Hydrogen Trapping Studies in Fusion First Wall Materials
Hydrogen trapping in the first wall of fusion reactors is one of the most serious materials problems facing the designers of magnetic confinement devices. The release of this trapped hydrogen from the near surface region of the first wall during a tokamak discharge has been shown to play a major role in the hydrogen fueling of today's plasmas, while trapped hydrogen isotopes will directly influence the on-site tritium inventory and the structural properties of future D-T reactors. This paper summarizes the laboratory ion implantation sttidies on the hydrogen trapping characteristics of first wall materials. Emphasis is placed on the near term materials problems associated with austenitic stainless steels that are currently in use in magnetic confinement devices. Hydrogen trapping experiments involving more exotic first wall structural and coating materials are briefly reviewed.
Hydrogen traps in the outgassing model of a stainless steel vacuum chamber
This article describes a model for hydrogen outgassing into a stainless steel vacuum chamber. It accounts for the geometry of the chamber components, the hydrogen dissolved in those components, and the processes of diffusion, recombination, and trapping. Strongly bound or “trapped” hydrogen, which occurs at heterogeneities such as dislocations and grain boundaries, can hold most of the dissolved hydrogen even though those locations comprise fewer than 0.1% of all lattice sites. Four simplifications allowed practical use of the model: (1) Each component was described as a one-dimensional object. (2) The hydrogen initially dissolved in each component was described as a uniform concentration. (3) Accurate, consistent values were used to describe diffusion and recombination in stainless steel types 304 and 316 [Grant et al., J. Nucl. Mater. 149, 180 (1987); 152, 139 (1988)]. (4) Only one type of hydrogen trap was considered, and trapping was ignored in components made from vacuum remelted stainless steel. The simple model was developed and validated by comparing it to outgassing measurements. Traps were required to describe the outgassing from a component made of drawn stainless steel 304. The initial hydrogen concentration in that component was comparable to concentrations found elsewhere by thermal desorption and almost 100 times larger than in the components made of vacuum remelted 316 stainless steel. The model's usefulness was illustrated by using it to predict the outgassing of a vacuum chamber made of type 304 stainless steel.
30 keV Ion Bombardment Apparatus for Study of Interaction of Light Ions with Surfaces
An ultrahigh vacuum system is described which was designed to study particles emitted from surfaces under bombardment by hydrogen ions. Stress has been laid on obtaining pressures ∼10−9 Torr in the presence of the ion beam. The beam is mass analyzed and its energy is variable from 5 to 30 keV. A maximum current of 300 μA over 0.1 cm2 is obtainable at the target, so that a beam/neutral gas bombardment ratio of better than 1000/1 can be achieved. A target mounting turntable is described which allows up to six targets to be bombarded in turn. A quadrupole mass filter is incorporated in the target chamber as a residual gas analyzer and as a means of identifying gas species resulting from the bombardment of surfaces.