Plasma-Facing Materials

Inside a fusion reactor, there is a material that stands closest to the burning plasma. It is the frontline wall known as the plasma-facing material (PFM). This page walks through, in order, why choosing this material is so difficult, the history of how the leading role shifted from carbon to beryllium and then to tungsten, the physics of how materials break down, and finally the change in thinking represented by liquid metals.

Start with Intuition (High School)

Imagine placing a solid wall right next to a plasma hotter than the center of the Sun, exceeding 100 million degrees. It seems like it would melt in an instant if it touched directly. In reality, a magnetic field holds the plasma suspended and keeps it away from the wall, so the wall does not reach 100 million degrees. Even so, the heat and particles leaking out of the plasma rain down on the wall, and its surface is exposed to an extremely harsh environment.

Let us organize what a plasma-facing material must do using some familiar analogies.

First, it must withstand heat. Just as a frying pan kept over a high flame should not melt, the material must hold its shape even under continuous intense heat. In the component called the divertor, which is designed to receive heat and particles in a concentrated way, this heat concentration becomes especially severe.

Second, it must resist erosion. When plasma particles strike the wall at high speed, wall atoms are knocked away, much as grains of sand slowly wear down a stone. This is called sputtering. If the wall erodes, its lifetime shortens, and the eroded atoms mix into the plasma and lower its temperature.

Third, it must not contaminate the plasma easily. When atoms released from the wall mix into the plasma, those atoms emit light and carry off energy. Heavier atoms in particular emit large amounts of light, so even a small admixture cools the plasma.

Fourth, it must not retain fuel easily. Fusion fuel uses a radioactive form of hydrogen called tritium. If the wall absorbs it and does not release it, precious fuel is lost, and radioactive material accumulates in the wall, creating a safety problem.

The tricky part is that these requirements often conflict with one another. Favoring one property comes at the expense of another. This is exactly why material selection has remained a major challenge in fusion.

Understand the Physics (Undergraduate)

From here, we look at several of these properties using equations and numbers.

How Much Heat Is Received

The heat flux $q$ received by the divertor surface reaches roughly $10$ to $20 \ \mathrm{MW/m^2}$ in steady-state operation. This is an intensity hundreds of times greater than the heat flux emitted by the surface of the Sun. Furthermore, during the periodic bursts at the plasma edge known as ELMs (edge-localized modes) and during disruptions, in which the plasma suddenly collapses, the flux can momentarily exceed $1 \ \mathrm{GW/m^2}$.

The rise in surface temperature depends strongly on the material’s thermal conductivity $\kappa$. For a steady heat flux $q$, the temperature difference $\Delta T$ between the front and back surfaces of a material of thickness $L$ can be roughly estimated as

$$\Delta T \approx \frac{q \, L}{\kappa}$$

The larger the thermal conductivity $\kappa$, the more the surface temperature rise can be suppressed for the same heat flux. Tungsten’s thermal conductivity is about $170 \ \mathrm{W/(m \cdot K)}$ at room temperature, which is high even among metals, and this is one reason it is chosen for high-heat-load regions.

How Much It Erodes

The ease with which sputtering occurs is expressed by the sputtering yield $Y$. This is a quantity indicating how many atoms, on average, are knocked out per incident particle. If $Y$ is $0.01$, it means that for every 100 particles that strike, 1 atom is eroded.

Physical sputtering is the phenomenon in which an incident particle mechanically knocks an atom away. It resembles a cue ball striking an object ball in billiards. Just as a minimum momentum is needed to knock the object ball away, the incident energy must exceed a certain value to knock out an atom. This minimum value is called the threshold energy. Heavier atoms, and atoms bound more strongly to the surface, require more energy to knock out, so the threshold is higher.

For tungsten, the threshold for deuterium incidence is high, about $200$ to $300 \ \mathrm{eV}$, so at the particle energies of a typical divertor plasma (roughly $5$ to $30 \ \mathrm{eV}$), physical sputtering hardly occurs. This is a major advantage. In contrast, light carbon has a low threshold and erodes even at low energies.

