melting was involved, the microhardness test revealed that the ODS substrate was insignificantly affected by laser heating during the offset laser cladding.

Infrared detection of tungsten cracking on actively cooled ITER-like component during high power experiment in WEST

The consequences of tungsten (W) damaging processes, such as cracking and melting, on divertor lifetime and plasma operation are high priority issues for ITER. A sustained melting experiment was conducted in WEST using a 2 mm deep groove geometry on the upstream mono-block (MB) to overexpose the sharp leading edge (LE) of the downstream MB. W-cracking has been evidenced for the first time with a very high spatial resolution infrared camera before tungsten melting was reached. These cracks develop when the monoblock temperature is about 2600 °C, thus higher than both ductile to brittle transition and softening threshold of tungsten, suggesting that these cracks are different from the ones observed in previous campaigns where brittle failure was involved, because of transient events on cold monoblock. Post-exposure analyses have been performed on the damaged monoblock, highlighting 12 main cracks on the LE, with a width varying from 33 µm to 77 µm, and an average spacing of 0.45 mm. Parallel heat flux about 90 MW/m2 has been derived from infrared temperature measurements, with a heat flux decay length on the target of 4 mm. The T-REX modelling code suggest here that with these thermal inputs, a crack can initiates due to thermal cycling without disruption, with a ductile failure, under 1 to 5 cycles for a tungsten DBTT varying from 400 °C to 500 °C.

FEATURES OF MODIFICATIONS IN THE RE-SOLIDIFIED SURFACES OF ADVANCED MATERIALS DUE TO HIGH-POWER PLASMA PULSES

The surface modification of advanced materials was studied through a series of repetitive plasma pulses caused tungsten melting. Features of the affected surface layers in reference materials (IGP W, AM W/WTa, Hastelloy, and EUROFER) for both fusion and fission applications were explored after exposure to plasma in the facilities (QSPA, MPC, and PPA) with different durations of plasma pulses. A detailed surface analysis was carried out with Scanning Electron Microscopy. It was found that the plasma treatment led to the formation of a modified layer as a result of the rapid re-solidification of the exposed surface. The fine cellular structures appeared in the re-solidified layers of the irradiated materials, with typical cell sizes ranging from 150 to 500 nm. An increase in the roughness of the exposed surfaces was attributed to the presence of the cracks and re-solidified layer.

Towards Selective Laser Melting of High-Density Tungsten

Selective laser melting (SLM) of tungsten (W) is challenging due to its high melting point and brittleness, resulting in defects including balling phenomenon, porosity and cracks. In this work, high-density crack-free SLM W was fabricated by employing cost-effective powders modified through air jet milling. The influence of the SLM processing parameters on microstructure, density, crack formation and the resulting mechanical properties of SLM W was investigated. Laser energy density and hatch distance were found to be the most important parameters in controlling porosity and crack formation of SLM W. The check-like microstructure in horizontal plane was induced by the difference in thermal gradients, which were caused by the movement of the heat source between overlapping regions and central regions of the molten pool. Combined efforts including powder modification through air jet milling, a 67° rotation scanning strategy, a hatch distance of 0.08 mm and a laser speed of 450 mm/s result in dense crack-free SLM W with relative density of 99.3%, microhardness of 403 HV50, and bending strength of 154 MPa. Additionally, the microstructure changed upon annealing at 1200 °C, accompanied by the reduced anisotropy of mechanical properties on both horizontal and vertical plane.

The CPS's pre-heating effect on the capability to withstand extreme plasma loads

This work presents experimental studies of plasma surface interactions (PSIs) during QSPA plasma impacts on the Sn Capillary Porous Structures (CPSs) in conditions, simulating disruption and ELM-like loads. The 3D tungsten target filled by Sn was exposed to the plasma streams with an energy density of up to 3 MJ/m2 which resulted in tungsten melting and evaporation. The initial temperature of a sample between plasma pulses had two options: remained at a room value for one sample, and reached 300 C which is close to Sn melting point for another one. The number of plasma irradiations reached 100 pulses. It is shown that the particle ejection was observed during PSI for pre-heated as well as cold CPS targets. The difference in values of the number of particles ejected from the targets, their velocities, and the start-time during exposition is discussed. Particles leave the target surface during all expositional pulses for both CPS targets. The threshold character of the ejection is more pronounced for the cold target. After the 100 pulses reduced Sn pool is observed. Moreover, areas without Sn occur on the exposed targets. Nevertheless, the severe damage of the W base of the CPS targets is not revealed during high-cycle powerful plasma loads.

