Materials breakthrough improves the timeline of plasma stability, brings endless energy closer.

This month in France, a donut-shaped fusion reactor the size of an eight-foot box called WEST has upgraded from a carbon interior to one made of tungsten, an improvement which successfully contained plasma hotter and longer than ever.

Compared to this latest result, our own sun burns at temperatures 30% less while still powering grid-scale systems running on solar panels whose efficiency maxes out at half that percentage – when harnessed, nuclear fusion presents exponentially more potential energy than is being commercially produced today, green or otherwise.

Carbon was originally used as the material for the inner walls of tokamak reactors – a Russian acronym for "toroidal chamber with magnetic coils" – but tungsten has quickly surpassed the mineral in this application.

Tungsten is an incredibly heat-resistant metal encasement that allows physicists to sustain hotter plasmas for longer and at higher energies and densities than carbon.

Fusion vs. fission

Fusion is a reaction by which atomic nuclei are combined (using the hydrogen isotopes of deuterium and tritium as fuel), producing a new, larger atomic element and a wealth of excess energy.

Fusion is a cleaner process than today's use of nuclear fission, which generates less energy and much more radioactive waste by splitting rather than combining fuel (uranium or plutonium) atoms.

Fusion is also inherently a fail-safe process; if the reaction is interrupted, fusion stops instantly, and it is impossible for such a reactor to have a meltdown. However, nuclear fusion is the same reaction that powers stars, which is still too energy-intensive to create and sustain without better equipment.

Carbon to tungsten

Tokamaks have been developed to enable controlled fusion reactions in a vacuum contained by a magnetic field. Because the temperature needed to make experimental fusion reactions possible is in the order of 100 million degrees Celsius, the interior wall of the vessel must be protected from extreme heat.

Historically, this was done using carbon materials, which have a very high melting point, but the atomic porosity of carbon itself retained a buildup of too much tritium particulate, which posed safety risks.

While carbon may not be the best material for reactors, tungsten has its own challenges; maintaining one of the highest melting points on the periodic table, it can become brittle. Even a small piece can cool the plasma, throwing off the entire process. Hence, tungsten-heavy alloys are also being studied to remedy this.

"The tungsten-wall environment is far more challenging than using carbon," Luis Delgado-Aparicio, the head of advanced projects at the Princeton Plasma Physics Laboratory, a partner on the WEST project, said. "This is, simply, the difference between trying to grab your kitten at home versus trying to pet the wildest lion."

A team of scientists at Pacific Northwest National Laboratory and Virginia Polytechnic Institute outlined the idea of petting the lion in a 2023 paper that makes a case for the use of tungsten-heavy alloys in advanced nuclear fusion reactors.

The next big thing

The walls of a fusion reactor are designed to channel excess fuel particles toward a reinforced chamber called the divertor, removing those diverted particles and any excess heat to protect the tokamak.

Situated at the bottom of the vacuum vessel, the divertor acts like an exhaust port, extracting heat and ash produced in the fusion reaction, minimizing plasma contamination, and protecting the integrity of the surrounding walls.

In April, the Korea Institute of Fusion Energy (KFE) announced that its KSTAR fusion reactor successfully sustained plasma at 100 million degrees Celsius thanks in large part to its tungsten divertor.

Previously, KSTAR demonstrated high-performance plasma operation for 30 seconds with temperatures over 100 million degrees, and now the goal is to achieve 300 seconds by the end of 2026 with this new divertor.

All this research will also feed into improved experiments run by the International Thermonuclear Experimental Reactor (ITER,) the world's largest industrial-scale nuclear fusion reactor, under construction since 2013 with the help of 35 nations bent on solving issues standing in the way of clean, global nuclear power.

"In KSTAR, we have implemented a divertor with tungsten material, which is also the choice made in ITER. We will strive to contribute our best efforts in obtaining the necessary data for ITER through KSTAR experiments," said KFE President Dr. Suk Jae Yoo.

Now, WEST has reported its own major breakthrough: its tungsten-clad tokamak successfully sustained a plasma reaction at 50 million degrees Celsius for a full six minutes at higher energies and densities than its carbon counterparts.

Like KSTAR, the WEST reactor's primary goal is to lay groundwork for ITER, also nearby in France, which hopes to finish construction by 2025.

Worldwide gains

All these reactors are part of the International Atomic Energy Agency's group for the Coordination on International Challenges on Long duration OPeration (CICLOP) program's objectives to gain experience with steady-state and long pulse operation, as the informative results will be essential to ITER and future international experiments.

ITER recently decided to make the switch from beryllium to tungsten for its own inner wall, anticipating further data collected from WEST will prove useful as ITER becomes fully operational.

"We will have the ability to measure the tungsten inside the plasma and to understand the transport of tungsten from the wall to the core of the plasma," said CICLOP Chair Xavier Litaudon.

Experiments to test the melting behavior of tungsten have additionally been run on the UK's retired JET tokamak, another CICLOP member, all of which will contribute key insights into the operation of tungsten divertors and their impacts on plasma performance.

It may seem like science fiction, but the science is sound, and continued international cooperation will help to speedily drive this and other game-changing technologies forward step by step into a global paradigm change, guaranteeing a long-term zero-carbon future.