Inside a fusion reactor there are areas where the plasma touches the wall, namely on the divertor target plates. There, the wall tiles experience very high heat loads and particle fluxes. Hence, among other properties, the wall material must have very high thermal conductivity and its erosion under the particle beam should be as low as possible. Currently, tungsten and fibrereinforced carbon (CFC) are the two considered options for ITER.

Disruptions and ELMs (see MHD article in this issue) are expected to occur at ITER, both causing high heat loads at the wall. With its high thermal conductivity and good mechanical properties, CFC offers the best properties with regard to withstanding stress of this nature and is therefore the reference option when it comes to starting ITER and learning how to master the powerful plasma. Also, due to the low atomic mass, carbon atoms do not disturb the plasma when eroded from the wall. Carbon erosion occurs mainly due to chemical erosion under hydrogen bombardment from the plasma, with the formation of volatile hydrocarbons. But erosion also causes carbon dust that accumulates at the divertor and which must be cleaned with high effort. Moreover, the dust can create hot spots on the wall, as has been shown at Tore Supra. Additionally, re-deposited carbon dust has an affinity for hydrogen and thus captures hydrogen, deuterium and tritium, the latter being lost as fuel for the fusion reaction. Because of these effects, it is predicted that CFC will be used as wall material for the ITER divertor target plates during the initial (nonactivated) phase of ITER only.

Particle emission patterns of undoped graphite (left) and Ti-doped graphite (right) during electron beam thermal shock loading under typical disruption conditions.

CFC is expensive and has long delivery times. Graphite is a more readily available carbon form, but so far has been considered unsuitable due to its low thermal conductivity and poor mechanical strength, in addition to the above mentioned erosion problems. Material scientists at the Spanish Centro de Estudios e Investigaciones Técnicas de Gipuzkoa (CEIT) have now found a way of overcoming these issues. This is achieved by doping graphite with titanium.

Among other promising properties, the new material shows higher thermal conductivity (220 W/mK at room temperature). Moreover, the Ti-doped graphite is isotropic, therefore its thermal conductivity is the same in all directions, whereas for CFC it is high only in one direction. Another important finding about Tidoped graphite is the reduced chemical erosion of the material, which is a factor of 4 to 10 less than that of pure carbon. Erosion experiments conducted at IPP Garching and thermal shock resistance tests done at Jülich have both yielded much better characteristics than pure carbon materials. Thermal shock resistance tests conducted at Jülich revealed similar behaviours as with CFC and much better performance than that of undoped graphite. Next, CEIT will assess the reproducibility of these properties by up-scaling to larger dimensions. CEIT will also look for an industrial partner willing to commercialise the new material.

Thanks to Carmen Garcia- Rosales from CEIT for her contribution to this article.

For more details on this work, please see Carmen Garcia-Rosales’ article on the EFDA website: graphites.pdf