Molecular dynamics study on melting point of tungsten nanostructures

We used molecular dynamics to study the melting point of tungsten nano-fibers. It is found that the nanoscale effect and helium impurity can significantly reduce the melting point. We use linear model and neural network model to fit the effect of two compound effects on melting point. For fuzz with 1%–4% helium and a diameter of 10–200 nm, its melting point can be reduced by 7.7%–16.4% with linear model and 6.1%–17.0% with neural network model. The actually reduction of melting point may be more than 20% due to the existence of a large number of holes in fuzz and the rough surface of fuzz. The reduction of melting point of tungsten fuzz can accelerate the dynamic phenomenon of fuzz annealing at low temperature, leading to the disappearance of fuzz. Our research is helpful to understand that the temperature limit of fuzz formation is lower than 2000 K to some extent. • Nanoscale effect and He impurity can significantly reduce the melting point of W. • The quantitative relationship of tungsten melting point change was obtained. • Research are helpful to understand the temperature limit of tungsten fuzz formation.

Experiments and modelling on ASDEX Upgrade and WEST in support of tool development for tokamak reactor armour melting assessments

The risk of metallic armour melting constitutes a major concern for ITER and DEMO. Combined computational and experimental effort has led to a successful understanding of melt dynamics in present-day tokamaks, but there remain unexplored melting regimes of relevance to future reactors. Analysis of recent poor-versus-efficient thermionic emitter sample exposures in ASDEX-Upgrade and ITER-like actively cooled tungsten leading edge exposures in WEST is presented. The coupled thermal response and melt dynamics is modelled with the new MEMENTO code employing the MEMOS-U physics model which has no adjustable parameters. In the weak Lorentz force regime accessed in the exposures, very close agreement between modelling and experiments is achieved not only for the deformation profiles, but also for the additional, unique to these experiments, constraints; simultaneous thermal response of two different materials in ASDEX-Upgrade and in-situ detection of melt build-up in WEST. • Two new melt experiments in ASDEX-Upgrade and WEST have accessed previously unexplored regimes. • The impact of thermionic emission in the energy and momentum balance of tungsten melting events is assessed. • Typically inaccessible in previous experiments secondary thermo-capillary effects are unmasked. • The coupled thermal response and melt dynamics is modelled with the new 5MF code employing the MEMOS-U physics model. • Enhanced value of the model validation effort due to existence of novel, unique to these experiments, constraints.

Theoretical simulation of the closed currents near non-uniformly strongly heated surface of tungsten due to thermo-emf

A problem of surface melting under the impact of plasmas is one of the most important in the development of future magnetic confinement fusion reactors. The expected high heat loads can lead to melting of tungsten chosen as the material of a divertor and the first wall of ITER. The tungsten melt can move under the action of forces and quickly deform a surface. This article shows that at high temperatures, a tungsten vapor can be considered as a conductive material with conductivity high enough to be taken into account during simulation of current through a sample. We describe the mechanism of current generation by the thermo-emf due to non-uniform heating of the vapor/condensed substance interface without external sources of charges. Even without direct contact with the external plasma, the density of this current is high enough to cause noticeable movement of the melt in an external magnetic field. Simulation shows that the melt moving observed at BETA may be caused not by a current of an electron beam but by non-uniformity of heating and can be reproduced by any other similar heat source in the external magnetic field.

