Material surface damage under the high heat fluxes typical of ELM bursts and disruptions

 

I.S. Landman1, V.M. Safronov2, B.N. Bazylev1, and S.E. Pestchanyi1

 

1.             Forschungszentrum Karlsruhe (FZK), Institute for Pulsed Power and Microwave Technology, P. B. 3640, 76021 Karlsruhe, Germany

2.             State Research Centre of Russian Federation Troitsk Institute for Innovation and Fusion Research (TRINITI), 142190, Troitsk, Moscow region, Russia

 

The divertor armor materials for the future tokamak ITER are expected to be carbon and tungsten. Carbon is going to be used as fiber composites (CFC) and tungsten either as brush-like structures or as thin plates. If assuming averaged heat loads, the thermo-mechanical properties of the armor combined of these components seems to be acceptable and provide rather low erosion rates. However, large disruptive pulse loads in which the heat flux W can exceed 102 GW/m2 on the time scale t of 1 ms, or tokamak operation in the ELMy H-mode at repetitive loads with W larger than 5 GW/m2 and t of 0.3 ms, can significantly deteriorate armors good performance, which highlights the importance of material investigations at these regimes.

In this work, the joint numerical and experimental investigations carried out in FZK and TRINITI of the erosion mechanisms at the mentioned heat fluxes are surveyed. The numerical modeling is based on the three-dimensional (3D) anisotropic thermo-mechanics code Pegasus-3D aimed for simulation of CFC, the surface melt motion code MEMOS-1.5D running now for tungsten targets also in the regimes with repetitive ELMs, and the radiation-magneto-hydrodynamic code FOREV-2D. FOREV-2D was newly upgraded by multi-fluid description of the impacting plasma and by the ITER magnetic field configuration, which provided adequate modeling of the off-normal heat loads. The tokamak simulation experiment is carried out at the plasma gun facilities MK-200 and QSPA which produce hydrogen streams in a rather wide range of pulse loads, with W from 10 to 3102 GW/m2 and t from 0.03 to 0.5 ms.

Essential experimental and numerical results are presented, such as testing of CFC NB31 and NS31 at the plasma guns both for disruption- and ELM-like heat loads and the application to these tests of the local overheating model of the brittle destruction of CFC, which was earlier suggested based on calculations with Pegasus. For the tungsten targets the testing experiments are discussed and a comparative analysis of the vaporization- and melt motion erosion mechanisms is given on the base of MEMOS calculations for repetitive high heat flux events. Numerical simulation predictions for the ITER off-normal regimes are produced based on heat loads obtained with FOREV and on validation of the numerical tools by comparison of the experimental and theoretical results.