Improvement of the CFC structure to withstand high heat flux

S. PESTCHANYI AND I. LANDMAN.

Forschungszentrum Karlsruhe, Institute for Pulsed Power and Microwave Technology

P.B. 3640, D-76021, Karlsruhe, Germany

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 1000-1500 K [1]. However, experimental and numerical investigation of these CFC erosion under extreme heat flux at not normal ITER events, like edge localized modes (ELM), and vertical displacement events (VDE) revealed their high erosion rates by brittle destruction in the temperature range of 3000 - 4000 K. This high erosion rate is explained by the new erosion mechanism due to local CFC overheating (LOEM) and enhanced brittle destruction of the overheated sites [2]. LOEM exists due to the complex structure of CFC, which consists of carbon matrix and carbon fibre reinforcement. Main component of the reinforcement, the pitch fibres, perpendicular to the surface, provides high heat conductivity of CFC. A large difference of fibre and matrix coefficients of thermal expansion is essential for LOEM. Enhancement of erosion in LOEM is due to the preferential cracking on the interfaces between fibres and matrix followed by thermal isolation of the PAN fibres from the matrix.

This work deals with the CFC structure optimisation with respect to the erosion rate minimisation. From the results of the numerical simulations done in [2] and in this work the conclusion follows that for a significant improvement of the CFC erosion resistance against the ITER off normal events (ELMs, VDEs and disruptions) the structure of CFC should have as less fibres parallel to the armour surface (e.g. weaving and needling fibres) as possible. The pitch fibres, which provide high thermal conductivity of CFC, do not cause erosion enhancement. The best way would be to have one-dimensional CFC with the pitch fibres perpendicular to the surface only. But, one-dimensional CFCs have too weak breaking strength and cleave asunder under high heat flux. The simulations performed using the code PEGASUS-3D [2] allowed proposition of a new three-dimensional CFC structure of minor distinction from NB31, but without its weak elements that cause the erosion enhancement. Simulation of the newly proposed CFC structure confirmed several times smaller erosion rate. Besides, one can expect that new CFC keep good enough macroscopic breaking strength due to the very small modification of CFC structure.

[1] G. Federici et al; Key ITER plasma edge and plasma–material interaction issues. Journal of Nuclear Materials Volumes 313-316, March 2003, 11-22

[2] S. Pestchanyi et al, 3-D simulation of macroscopic erosion of CFC under ITER off-normal heat loads, Fusion Engineering and Design. V. 66-68C pp. 271-276.

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