Tungsten Isotope Helps Investigate How to Armour Future Fusion Reactors

Tungsten (W) owns the highest melting point and lowest vapor pressure of all metals on the periodic table, as well as very high tensile strength, the properties that make it well-suited to take abuse for long periods of time, and a good candidate for armouring future fusion reactors. Zeke Unterberg and his team at the Department of Energy's Oak Ridge National Laboratory (ORNL) are currently working with this candidate metal.

Researchers are focused on understanding how W would work inside a fusion reactor, a device that heats light atoms to temperatures hotter than the sun's core so that they fuse and release energy. Hydrogen gas in a fusion reactor is converted into hydrogen plasma, which is then confined to a small area or laser by a strong magnetic field.

Unterberg, a senior researcher at ORNL's Fusion Energy Division, said: "You don't want to put something in your reactor that only lasts a couple of days. You want to have a sufficient lifetime. We put tungsten in areas where we anticipate there will be very high plasma bombardment."

In 2016, Unterberg and his team began experiments in the fusion reactor tokamak at the DIII-D National Fusion Facility at the DOE Office of Science user facility in San Diego, USA. The fusion reactor uses a magnetic field to contain the plasma loop.

They wanted to know whether W could be used to armor the tokamak's vacuum chamber for fusion reactors, protecting it from rapid destruction caused by the effects of plasma-without severely contaminating the plasma itself. This contamination, if not sufficiently managed, could eventually extinguish the fusion reaction.

Unterberg said: "We were trying to determine what areas in the chamber would be particularly bad: where the W was most likely to generate impurities that can contaminate the plasma."

To discover this, the researchers used the enriched tungsten isotope W-182 and unmodified isotope to track the corrosion, migration, and redeposition of W inside the divertor. Looking at the movement of W within the shunt-the area in the vacuum chamber designed to transfer plasma and impurities-gives them a clearer picture of how W is corroded from inside the tokamak and interacts with the plasma. The enriched W isotope has the same physical and chemical properties as conventional W.

This setup allowed the researchers to collect samples on special probes temporarily inserted in the chamber for measuring impurity flow to and from the vessel armor, which could give them a more precise idea of where the W that had leaked away from the divertor into the chamber had originated. Unterberg said, "using the enriched isotope gave us a unique fingerprint."

The Unterberg team found that, as they expected, when tungsten is placed far away from the high-flux hit point, exposure to low-frequency ELM with higher energy content and surface contact with each event greatly increases contamination possibility. the research team found that this divertor far-target region was more prone to contamination the SOL even though it generally has lower fluxes than the strike-point. These seemingly counterintuitive results are being confirmed by ongoing divertor modeling efforts in relation to this project and future experiments on DIII-D.

The team published research online earlier this year in the journal Nuclear Fusion. The research may immediately benefit the Joint European Torus, or JET, and ITER, now under construction in Cadarache, France, both of which use tungsten armor for the divertor for fusion reactors.

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