RF-Specific Tungsten Erosion and Global Transport in ITER under Neon Seeding

Inside the massive, doughnut-shaped machine known as a fusion reactor, scientists are trying to recreate the power of the sun. To do this, they must heat a cloud of gas, called plasma, to temperatures far hotter than the core of the sun. One of the most powerful tools for heating this plasma is a system that blasts it with high-frequency radio waves, similar to the energy used in a microwave but on a vastly different scale. However, these powerful waves can sometimes behave unpredictably when they hit the metal walls of the reactor. Just as a radio signal can create static on a speaker, these waves can build up strong electric fields right at the surface of the machine's components. If these fields become too strong, they can act like a violent accelerator, slamming atoms from the reactor walls into the plasma and potentially poisoning the reaction.

The International Thermonuclear Experimental Reactor, or ITER, is the world's largest attempt to build a working fusion power plant. It will use a special metal called tungsten for its inner walls because the metal can withstand extreme heat. But tungsten is heavy, and even a tiny amount of it floating in the super-hot plasma can cool the reaction down and stop the energy production. Scientists have long known that the radio waves used to heat the plasma could cause the tungsten walls to wear away, but they lacked a complete picture of exactly how much damage would occur or where the worn-off material would go. A new study by researchers from Oak Ridge National Laboratory, the ITER Organization, and several other institutions has finally filled in these missing pieces. By building a sophisticated computer model that simulates the entire reactor environment, they have predicted exactly how the radio waves interact with the walls and how much tungsten will be eroded under the specific conditions ITER plans to use.

The researchers focused on a scenario where the reactor is filled with neon gas, a technique used to help manage the intense heat flowing out of the plasma. In this environment, the radio waves create a complex, invisible electric field right next to the antenna that sends the waves into the plasma. This field acts like a ramp, accelerating ions—charged atoms of the neon gas and the fuel itself—toward the tungsten walls. When these fast-moving ions hit the wall, they knock tungsten atoms loose. The team used a detailed simulation framework called STRIPE to track this process from start to finish. They combined a model of the plasma flow with calculations of how the radio waves behave, how the ions accelerate, and how the knocked-off tungsten atoms move through the machine.

The results of the simulation were striking. Under the influence of the radio waves, the electric fields near the antenna walls can reach voltages of up to 3,000 volts. This is far higher than the natural electric fields that exist in the plasma without the radio waves. Because of this extra push, the amount of tungsten being knocked off the walls increases dramatically. The study found that the total amount of tungsten eroded from the antenna area is about 64 times greater than it would be if the radio waves were not present. This creates a source of roughly 3.34 billion billion tungsten atoms every second. However, the story does not end with the erosion. The researchers also tracked where these atoms went. They found that about 90 percent of the eroded tungsten escapes the immediate area and travels into the rest of the reactor, while only about 10 percent lands back on the wall nearby.

Perhaps the most surprising finding was not just how much tungsten was lost, but what was doing the knocking. Scientists had assumed that the most highly charged neon atoms would be the main culprits because they would gain the most speed from the electric fields. Instead, the simulation showed that a mix of neon atoms with lower and medium levels of charge were responsible for the majority of the damage. This happened because there were simply far more of these moderately charged atoms hitting the wall, even though each individual atom hit with less force than the highly charged ones. The study also revealed that the location of the worst damage was not exactly where the electric field was strongest. The erosion was highest where the strong electric field overlapped with a high density of incoming plasma particles. It was the combination of a strong push and a heavy crowd of particles that caused the most wear, rather than the electric field alone.

Despite the massive increase in erosion caused by the radio waves, the researchers concluded that this specific source of tungsten is unlikely to be the main problem for the reactor. When they compared the amount of tungsten coming from the antenna to the amount coming from the rest of the reactor walls, the antenna contribution was tiny. The erosion from the antenna was about 1,000 times smaller than the erosion from the bottom of the reactor (the divertor) and more than 100 times smaller than the erosion from the main chamber walls. Even with the radio waves making the antenna a much more active source of tungsten, the total amount of metal entering the plasma from this area remains small enough that it should not dominate the overall contamination of the reactor.

The study also looked at how the tungsten atoms moved through the reactor after being knocked loose. After just a fraction of a second, about 22 percent of the mobile tungsten atoms that had entered the plasma were found in the confined region where the fusion reaction happens. This corresponds to a very low concentration, suggesting that while the radio waves do create a local source of impurities, the reactor's natural flow of plasma is effective at diluting them. The work demonstrates that predicting the behavior of a fusion reactor requires looking at the entire system together. You cannot simply calculate how much a wall erodes in one spot; you must understand how the radio waves, the plasma flow, the magnetic fields, and the movement of impurities all interact. By successfully linking these different physical processes, the researchers have provided a powerful new tool for designing future reactors. Their findings suggest that while the radio waves will cause significant local wear on the antenna, they will not overwhelm the reactor with tungsten, giving engineers confidence that the ITER design can manage these interactions effectively.