There is another mode of erosion called chemical sputtering. This is the phenomenon in which incident particles react chemically with wall atoms to form easily volatile molecules that fly away. A representative example is carbon and hydrogen reacting to form hydrocarbon gases such as methane. Because it occurs even at low energies, it became a weakness of carbon materials. Tungsten does not form stable compounds with hydrogen, so it is essentially immune to chemical sputtering.

How Much It Contaminates the Plasma

When impurity atoms released from the wall mix into the plasma core, they carry off energy through radiation. This radiative loss increases sharply as the atomic number $Z$ grows. Roughly speaking, you can think of the tolerable impurity concentration as dropping steeply as $Z$ increases.

Low-$Z$ materials such as carbon ($Z = 6$) and beryllium ($Z = 4$) do not cool the plasma much even if some mixes in. Conversely, high-$Z$ materials such as tungsten ($Z = 74$) cool the plasma when only a tiny amount (a concentration of about $10^{-4}$) mixes in. Here lies the fundamental conflict between erosion resistance (favoring high $Z$) and low contamination (favoring low $Z$).

Comparing the Three Main Materials

Let us organize this conflict in terms of the three materials that have played the leading role in history.

Carbon-based materials (graphite and carbon fiber composites, CFC) have no melting point and sublimate above about $3900 \ \mathrm{K}$, so there is no worry about melting and scattering, and being low $Z$ they do not readily contaminate the plasma. They were the star pupil and the standard choice for divertors until the 1990s, but they carried the fatal weaknesses of chemical sputtering and tritium accumulation.

Beryllium ($Z = 4$, melting point about $1560 \ \mathrm{K}$) is the lightest metal. Even when mixed into the plasma, its radiative loss is small, and it also has an oxygen-getter effect, bonding with oxygen to reduce residual oxygen. However, its melting point is low, and it is toxic.

Tungsten ($Z = 74$, melting point $3695 \ \mathrm{K}$) has the highest melting point of all metals, a high sputtering threshold, and retains almost no hydrogen. Its weaknesses are that, being high $Z$, it readily contaminates the plasma, and the embrittlement (becoming brittle) described later.

Deepen the Theory (Graduate)

Using the history of how the leading role shifted from carbon to tungsten as our axis, we dig deeper into the physics of how materials break down.

From Carbon to Tungsten: Why the Lead Changed

Carbon’s downfall was tritium co-deposition. Carbon ejected by chemical sputtering redeposits as a film in the low-temperature regions of the plasma, entraining hydrogen isotopes as it does. Because this co-deposition layer takes up tritium at a ratio of roughly one to one with hydrogen, tritium keeps accumulating inside the reactor as operation continues. In devices that handle large amounts of tritium, such as ITER, a safety upper limit on the in-vessel tritium inventory is set (about $700 \ \mathrm{g}$ for ITER), and with carbon this limit was predicted to be reached within a few hundred shots. This was the decisive reason ITER abandoned the use of carbon.

This prediction was confirmed on a real machine. From 2010 to 2011, Europe’s JET carried out a refit (the ITER-like wall) in which its previous carbon wall was completely replaced with a beryllium first wall and a tungsten divertor. As a result, the rate of tritium accumulation inside the vessel dropped to roughly one-twentieth of that in the carbon-wall era. This demonstration reinforced the decision to make ITER’s divertor from tungsten.

Note that ITER was initially designed to use beryllium for the first wall and tungsten for the divertor, but in subsequent design reviews consideration has proceeded toward expanding the scope of tungsten use. Here we describe, as an established fact, the path up to the choice of tungsten for the divertor.

Melting and Recrystallization Embrittlement

Although tungsten has a high melting point, if the melting point is locally exceeded during a disruption or the like, the surface melts. Molten metal moves under surface tension and electromagnetic forces, leaving behind unevenness and cracks after it solidifies. Melt splashing also becomes a source of impurities entering the plasma.

