Evaluation of the high heat flux behaviour of metals and graphites by use of a hydrogen beam

Abstract

High heat fluxes to in-vessel components of fusion devices during normal operation and under abnormal operation conditions are one of the governing issues in materials selection for and design of such components. Their failure under high heat loads during service can severely influence the further operability of the entire fusion device. In order to determine the response of materials to high heat fluxes an experimental program was carried out on metals and carbon materials using the 10 MW Neutral Beam Injection Test Stand of the Institute of Plasma Physics of Nagoya University. Stainless steel, aluminium, copper, and molybdenum samples as well as 13 grades of fine grain graphites were subjected to hydrogen beam exposure with power densities of 16 to 106 MW/m2 and pulse durations of 50 to 950 ms. The threshold values for the occurance of damage and the resulting damage were determined and documented. The main damage observed on samples was melting, erosion, and crack formation. The high heat flux resistance of the materials tested were compared comprehensively with each other. Among metals, copper and molybdenum showed the highest heat flux resistance. Graphites with a low coefficient of thermal expansion and a coarse microstructure had the highest resistance against crack initiation and further crack propagation. Processes leading to material damage were identified and, in the case of damage on graphite, models of cracking and erosion processes were developed. The implication of the experimental results for material selection and design of first wall components under the high heat flux aspect was examined.