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Investigation of Helium Bubble Behavior in Plasma-Facing Micro-Engineered Tungsten: Theory & Experiment


Understanding the interaction mechanisms between low-energy helium plasma and solid materials has significant implications in several advanced technologies; primary amongst them are fusion energy and space electric propulsion. Since the penetration of low-energy helium is very shallow, it may be advantageous to manipulate the surface of the solid by micro-engineering techniques so as to provide a degree of control over the resulting plasma-induced damage. Helium plasma exposure of tungsten in fusion devices causes extensive damage due to the creation of nanometer-scale subsurface bubbles. We investigate here the formation mechanisms of helium bubbles in tungsten that has been micro-engineered utilizing both theory and experiments.

Experiments on four different micro-engineered surfaces were conducted at the PISCES facility at the University of California, San Diego (UCSD), where they were exposed to various fluences of low-energy helium plasma, with planar surface as control. These samples included two surfaces covered with uniform micro-pillars; one with cylindrical pillars 1 micron in diameter and 25 micron in height, and one with dendritic conical pillars 4 - 10 micron in diameter and 20 micron in height. Additionally, two samples with reticulated open-cell foam geometry, one at 45 pores per inch (PPI), and the other at 80 PPI were fabricated with Chemical Vapor Deposition (CVD). The samples were exposed to helium plasma at 30 - 100 eV ion energy, 823 - 1123 K temperature, and 5 x 10^25 - 2 x 10^26 m^-2 ion fluence. Results show that whereas nanometer-scale tendrils (fuzz) would grow on planar tungsten surfaces, the fuzz formation on micro-engineered surfaces was greatly reduced due to backscattering of ions and the increased surface area. A 20% decrease in the ion incident angle on pillar type surfaces leads to ~30% decrease in bubble size, down to 30 nm in diameter. W fuzz was found to be absent from pillar sides due to high ion reflection. In foam samples, 28% higher PPI is observed to have 24.7% - 36.7% taller fuzz, and 17.0% - 25.0% larger subsurface bubbles. These are found to be an order of magnitude smaller than those in planar surfaces of similar environment. The helium bubble density was found to increase with ion energy in pillars, roughly from 8.2% to 48.4%, and to increase with increasing PPI, from 36.4% - 116.2%, and with bubble concentrations up to 9.1 x 10^21 m^-3. Geometric shadowing effects in or near surface ligaments were observed in all foam samples, with near absence of helium bubbles or fuzz in deeper layers of the foam.

Then a multiscale model of helium bubble evolution in plasma-facing materials is developed. The model links different stages of helium bubble evolution: deposition, nucleation, growth, motion, and coalescence. Helium deposition is simulated with the SRIM Monte Carlo program to give spatial information on helium and displacement damage distributions near the surface. This deposition profile is then introduced into a space-dependent rate theory of bubble nucleation and growth to describe the early stages of the distribution and size of helium bubbles. The coarsening stage of bubble evolution as a result of whole bubble motion, interaction, and coalescence is modeled by a new Object Kinetic Monte Carlo (OKMC) model, for which initial conditions are taken from the mean-field rate theory calculations. The model is compared to experimental data on low-energy helium plasma interaction with micro-engineered tungsten (W), and on high-energy helium ion deposition in flat W samples. The novel features of the multiscale model are: (1) space-dependent rate theory; (2) OKMC model of bubble motion in stress and temperature fields; and (3) application of the model to micro-engineered materials, and comparison with experiments on the same time-scale. At low helium ion energy, it is found that the mechanism of trap mutation is essential in achieving good agreement with experimental measurements. On the other hand, good agreement with experiments at high incident ion energy and temperature showed the importance of bubble coalescence and coarsening as main mechanisms. The results of the model are compared with experiments on flat W surfaces irradiated at high ion energy (30 keV), and with micro-engineered W, results are compared with the above mentioned experimental exposure on micro-pillars at 100 eV. The predicted average bubble radius and density are in qualitative agreement with experimental results.

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