Estimation of CFCs as the divertor material
S. Pestchanyia, I. Landman and V. Safronov1
Forschungszentrum Karlsruhe, Institut fuer
Hochleistungsimpuls – und Mikrowellentechnik Postfach 36 40 D-76021 Karlsruhe,
Germany
1. State research centre of RF TRINITI, 142190, Troitsk, Russia
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. Preferential 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.
a)
corresponding author, e-mail address
sergey.pestchanyi@ihm.fzk.de
phone +49 (0)7247 82 3408, fax +49
(0)7247 82 4874