Effects of hydrogen permeation on W, Mo and Cu Langmuir probes at ISTTOK
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1125-R07-04
Effects of hydrogen permeation on W, Mo and Cu Langmuir probes at ISTTOK D. Nunes1,3, P.A. Carvalho1,3, R. Mateus1, I.D. Nogueira3, A. Moita de Deus3, J.B. Correia2, N. Shohoji2, R.B. Gomes1, H. Fernandes1, C. Silva1, N. Franco4, E. Alves4 1
Euratom/IST, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 2 LNEG, Departamento de Materiais e Tecnologias de Produção, Estrada do Paço do Lumiar, 1649-038 Lisboa, Portugal 3 ICEMS, Departamento de Engenharia de Materiais, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 4 ITN, Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal
ABSTRACT The microstructures of tungsten, molybdenum and copper wires used as Langmuir probes at ISTTOK edge plasma have been investigated. The probes cross-sections evidenced extensive grain growth, intergranular bubbles and increased hardness at the plasma exposed regions. Internal surfaces of large bubbles exhibited slip bands resulting from plastic deformation induced by high H2 pressure. Elastic recoil detection analysis was used to measure H concentration profiles. The present results suggest that H2 bubble formation in first wall components under long-term high thermal loads should be closely monitored in nuclear fusion devices. Strategies for H damage mitigation are proposed and discussed. INTRODUCTION Both reliability and long lifetime are essential requirements for materials of first wall components. Tungsten presents great potential to fulfill these requirements due to its high melting point, high threshold for sputtering and good heat load capability [1]. These characteristics combined with low neutron activation and low tritium inventory render tungsten as potentially suitable for high flux components and high-power density structural applications in fusion reactors. Molybdenum, a similar refractory metal, is also considered a potential plasma facing material [2]. Copper alloys, on the other hand, have been chosen as heat-sink materials for ITER first wall panels due to their favorable thermal conductivity, mechanical strength and radiation resistance [3]. However, a comprehensive understanding of the damage caused by H plasmas to these materials is still required. Energetic hydrogen ions are able to penetrate the surface barrier of metals and this ion driven permeation process can exceed gas driven permeation by several orders of magnitude. The solubility of hydrogen isotopes in tungsten, molybdenum and copper is low and hydrogen atoms are likely to come out of solution at traps such as vacancies, voids and grain boundaries, where they achieve a lower potential energy than that attained among crystalline sites [4]. The traps eventually become saturated and, even for the low gas pressure in the fusion chamber, untrapped hydrogen atoms diffuse out of the implant zone deeper into the material. At saturated traps, molecular recombination tends to occur resulting in bubbles and blisters with increasing
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