Energy: Fusion, from ENEA innovative laser

ENEA, in collaboration with some European research institutions, has developed an innovative laser system that, for the first time in the world, has allowed the chemical composition of the internal walls of the Joint European Torus (JET), one of the largest experimental nuclear fusion reactors ever built, to be studied in-situ, after an experimental campaign.

The activity was funded by the EUROFUSION Consortium and involved, in addition to ENEA as a one of the leading partners, prestigious institutions, including: the research institutions of Germany and Finland Forschungszentrum Jülich and VTT Technical Research, the United Kingdom Atomic Energy Authority (UKAEA), the Polish Institute of Plasma Physics and Laser Microfusion in Warsaw (IPPLM), as well as some European universities such as Comenius University in Bratislava (Slovakia), the University of Tartu (Estonia) and the Institute of Solid State Physics of the University of Latvia.

Specifically, the research team demonstrated the feasibility of the LIBS (Laser-Induced-Breakdown Spectroscopy) technique for remote and real-time monitoring of components exposed to plasma, without removing or manipulating them for analyses. The adopted technique allows for a qualitative and quantitative analysis of the internal walls of the reactor, also detecting the presence of Hydrogen and its isotopes, deuterium and tritium ^[1], the reactor fuel.

Compact, lightweight and versatile, the system created by ENEA and the European partners was positioned inside the vacuum chamber of JET, where it performed analyses in hundreds of points, demonstrating that the technique can be used on any type of sample of the first wall of the reactors and that it is able to preserve them, substantially intact, for subsequent analyses.

Installed on the remote-controlled robotic arm of JET, the LIBS system hits the target sample, generating a small explosion that vaporizes a microscopic portion of it. The atoms of the target, thus vaporized, emit a light that, analyzed by a spectrometric system, determines its chemical composition.

“In future nuclear fusion devices such as ITER and DEMO, of which JET was the most representative precursor, the plasma can cause erosion in some parts of the surface of the internal wall of the reactor, the eroded material can be transported far from the area of ​​origin and deposited in other areas of the wall”, explains Salvatore Almaviva, researcher at the ENEA Nuclear Department of the Frascati research center, involved in the study. “The experiments indicate that the material deposited on the surfaces is composed of residues of the structural materials of the walls of the chamber and of the reactor fuel itself, namely deuterium and tritium: the latter, in particular, must be carefully monitored to reduce risks in operations and minimize the amount of fuel immobilized in the reactor structures. The technique that has proven effective on JET could also play an important role for future reactors under construction or in the design phase such as ITER, DTT and DEMO”, concludes Almaviva.

Until now, this type of monitoring was carried out through laboratory analysis at the end of the experimental campaigns, but in the next generation reactors, in-situ monitoring will be essential, conducted without removing the elements of the internal wall, so as not to compromise the operation of the reactor by avoiding its shutdown for long periods.

In addition to the development and use of the LIBS system, ENEA is also playing a leading role in the processing of experimental results which, from a preliminary analysis, highlights a low contamination of the internal components of the JET with tritium, but clearly highlights the phenomena of erosion and redeposition of the materials of the internal walls described above.

ENEA is at the forefront in this sector also for its many years of pioneering experience in the development of LIBS systems for fusion devices: the first operational prototype of this kind was in fact developed and tested for the first time ever on the experimental reactor FTU (Frascati Tokamak Upgrade) operational until a few years ago at the ENEA research center in Frascati.

Further details on the project results will be presented at the 20th International Conference on Materials and Components for Fusion Applications to be held in May 2025 in Slovenia.