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In nuclear fusion power research, the plasma-facing material (or materials) (PFM) is any material used to construct the plasma-facing components (PFC), those components exposed to the plasma within which nuclear fusion occurs, and particularly the material used for the lining the first wall or divertor region of the reactor vessel.
Plasma-facing materials for fusion reactor designs must support the overall steps for energy generation, these include:
In addition PFMs have to operate over the lifetime of a fusion reactor vessel by handling the harsh environmental conditions, such as:
Currently, fusion reactor research focuses on improving efficiency and reliability in heat generation and capture and on raising the rate of transfer. Generating electricity from heat is beyond the scope of current research, due to existing efficient heat-transfer cycles, such as heating water to operate steam turbines that drive electrical generators.
Current reactor designs are fueled by deuterium-tritium (D-T) fusion reactions, which produce high-energy neutrons that can damage the first wall, [1] however, high-energy neutrons (14.1 MeV) are needed for blanket and Tritium breeder operation. Tritium is not a naturally abundant isotope due to its short half-life, therefore for a fusion D-T reactor it will need to be bred by the nuclear reaction of lithium (Li), boron (B), or beryllium (Be) isotopes with high-energy neutrons that collide within the first wall. [2]
Most magnetic confinement fusion devices (MCFD) consist of several key components in their technical designs, including:
The core fusion plasma must not actually touch the first wall. ITER and many other current and projected fusion experiments, particularly those of the tokamak and stellarator designs, use intense magnetic fields in an attempt to achieve this, although plasma instability problems remain. Even with stable plasma confinement, however, the first wall material would be exposed to a neutron flux higher than in any current nuclear power reactor, which leads to two key problems in selecting the material:
The lining material must also:
Some critical plasma-facing components, such as and in particular the divertor, are typically protected by a different material than that used for the major area of the first wall. [3]
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Materials currently in use or under consideration include:
Multi-layer tiles of several of these materials are also being considered and used, for example:
Graphite was used for the first wall material of the Joint European Torus (JET) at its startup (1983), in Tokamak à configuration variable (1992) and in National Spherical Torus Experiment (NSTX, first plasma 1999). [11]
Beryllium was used to reline JET in 2009 in anticipation of its proposed use in ITER. [12]
Tungsten is used for the divertor in JET, and will be used for the divertor in ITER. [12] [13] It is also used for the first wall in ASDEX Upgrade. [14] Graphite tiles plasma sprayed with tungsten were used for the ASDEX Upgrade divertor. [15] Studies of tungsten in the divertor have been conducted at the DIII-D facility. [16] These experiments utilized two rings of tungsten isotopes embedded in the lower divertor to characterize erosion tungsten during operation. Molybdenum is used for the first wall material in Alcator C-Mod (1991).
Liquid lithium (LL) was used to coat the PFC of the Tokamak Fusion Test Reactor in the Lithium Tokamak Experiment (TFTR, 1996). [8]
Development of satisfactory plasma-facing materials is one of the key problems still to be solved by current programs. [17] [18]
Plasma-facing materials can be measured for performance in terms of: [9]
The International Fusion Materials Irradiation Facility (IFMIF) will particularly address this. Materials developed using IFMIF will be used in DEMO, the proposed successor to ITER.
French Nobel laureate in physics Pierre-Gilles de Gennes said of nuclear fusion, "We say that we will put the sun into a box. The idea is pretty. The problem is, we don't know how to make the box." [19]
Solid plasma-facing materials are known to be susceptible to damage under large heat loads and high neutron flux. If damaged, these solids can contaminate the plasma and decrease plasma confinement stability. In addition, radiation can leak through defects in the solids and contaminate outer vessel components. [1]
Liquid metal plasma-facing components that enclose the plasma have been proposed to address challenges in the PFC. In particular, liquid lithium (LL) has been confirmed to have various properties that are attractive for fusion reactor performance. [1]
Tungsten is widely recognized as the preferred material for plasma-facing components in next-generation fusion devices, largely due to its unique combination of properties and potential for enhancement. Its low erosion rates make it particularly suitable for the high-stress environment of fusion reactors, where it can withstand the intense conditions without degrading rapidly. Additionally, tungsten's low tritium retention through co-deposition and implantation is crucial in fusion contexts, helping to minimize the accumulation of this radioactive isotope. [20] [21] [22]
Another key advantage of tungsten is its high thermal conductivity, essential for managing the extreme heat generated in fusion processes. This property ensures efficient heat dissipation, reducing the risk of damage to the reactor's internal components. Furthermore, the potential for developing radiation-hardened alloys of tungsten presents an opportunity to enhance its durability and performance under the intense radiation conditions typical in fusion reactors.
