Gas trapping and release in polycrystalline nickel preimplanted with helium
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INTRODUCTION
THE behavior of helium and hydrogen isotopes in materials is one of the important factors in the choice of materials for the first wall of thermonuclear fusion reactors. The interaction of the hydrogen plasma with the wall has a major influence on both the plasma performance and the properties of the first wall materials. Helium, which will build up in the first wall owing to several mechanisms, can drastically affect the recycling between the fuel plasma and the wall through an interaction of the hydrogen isotope atoms with helium bubbles or with helium-induced defects. Trapping and release processes will also affect the tritium inventory in the first wall, limiter/diverters, and tritium permeation to the coolant. It is well established that hydrogen may be trapped at particular defect sites such as inclusions, precipitates, grain boundaries, or dislocations. Direct evidence such as that obtained by autoradiography techniques, tl'21 and indirect evidence as in permeation experiments I31or deuterium depth profile measurementsta'sJ are numerous. One of the most important trapping effects was observed to be the result of an interaction between the hydrogen isotopes and the helium impurity, t6,7] To understand the helium effect, extensive investigations have been carried out into the microstructure changes introduced by helium implantation, t8'91 Many studies have been conducted to characterize the shape and dimension of the bubbles as well as to determine the helium density inside the bubbles. Transmission electron microscopy (TEM) observationst8-~~ show a microstructure which contains many small spherical bubbles in a diameter range of 1 to 2 nm. It was shown t81 that the helium gas bubbles lie on a superlattice which E. ABRAMOV, Doctor, and D. ELIEZER, Professor, are with the Department of Materials Science and Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. E. ABRAMOV is also at the Nuclear Research Centre-Negev, Beer-Sheva 84190, Israel. Manuscript submitted May 26, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
has an fcc structure with principal axes aligned with those of the metal matrix. The lattice constants based on diffraction from the superlattice planes for nickel and stainless steels were found to be 6 to 7 nm. In experimental works, tl~ the density of helium inside the bubbles was estimated to be 2 • 10 29 atm/m 3. Using the equation of state of helium to high p r e s s u r e , [121 o n e can find that the pressure inside the bubbles is about 3.5 x 101~N / m E (350 kbar), for which helium should be solid at room temperature. Wampler and Myers tL3j determined the trap strengths in nickel preimplanted with helium using a deuterium gas charging method. This technique avoids ion beam damage that occurs in experiments where deuterium charging by ion implantation is used. Wilson et al.lTi studied the dependence of trapped deuterium in steels on the preimplanted helium dose. They found that the deuterium trapping efficiency decreased significantly at helium concen
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