Estimation of carbon fibre composites as ITER divertor armour

Carbon fibre composites (CFC) of NB31 and NS31 grades developed for the divertor armour of the future tokamak ITER have shown a high thermal conductivity and a low erosion rate appropriate for the tokamak stationary regimes with characteristic temperatures of 1000-2000 K. However, for the expected slow transient loads of 20 MW/m2 simulation experiments of Bonal et al on electron beam facilities demonstrated high erosion rates of NB31 in the temperature range of 3000 - 3500 K. At such temperatures, brittle destruction erosion of fine grain graphite armour is much smaller. As demonstrated by numerical simulations for NB31 with the Pegasus-3D code, the high erosion rate of NB31 is due to the new erosion mechanism. The local overheating erosion mechanism (LOEM) exists in rather complex structure of CFC, consists of fibre framework and carbon matrix. The discovered LOEM is due to the preferential cracking on the fibre surface and thermal isolation of the fibres parallel to the armour surface from matrix. A large difference of fibre and matrix coefficients of thermal expansion is essential for LOEM. At the QSPA facility in TRINITI, new experiments simulating ITER type I ELMs heat load are performed. ... mehrPreferential erosion of the fibre bundles parallel to the heated surface of NB31 is demonstrated, which substantiates the LOEM model. Thermophysical properties of the CFC fibres and matrix material at high temperatures are hardly known and their measurements are difficult. Comparison of experimental results with dedicated numerical simulation and the new estimations of NB31 properties are performed. The Pegasus-3D calculations for off-normal event conditions are carried out and the conclusions on the CFC applicability for ITER are drawn.