Despite these benefits, tungsten is not without its drawbacks. One notable issue is its tendency to contribute to high core radiation, a significant challenge in maintaining the plasma performance in fusion reactors. Nevertheless, tungsten has been selected as the plasma-facing material for the ITER project's first-generation divertor, and it is likely to be used for the reactor's first wall as well.
Understanding the behavior of tungsten in fusion environments, including its sourcing, migration, and transport in the scrape-off-layer (SOL), as well as its potential for core contamination, is a complex task. Significant research is ongoing to develop a mature and validated understanding of these dynamics, particularly for predicting the behavior of high-Z (high atomic number) materials like tungsten in next-step tokamak devices.
To address tungsten's intrinsic brittleness, which limits its operational window, a composite material known as W-fibre enhanced W-composite (Wf/W) has been developed. This material incorporates extrinsic toughening mechanisms to significantly increase toughness, as demonstrated in small Wf/W samples.
In the context of future fusion power plants, tungsten stands out for its resilience against erosion, the highest melting point among metals, and relatively benign behavior under neutron irradiation. However, its ductile to brittle transition temperature (DBTT) is a concern, especially as it increases under neutron exposure. To overcome this brittleness, several strategies are being explored, including the use of nanocrystalline materials, tungsten alloying, and W-composite materials.
Particularly notable are the tungsten laminates and fiber-reinforced composites, which leverage tungsten's exceptional mechanical properties. When combined with copper's high thermal conductivity, these composites offer improved thermomechanical properties, extending beyond the operational range of traditional materials like CuCrZr. For applications requiring even higher temperature resilience, tungsten-fibre reinforced tungsten-composites (Wf/W) have been developed, incorporating mechanisms to enhance toughness, thereby broadening the potential applications of tungsten in fusion technology.
Lithium (Li) is an alkali metal with a low Z (atomic number). Li has a low first ionization energy of ~5.4 eV and is highly chemically reactive with ion species found in the plasma of fusion reactor cores. In particular, Li readily forms stable lithium compounds with hydrogen isotopes, oxygen, carbon, and other impurities found in D-T plasma. [1]
The fusion reaction of D-T produces charged and neutral particles in the plasma. The charged particles remain magnetically confined to the plasma. The neutral particles are not magnetically confined and will move toward the boundary between the hotter plasma and the colder PFC. Upon reaching the first wall, both neutral particles and charged particles that escaped the plasma become cold neutral particles in gaseous form. An outer edge of cold neutral gas is then “recycled”, or mixed, with the hotter plasma. A temperature gradient between the cold neutral gas and the hot plasma is believed to be the principal cause of anomalous electron and ion transport from the magnetically confined plasma. As recycling decreases, the temperature gradient decreases and plasma confinement stability increases. With better conditions for fusion in the plasma, the reactor performance increases. [23]
Initial use of lithium in 1990s was motivated by a need for a low-recycling PFC. In 1996, ~ 0.02 grams of lithium coating was added to the PFC of TFTR, resulting in the fusion power output and the fusion plasma confinement to improve by a factor of two. On the first wall, lithium reacted with neutral particles to produce stable lithium compounds, resulting in low-recycling of cold neutral gas. In addition, lithium contamination in the plasma tended to be well below 1%. [1]
Since 1996, these results have been confirmed by a large number of magnetic confinement fusion devices (MCFD) that have also used lithium in their PFC, for example: [1]
The primary energy generation in fusion reactor designs is from the absorption of high-energy neutrons. Results from these MCFD highlight additional benefits of liquid lithium coatings for reliable energy generation, including: [1] [23] [8]
Newer developments in liquid lithium are currently being tested, for example: [9] [10]
Silicon carbide (SiC), a low-Z refractory ceramic material, has emerged as a promising candidate for structural materials in magnetic fusion energy devices. While the remarkable properties of SiC once attracted attention for fusion experiments, past technological limitations hindered its wider use. However, the evolving capabilities of SiC fiber composites (SiCf/SiC) in Gen-IV fission reactors have renewed interest in SiC as a fusion material. [24]