Edge-localized-mode simulation in CFETR steady-state scenario

The EPED1 model and self-consistent core-pedestal coupling in integrated modeling are used to design the pedestal structure of the China Fusion Engineering Testing Reactor (CFETR) steady-state scenario. The key parameters, such as β p and q 95, are based on the grassy edge-localized-mode (ELM) experimental database. In this work, we use the BOUT++ six-field two-fluid code to simulate the onset of the ELM in the CFETR steady-state scenario. The ELM size is around 0.2% in nonlinear simulations, which is in the experimental range of the grassy ELM discharges, 0.1%–1% observed in multiple tokamak devices. Linear and nonlinear simulations show that the dominant high-n ballooning modes peak around n = 40. Compared to type-I ELM crashing dynamics, grassy ELM crashing has a smaller initial crash and is then followed by three phases of turbulence spreading, which are dominated by multi-modes, a high-n mode of n = 45 and low-n mode of n = 5, respectively. In contras to type-I ELM, the perturbation of the high-n mode has a narrow width around ψ = 0.95, and magnetic island formation and reconnection occur only beyond ψ = 0.95, leading to a small initial crash. Mode–mode interaction in the multi-mode coexistence stage stops the growth of individual modes and reduces the transport of particles and heat, and these are the two reasons why the ELM size is small. In–out asymmetry of transient heat flux with a ratio of E out/E in = 3.5 is found during grassy ELM crash. The rise and delay times of the heat flux match the calculation from the free-streaming model. To evaluate the erosion of the divertor target, the energy fluence at the outer divertor target is calculated, which is 0.029 MJ m−2, 5.5 times smaller than the tungsten melting limit 0.16 MJ m−2. The calculated energy fluency still follows the experimental scaling law from type-I ELM experiments. The fluctuation eddies in the toroidal direction show a filament structure at the outer mid-plane. Parallel heat flux patterns with a toroidal mode number n = 10 are found at the outer divertor with an amplitude of 680 MW m−2.

Sustained W-melting experiments on actively cooled ITER-like plasma facing unit in WEST

The consequences of tungsten (W) melting on divertor lifetime and plasma operation are high priority issues for ITER. Sustained and controlled W-melting experiment has been achieved for the first time in WEST on a poloidal sharp leading edge of an actively cooled ITER-like plasma facing unit (PFU). A series of dedicated high power steady state plasma discharges were performed to reach the melting point of tungsten. The leading edge was exposed to a parallel heat flux of about 100 MW.m−2 for up to 5 s providing a melt phase of about 2 s without noticeable impact of melting on plasma operation (radiated power and tungsten impurity content remained stable at constant input power) and no melt ejection were observed. The surface temperature of the MB was monitored by a high spatial resolution (0.1 mm/pixel) infrared camera viewing the melt zone from the top of the machine. The melting discharge was repeated three times resulting in about 6 s accumulated melting duration leading to material displacement from three similar pools. Cumulated on the overall sustained melting periods, this leads to excavation depth of about 230 μm followed by a re-solidified tungsten bump of 200 μm in the JxB direction.

Deuterium and helium retention and corresponding modifications of W under heat loads relevant to ITER transient plasma events: Part I. The power load below the tungsten melting temperature

In order to simulate ITER transient events with surface heat load parameters relevant to edge-localized-mode (ELM) impacts, tungsten samples were exposed to pulsed heat loads using pure deuterium (D) and with 10% helium (He) seeding plasmas in quasi-stationary high-current plasma gun QSPA-T. The pulse duration was 1 ms that is relevant to ELMs and number of pulses was varied from one to thirty. The power load was 0.7 MJ/m2 that is below the tungsten melting temperature (Tm). Two tungsten samples were used, namely, polycrystalline tungsten without (W) and with pre-existing He-induced W ‘fuzz’ (Wf). Similar to the steady state plasma, the presence of He in tungsten leads to a reduction of the D retention during transient events at temperature below Tm. We explained it as interruption of the D diffusion towards to the bulk of tungsten by (i) the strain field induced by He bubbles and (ii) the formation of interconnected He bubbles at high temperature which leads to an open porosity for accelerated D desorption, thus, decreasing the D influx into the tungsten bulk. But as the He retention in Wf decreases below 1019 He/m2, the effect of He on the D retention after the plasma gun irradiation disappears: the D retention in W and Wf is the same after 30 pulses of the exposure to pure D plasma. In both cases of pure D and He seeded D plasma gun exposures, the D retention is higher compared to the steady state plasma exposure at sample temperature above 600 K.

