BLANC - Programme blanc

Carbone et Autres Matériaux pour ITER – CAMITER

Submission summary

Scientific description - - - A list of acronyms is added in annexe 1 - - Annexe 2 is the "FICHE DE VISA pour proposition ANR" written for the CEA administration. - - 1 - Background and objective - - ITER (International Thermonuclear Experimental Reactor) is the project to check the feasibility of controlled magnetic fusion as an energy source for future, the next step being reactor DEMO. Tokamak ITER will be built in Cadarache, the place where is operating Tore Supra, the French tokamak. Tokamaks are toroidal machines where a complex magnetic structure confines a hot plasma to achieve fusion between D (deuterium) and T (tritium) nuclei. Using tritium needs serious safety environment. In Tore Supra, only D is used and in what follows, the word "hydrogen" often generically designates the tokamak fuel (i.e. D or T isotopes). - In the neighbouring of tokamak walls, the plasma is colder in the core but Plasma Facing Components nevertheless receive high particle and heat fluxes. Carbon is the best choice to face these fluxes due to its thermal and mechanical properties. Its low atomic number is also an important advantage, limiting the drawbacks in case of plasma pollution. For this reason, carbon is widely used for PFC and is also the nowadays retained element for the most exposed parts in ITER, whereas tungsten and beryllium are the two other materials retained for the wall. - In Tore Supra, there is a limiter which is a 15 m2 surface covered with tiles of a carbon/carbon composite usually named CFC (Carbon Fiber Composite), limiting the plasma at the bottom of the chamber. This limiter is actively cooled by a pressurized water loop to ensure a limited temperature increase during the discharges. This configuration allows long duration discharges, Tore Supra therefore setting energy record. - Carbon has nevertheless major drawbacks which are the complex erosion/deposition phenomena occurring under irradiation. Particle fluxes lead to physical erosion (sputtering of carbon) or chemical erosion (erosion of species where carbon is chemically bonded with hydrogen) of the carbon PFC, and deposition (of carbon) or co-deposition (of carbon and hydrogen) are observed in various locations in the machine. Depending on the edge plasma conditions, the deposits can be more or less amorphous and more or less rich in hydrogen. The existence of such deposits is unacceptable for safety reasons in the case of ITER, for which they will be tritiated. - Controlling deposition through controlling plasma wall interaction is therefore a central issue for magnetic fusion. - Besides, in Tore Supra, the particle balance analysis has shown that surprisingly, the walls act as a reservoir, half of the injected hydrogen being continuously retained. Three mechanisms can be at the origin of this wall retention: (i) implantation, (ii) co-deposition and (iii) diffusion. Implantation concerns only the first nanometers of the wall and therefore rapidly saturates, contrary to co-deposition which "creates" a new wall material with hydrogenated deposits. Quantitative estimation has been done for Tore Supra both by measuring the D/C ratio of the deposits and by estimating their total amount. This amount is far from enough (by nearly one order of magnitude) to explain the observed retention. Diffusion was expected to be negligible but laboratory experiments have shown that diffusion far in the bulk can occur for irradiated CFC in case of large fluence. - - It is therefore crucial to develop research along the three following axes: (i) understanding the phenomena occurring with carbon (wall and deposits), (ii) studying the new compounds which could be created mixing carbon, tungsten, beryllium, hydrogen..., and (iii) exploring alternative materials. - - The objective of the project is therefore to study the fusion relevant mechanisms of plasma/surface interaction, and to bridge the gap between fundamental work on atom or ion interacting with perfect surfaces, and work conducted ...

Project coordination

Pascale ROUBIN (Université)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.



Help of the ANR 693,430 euros
Beginning and duration of the scientific project: - 48 Months

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