Fuel retention study in fusion reactor walls by micro-NRA deuterium mapping

Nuclear Reaction Analysis (NRA) with a He-3 ion beam is a powerful analytical technique for analysis of light elements in thin films. The main motivation for 3He focused beam applications is lateral mapping of deuterium using the nuclear reaction D(He-3,p)He-4 in surfaces exposed to a tokamak plasma, where a lateral resolution in the pm-range provides unique information for fuel retention studies. At the microprobe at the Jozef Stefan Institute typical helium ion currents of 300 pA and beam dimensions of 4 x 4 mu m(2) can be obtained. This work is focused on micro-NRA studies of plasma-facing materials using a set-up consisting of a silicon partially depleted charge particle detector for NRA spectroscopy applied in parallel with a permanently installed X-ray detector, an RBS detector and a beam chopper for ion dose monitoring. A method for absolute deuterium quantification is described. In addition, plasma-deposited amorphous deuterated carbon thin films (a-C:D) with known D content were used as a reference. The method was used to study deuterium fuel retention in carbon fibre composite materials exposed to a deuterium plasma in the Tore Supra and TEXTOR tokamaks. The high lateral resolution of micro-NRA allowed us to make a detailed study of the influence of topography on the fuel retention process. We demonstrated that the surface topography plays a dominant role in the retention of deuterium. The deep surfaces inside the castellation gaps showed approximately two orders of magnitude lower deuterium concentrations than in areas close to the exposed surface. (C) 2011 Elsevier B.V. All rights reserved.