MODIFICATION AND ALLOYING EFFECTS IN EUROFER STEEL UNDER POWERFUL PULSED PLASMA IMPACTS

Experimental studies of surface modification of Eurofer samples have been performed with a quasi-stationary plasma accelerator QSPA Kh-50. The heat load on the surface was near the tungsten melting threshold (i.e. about 0.6 MJ/m2). The plasma streams exposures result in modification of steel-based materials and formation of cellular submicron structures in re-solidified layers. Phase characterized by body-centered cubic lattice appeared due to recrystallization of affected material.

Disruption mitigation in tokamak reactor via reducing the seed electrons of avalanche

Disruption mitigation technology remains the key factor for ensuring safe and reliable operation in future large tokamaks including ITER. A novel approach analyzed in this paper aims at mitigating the runaway avalanche (RA) development by reducing the population of runaway seeds using the tungsten projectile injection into the ITER plasma just after the thermal quench. As opposed to the scenario currently discussed by the ITER team, the approach does not involve injecting a large mass of deuterium into the tokamak vacuum vessel for collisional suppression of the RA current generated. This should significantly reduce the operational problems of the ITER technological systems dealing with fueling and pumping, isotope separation, plasma heating, etc. Collecting seeds and runaways by the projectile injected might reduce the avalanche current in ITER below the level of 1 MA and its magnetic energy by more than two orders of magnitude. A tungsten rod being 8 mm inside of the square section and 80 mm in length that crosses the plasma volume with the velocity of 0.8 km s−1 radially or tangentially seems appropriate for ITER disruption conditions. Simulations of the projectile interaction with the ITER post TQ plasma revealed that its surface temperature remains below the tungsten melting point. It is also found that a railgun using the toroidal magnetic field of the tokamak is the optimal way to accelerate such a projectile.

Non-thermal melting of tungsten under intense electronic excitations

Non-thermal effect caused by ultrafast lasers and swift ions in materials are very intriguing, which is of both scientific interest and technological importance. However, the underlying physics of non-thermal effect on ultrafast process remains unclear and the proposed mechanisms have been controversial. Based on the perturbation approximation under tight-binding theory, the non-thermal effect on tungsten (W) are extensively studied. We demonstrate that the non-thermal effect stemmed from the intense electronic excitations induce dramatic decrease in the melting point of W crystal, as well as non-thermal melting inside the W slab. Our analysis shows that the non-thermal forces are essentially responsible for the drop of melting point of the bulk system. Remarkably, the non-thermal effect combined with surface effect on a W film enhance the ordering of the direction of atomic motion near the surface, preventing melting near the surface area, but leading to non-thermal melting in the interior area of the film. Our work also exhibits a unified relationship between the non-thermal melting and the interatomic forces. This relationship is universal in metals and semiconductors irradiated by ultrafast lasers or swift ions, and has been well established long before.

Interpretation of temperature distribution observed on W-ITER-like PFUs in WEST monitored with a very-high-resolution IR system

