Energy Threshold of Brittle Destruction for Carbon Based Materials

B. N. Bazylev^a, Y. Koza^b, I.S. Landman^a, J. Linke^a S. E. Pestchanyi^a H. Wuerz^a

^aForschungszentrum Karlsruhe, Institute for Pulsed Power and Microwave Technology, P.O. Box 3640, 76021 Karlsruhe, Germany

^bForschungszentrum Juelich, EURATOM-Association, 52425 Juelich, Germany

In the future tokamak ITER the carbon fiber composites (CFC) are foreseen as one of divertor armour materials. The main deficiency of CFC is that their brittle destruction (BD) can occur at the heat fluxes of impacting SOL plasma above 0.5 GW/m², which are typical of the tokamak operation in the ELMy H-mode. These rather large heat loads applied to the NS31 and NB31 CFC targets are successfully simulated at the electron beam facility JUDITH for pulse- and quasi-stationary regimes on the spatial scale of 2 cm by means of scanning the e-beam.

To support these experimental investigations of BD, in this work a semi-empirical three-dimensional (3D) model based on the energy threshold of BD is developed and applied for analysis of the graphite erosion obtained at JUDITH in wide ranges of heat flux and pulse duration. Earlier [1] a 2D-version of this model demonstrated a rather good agreement both with graphite erosion at JEBIS and, on a small scale, with the advanced model [2] that can be used for simulation of CFC only on the scale of less than 0.1 mm. Now the real experimental conditions at JUDITH taking into account the e-beam sweeping are modeled. The effective thermal conductivity, which takes into account the crack formation in the matrix of CFC [3], is implemented.

The joint experimental and numerical work confirmed that BD is a threshold phenomenon both for fine grain graphite and the CFC materials, and the BD threshold of 10 MJ/kg is verified for the fine grain graphite R6650. From the JUDITH experiments and the numerical simulation dedicated to different experimental regimes including cyclic pulses of a few milliseconds follows that this BD threshold is rather constant, which allows application of the threshold model to the much shorter ELMy time scale of 0.1 – 0.5 ms.

References

[1] H.Wuerz et al, Vertical Target and FW Erosion during Off-Normal Events and Impurity Production and Transport during ELMs typical for ITER-FEAT. J.Nucl.Mater. Vol. 290-293 (2001), pp. 1138-1143.

[2] S. E.Pestchanyi, H. Wuerz, Brittle Destruction of Carbon Based Materials under off-normal ITER-FEAT Conditions. Physica Scripta, Vol 91, (2001), pp. 84-89

[3] S. E.Pestchanyi, I.S. Landman, Effective thermal conductivity of graphite materials with cracks. These Proceedings.