Modern versions of SiCf/SiC combine many desirable attributes found in carbon fiber composites, such as thermo-mechanical strength and high melting point. These versions also present unique benefits: they exhibit minimal degradation of properties when exposed to high levels of neutron damage. However, tritium retention in silicon carbide plasma-facing components is about 1.5-2 times higher than in graphite, leading to reduced fuel efficiency and increased safety risks in fusion reactors. SiC traps more tritium, limiting its availability for fusion and increasing the potential for hazardous buildup, which complicates tritium management. [25] [26] Additionally, the chemical and physical sputtering of SiC is still significant and contributes to the key issue of increasing tritium inventory through co-deposition over time and with particle fluency. For those reasons, carbon-based materials have been ruled out in ITER, DEMO, and other devices. [27] SiC has demonstrated a tritium diffusivity lower than that observed in other structural materials, a property that can be further optimized by applying a thin layer of monolithic SiC on a SiC/SiCf substrate. [28] [29]
Siliconization, as a wall conditioning method, has been demonstrated to reduce oxygen impurities and enhance plasma performance. [30] [31] Current research efforts focus on understanding SiC behavior under conditions relevant to reactors, providing valuable insights into its potential role in future fusion technology. Silicon-rich films on divertor PFCs were recently developed using Si pellet injections in high confinement mode scenarios in DIII-D, prompting further research into refining the technique for broader fusion applications. [32]
A tokamak is a device which uses a powerful magnetic field generated by external magnets to confine plasma in the shape of an axially symmetrical torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. The tokamak concept is currently one of the leading candidates for a practical fusion reactor.
Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors. Research into fusion reactors began in the 1940s, but as of 2024, no device has reached net power, although net positive reactions have been achieved.
A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear power. Neutron source variables include the energy of the neutrons emitted by the source, the rate of neutrons emitted by the source, the size of the source, the cost of owning and maintaining the source, and government regulations related to the source.
This timeline of nuclear fusion is an incomplete chronological summary of significant events in the study and use of nuclear fusion.
The Joint European Torus (JET) was a magnetically confined plasma physics experiment, located at Culham Centre for Fusion Energy in Oxfordshire, UK. Based on a tokamak design, the fusion research facility was a joint European project with the main purpose of opening the way to future nuclear fusion grid energy. At the time of its design JET was larger than any comparable machine.
ITER is an international nuclear fusion research and engineering megaproject aimed at creating energy through a fusion process similar to that of the Sun. It is being built next to the Cadarache facility in southern France. Upon completion of construction of the main reactor and first plasma, planned for 2033–2034, ITER will be the largest of more than 100 fusion reactors built since the 1950s, with six times the plasma volume of JT-60SA in Japan, the largest tokamak operating today.
JT-60 is a large research tokamak, the flagship of the Japanese National Institute for Quantum Science and Technology's fusion energy directorate. As of 2023 the device is known as JT-60SA and is the largest operational superconducting tokamak in the world, built and operated jointly by the European Union and Japan in Naka, Ibaraki Prefecture. SA stands for super advanced tokamak, including a D-shaped plasma cross-section, superconducting coils, and active feedback control.
The T-15 is a Russian nuclear fusion research reactor located at the Kurchatov Institute, which is based on the (Soviet-invented) tokamak design. It was the first industrial prototype fusion reactor to use superconducting magnets to control the plasma. These enormous superconducting magnets confined the plasma the reactor produced, but failed to sustain it for more than just a few seconds. Despite not being immediately applicable, this new technological advancement proved to the USSR that they were on the right path. In the original shape, a toroidal chamber design, it had a major radius of 2.43 m and minor radius 0.7 m.