During the 2019 experimental campaign, the WEST tokamak was partially equipped with ITER-like plasma facing units (PFUs) made of discrete monoblocks (MBs) in one of the twelve lower divertor sectors. The magnetic field lines can enter the gaps between two MBs and strike their leading edges (LEs) with near normal incidence, leading to high localized heat flux, temperature and thermomechanical stress during both stead-state operation and transients. Exposed leading edges are a particular matter of concern because of potential crack formation, recrystallization or even melting of tungsten. During the 2019 experimental campaign in WEST, five of them were misaligned (vertical misalignment h = 0.30 ± 0.1 mm) with their poloidal leading edges exposed to the plasma heat flux. A medium wavelength IR filter (MWIR: 3.9 ± 0.1 μm) was installed in the very high resolution infrared system, featuring a submillimeter spatial resolution (∼0.1 mm/pixel). This system has a temperature detection threshold of Tthreshold, BB ≈ 250 °C. In this paper, thermal analysis will be presented with a specific focus on overheating of poloidal and toroidal edges using post-mortem measured emissivity maps. The study reveals an unexpected hot spot in the top LE corner of misaligned PFUs at the outer strike point (OSP). Photonic calculations were performed with different emissivity on top and lateral surfaces, in order to consider the complex reflectance characteristics in the toroidal gaps. During this experiment, the Larmor radius (0.35 mm) is comparable to the gap size. Thus, the helical trajectory of the particles may significantly affect the heat load distribution in the edges vicinity. Heat load simulations were performed with ion orbit modeling that consider the Larmor radius, to study its impact on the thermal distribution of the MB. The photonic simulations showed that it was possible to observe a false hot spot at the top LE corner, with both optical approximation (OA) and ion orbit (IO) modeling, due to reflections and different emissivity values.

The MEMOS-U code description of macroscopic melt dynamics in fusion devices

The MEMOS-U physics model, addressing macroscopic melt motion in large deformation and long displacement regimes, and its numerical schemes are presented. Discussion is centred on the shallow water application to the metallic melts induced by hot magnetized plasmas, where phase transitions and electromagnetic responses are pivotal. The physics of boundary conditions with their underlying assumptions are analysed and the sensitivity to experimental input uncertainties is emphasized. The JET transient tungsten melting experiment (Coenen et al 2015 Nucl. Fusion 55 023010) is simulated to illustrate the MEMOS-U predictive power and to highlight key aspects of tokamak melt dynamics.

Edge localized mode characteristics and divertor heat flux during stationary and transient phase for CFETR hybrid scenario

The study of edge localized mode (ELM) behavior, stationary heat flux and transient heat flux during the ELM crash phase is performed for a China Fusion Engineering Test Reactor (CFETR) 1 GW hybrid mode operation scenario ( = 7.2 m, = 6.5 T, = 13.78 MA). Modeling and simulation start with a scenario obtained by multi-code integrated modeling on the one modeling framework for integrated tasks framework. Linear stability and nonlinear simulations of ELM dynamics are carried out using the BOUT++ six-field reduced magnetohydrodynamic module, which show a much smaller ELM energy loss (ΔELM ~ 0.13%) compared to that of a Type-I ELM. Parametric analysis of the weak linear growth rate and small ELM energy loss characteristics shed light on physics corresponding to a grassy ELM regime for CFETR 1 GW hybrid scenario. The transient heat flux on the divertor target during this small ELM phase is investigated using BOUT++. We found that upstream radial transport in the scrape-off-layer (SOL) induced by small ELMs is weak, which keeps it in the drift-dominated region. However, the heat flux width is still broadened to = 4.64 mm by the increase of separatrix temperature during ELM nonlinear evolution. The impact of transient peak heat load and ELM energy fluence on tungsten melting and net erosion rate of divertor target is evaluated for the first-time using physics-based transport that connects the pedestal with the SOL. Energy fluence caused by a single ELM pulse is below the tungsten melting limit, while tungsten erosion would exceed the material requirements. We conclude that external mitigation methods, such as divertor detachment and advanced divertor geometry are likely needed for the steady state operation of CFETR.

Selective laser melting of tungsten: Effects of hatch distance and point distance on pore formation

An experimental study was conducted on the effects of point distance and hatch distance on pore formation in selective laser melting of tungsten. The Archimedes’ method was used to measure the apparent and closed porosities. Apparent (open) porosity and closed porosity were found to follow different trends: the apparent porosity increased but the closed porosity decreased as hatch distance or point distance increased. The open pores are attributed to the lack of fusion at the large hatch distance or point distance while the closed pores are attributed to the possible spattering and keyhole formation at the smaller hatch distance or point distance. The results led to understanding the effects of hatch distance and point distance on the formation of open and closed pores and provided insight on the optimization of process parameters to further increase the relative bulk density.