More troublesome is recrystallization, which proceeds at temperatures far below the melting point. Worked tungsten is made of fine crystal grains that hold internal strain, and in this state it has a certain degree of ductility. However, when heated to about $1500$ to $1700 \ \mathrm{K}$, the microstructure reorganizes into large, strain-free crystal grains. As the grains coarsen, grain boundaries decrease, the hardness drops, and at the same time the material becomes prone to cracking.

Tungsten has a ductile-to-brittle transition temperature (DBTT), a boundary above which it switches from a tough state to a brittle state. Tungsten’s DBTT is inherently higher than room temperature, making it a difficult material that is prone to cracking when handled at ordinary temperatures. Recrystallization pushes this DBTT even higher, increasing the risk of brittle fracture when operation is stopped and the material cools.

Helium Irradiation and Fuzz Structures

A phenomenon peculiar to tungsten is the formation of fuzz through helium irradiation. When helium ions produced by fusion burning rain down on the tungsten surface at relatively low energies (a few tens of $\mathrm{eV}$), the helium, unlike hydrogen, barely dissolves in tungsten and instead gathers at atomic vacancies to form tiny bubbles. When the surface temperature is in the range of about $1000$ to $2000 \ \mathrm{K}$, these bubbles grow and coalesce, and the surface transforms into a fluffy, cotton-like structure covered with countless nanometer-scale fibers. This is fuzz.

Because fuzz effectively makes the surface sponge-like, thermal conduction worsens, and the surface overheats easily even under a small amount of heat. There are also concerns that the brittle fibers may flake off into dust or become starting points for arc discharges. The conditions for fuzz formation and its subsequent behavior are an important theme, with research continuing that simulates actual reactor environments.

Combined Effects with Neutron Irradiation

The $14.1 \ \mathrm{MeV}$ neutrons produced in D-T reactions are electrically neutral, so they are not bent by the magnetic field and penetrate deep into the wall, knocking atoms away and creating cascades of defects within the lattice. This amount of damage is measured in a unit called dpa (displacements per atom), which expresses how many times, on average, each atom has been knocked out. For a DEMO-class divertor, this is estimated to reach $5$ to $10 \ \mathrm{dpa}$ per year.

Neutron irradiation harms materials in two ways. One is irradiation hardening, in which the defects that form impede the motion of dislocations, making the material hard and brittle, which raises the DBTT further. The other is that the reaction between neutrons and atomic nuclei (transmutation) generates helium even inside the reactor material, and this gathers at grain boundaries and weakens them.

What is especially important is that these effects act not in isolation but overlapping one another. At the surface, helium from the plasma creates fuzz; inside, neutrons produce defects and transmutation helium; and high heat loads drive recrystallization. Evaluating how a material behaves when multiple such degradation mechanisms proceed simultaneously, in an environment close to actual operating conditions, is the central difficulty of materials research.

Research Frontier (PhD)

We introduce the directions of current research, along with their keywords.

The development of tungsten alloys is one pillar. Oxide dispersion-strengthened tungsten (ODS-W) disperses fine oxide particles to suppress recrystallization and grain coarsening, seeking to maintain strength at high temperatures. W-Re alloys, made by adding rhenium to tungsten, improve ductility, but the fact that rhenium increases through transmutation under neutron irradiation, as well as resource constraints, are discussed as challenges. High-entropy alloys, which mix multiple elements in roughly equal amounts, and W_f/W composites (tungsten fiber-reinforced tungsten), which are reinforced with fibers to increase energy absorption at fracture, are also being studied as candidates for overcoming brittleness.

The idea that seeks to fundamentally avoid the limits of solid materials is the liquid metal divertor. If the surface is covered by a flow of liquid metal, a fresh surface is always supplied even as it erodes, so the concept of wear disappears, there is no worry about melting, and internal irradiation damage is washed away each time.