Magnetic confinement fusion (MCF) is an approach to generate thermonuclear fusion power that uses magnetic fields to confine fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of controlled fusion research, along with inertial confinement fusion.
DEMO, or a demonstration power plant, refers to a proposed class of nuclear fusion experimental reactors that are intended to demonstrate the net production of electric power from nuclear fusion. Most of the ITER partners have plans for their own DEMO-class reactors. With the possible exception of the EU and Japan, there are no plans for international collaboration as there was with ITER.
ASDEX Upgrade is a divertor tokamak at the Max-Planck-Institut für Plasmaphysik, Garching that went into operation in 1991. At present, it is Germany's second largest fusion experiment after stellarator Wendelstein 7-X.
The WEST, or Tungsten Environment in Steady-state Tokamak, is a French tokamak that originally began operating as Tore Supra after the discontinuation of TFR and of Petula. The original name came from the words torus and superconductor, as Tore Supra was for a long time the only tokamak of this size with superconducting toroidal magnets, allowing the creation of a strong permanent toroidal magnetic field. After a major upgrade to install tungsten walls and a divertor, the tokamak was renamed WEST.
Hybrid nuclear fusion–fission is a proposed means of generating power by use of a combination of nuclear fusion and fission processes.
SST-1 is a plasma confinement experimental device in the Institute for Plasma Research (IPR), an autonomous research institute under Department of Atomic Energy, India. It belongs to a new generation of tokamaks with the major objective being steady state operation of an advanced configuration plasma. It has been designed as a medium-sized tokamak with superconducting magnets.
The Lithium Tokamak Experiment (LTX), and its predecessor, the Current Drive Experiment-Upgrade (CDX-U), are devices dedicated to the study of liquid lithium as a plasma-facing component (PFC) at Princeton Plasma Physics Laboratory.
In plasma physics and controlled nuclear fusion, plasma-surface interactions concern the physical, chemical, and mechanical processes at the interface between plasma and solid surfaces.
In magnetic confinement fusion, a divertor is a magnetic field configuration which diverts the heat and particles escaped from the magnetically confined plasma to dedicated plasma-facing components, thus spatially separating the region plasma-surface interactions from the confined core. This requires establishing a separatrix-bounded magnetic configuration, typically achieved by creating poloidal field nulls (X-points) using external coils.
The ARC fusion reactor is a design for a compact fusion reactor developed by the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC). ARC aims to achieve an engineering breakeven of three. The key technical innovation is to use high-temperature superconducting magnets in place of ITER's low-temperature superconducting magnets. The proposed device would be about half the diameter of the ITER reactor and cheaper to build.
The China Fusion Engineering Test Reactor, or CFETR, is a proposed tokamak fusion reactor, which uses a magnetic field in order to confine plasma and generate energy. As of 2015, tokamak devices are leading candidates for the construction of a viable and practical thermonuclear fusion reactor. These reactors may be used to generate sustainable energy while ensuring a lower environmental impact and a smaller carbon footprint than fossil fuel-based power plants.
The history of nuclear fusion began early in the 20th century as an inquiry into how stars powered themselves and expanded to incorporate a broad inquiry into the nature of matter and energy, as potential applications expanded to include warfare, energy production and rocket propulsion.
Abstract: The paper gives a short overview on tungsten (W) coatings deposited by various methods on carbon materials (carbon fibre composite – CFC and fine grain graphite – FGG). Vacuum Plasma Spray (VPS), Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD)... A particular attention is paid to the Combined Magnetron Sputtering and Ion Implantation (CMSII) technique, which was developed during the last 4 years from laboratory to industrial scale and it is successfully applied for W coating (10–15 μm and 20–25 μm) of more than 2500 tiles for the ITER-like Wall project at JET and ASDEX Upgrade.... Experimentally, W/Mo coatings with a thickness up to 50 μm were produced and successfully tested in the GLADIS ion beam facility up to 23 MW/m2. Keywords: Tungsten coating; Carbon fibre composite (CFC); ITER-like wall; Magnetron sputtering; Ion implantation