Lithium (Li) is being studied as a candidate liquid metal. Lithium is the lightest metal, so it does not readily contaminate the plasma even when mixed in, and it strongly adsorbs hydrogen, giving it a wall-pumping effect that suppresses particle recycling at the plasma edge. In devices such as NSTX, improvements in confinement performance have been reported with the introduction of a lithium wall. Another candidate, tin (Sn), is chemically stable and has a low vapor pressure, giving it the feature that contamination through evaporation into the plasma is easier to suppress.

How to hold and supply the liquid metal is also a research theme. A representative approach is the capillary porous system (CPS), which holds liquid metal by soaking it into porous tungsten or a metal mesh through capillary force, where the key is to control the liquid metal so that it does not scatter through the interaction of the magnetic field and electric current.

Efforts to obtain material data under realistic environments are also important. Because a powerful neutron source that reproduces the neutron spectrum of a fusion reactor on the ground does not yet exist, the IFMIF-DONES (International Fusion Materials Irradiation Facility - DEMO Oriented Neutron Source) project, which uses neutrons generated by an accelerator, is being advanced. Its purpose is to build the database of materials irradiated to the high dpa levels needed for DEMO design.

Check Your Understanding

Q1. In plasma-facing materials, why do erosion resistance and low contamination tend to conflict?

A Erosion resistance favors heavy materials with a large atomic number Z, but high-Z materials cause large radiative losses when they mix in and cool the plasma, making them unfavorable for low contamination B All erosion-resistant materials have low melting points, and all non-contaminating materials have high melting points C The lighter a material is, the more erosion-resistant it is, and at the same time its radiative loss increases D Both erosion resistance and low contamination are determined solely by thermal conductivity, so they cannot both be achieved

Q2. Which of the following correctly describes the difference between physical sputtering and chemical sputtering?

A Physical sputtering occurs through chemical reactions, and chemical sputtering occurs through mechanical knock-out B Physical sputtering is the phenomenon in which an incident particle mechanically knocks an atom away and requires a threshold energy, while chemical sputtering forms volatile molecules through chemical reactions and occurs even at low energies C They are the same in that neither occurs unless a threshold energy is exceeded D Physical sputtering is caused by neutrons, and chemical sputtering is caused by photons

Q3. What was the main reason ITER abandoned the use of carbon materials?

A Carbon has a low melting point and would melt under the heat flux of steady-state operation B Carbon has a large atomic number and cools the plasma with only a small admixture C Carbon accumulates large amounts of radioactive tritium in co-deposition layers, and it was predicted to reach the safety upper limit of the in-vessel tritium inventory within a short time D Carbon has such a high sputtering threshold that it does not erode at all and cannot be machined

Q4. Which is the correct combination of the conditions and problems for tungsten's recrystallization embrittlement and helium fuzz?

A Recrystallization coarsens crystal grains at about 1500 to 1700 K, raising the DBTT and making brittle fracture likely on cooling, while fuzz turns the surface into nanofibers under low-energy helium irradiation at about 1000 to 2000 K, causing degraded thermal conduction and dust generation B Both recrystallization and fuzz occur only when the melting point of 3695 K is exceeded, and melting is the problem in both cases C Recrystallization refines the crystal grains and strengthens the material, and fuzz enhances the surface thermal conduction, so both improve performance D Recrystallization occurs only under neutron irradiation, and fuzz occurs only under hydrogen irradiation

Q5. Which is a point where a liquid metal divertor is fundamentally advantageous over solid materials?

A It has higher thermal conductivity than solids, so the surface temperature does not rise at all B Because the surface is liquid and constantly renewed, the concept of wear disappears, there is no worry about melting, and internal irradiation damage is washed away each time C Because its atomic number is large, its sputtering threshold is high and it does not evaporate at all D It eliminates the need for a magnetic field, so it is fine to let the plasma touch the wall directly

Related Topics

• First Wall - The design of the first wall where plasma-facing materials are used
• Divertor - The component that receives heat and particles in a concentrated way
• Structural Materials - Materials that support the blanket and vacuum vessel