Fusion Wall Development Research by Neutron Depth Profiling

In an inertial fusion energy (IFE) reactor, the plasma-facing surface of the chamber, the first wall, is subjected to high doses of light ions and high temperatures produced by
²H + ³H → ⁴He + n (DT) fusion reactions. Tungsten is a promising armor material for the first wall surface but its longevity is limited by degradation. Accumulation of helium in the wall results in bubble formation causing blistering and repeated surface exfoliation. Therefore, it is important to understand the characteristics of tungsten as a function of helium dose and temperature at fusion conditions. The objective of this study was to irradiate a novel nano-tungsten material using the helium threat energy spectrum and study the helium distribution by Neutron Depth Profiling (NDP) following systematic heat treatments.

The minimum first wall operating temperature is approximately 750 °C and can routinely spike to 2400 °C. In our previous studies of single crystal, polycrystalline, and amorphous tungsten, surface blistering occurred with 1.3 MeV He ions at doses around 10²¹/m², and exfoliation of tungsten occurred at does around 10²²/m². When such a high dose is implanted in equally divided smaller doses (100 steps of 10¹⁸/m² He to reach a total dose of 10²⁰/m² He) where the material was held at 850°C then flash heated to 2000°C for 10 s after each implantation step, the results showed reduced helium retention. In this study, tungsten samples were prepared by vacuum plasma spray (VPS) using micrometer size (20 μm to 50 μm) feedstock powders sprayed onto steel substrates. The substrates were removed using etching techniques. The upper surface of the W was treated to create nano-sized cavities with the theory that the structure would facilitate rapid He release. 

Figure 1. Photomicrograph of a nano-tungsten coating deposited by Scott O'Dell of Plasma Processes, Inc.

To generate the threat spectrum, a 900 keV He⁺ beam generated from AN-2500 Van de Graaff accelerator was transmitted through a 1.5 μm aluminum foil. From energy loss during transmission through the aluminum foil, the He energy decreased and became broader. The foil was tilted from 0 to 60 degrees to the sample, thereby generating a series of He energy spectra and the He threat spectrum was generated. A computer program controlled the foil tilt (≈0.5 degree accuracy), duration, sample temperature, and dosimetry simultaneously. Samples were resistively heated while monitoring the temperature using an infrared thermometer. Samples preparation consisted of a cyclic procedure of implanting a fraction of the helium threat spectrum at 850 °C ± 10 °C then flash heating to 2000 °C ± 20 °C for 10 s thus introducing a total dose of 10²⁰ or 10²² He/m². Some samples were implanted with the total dose in a single step and heated to 2000 °C for a time equivalent to multiple steps.

Each implanted sample was analyzed by NDP technique, which utilizes the ³He(n,p)T reaction (5333 barns). This reaction simultaneously produces 572 keV protons and 191 keV recoil tritons. Measurement of the proton energy spectrum establishes the ³He depth distribution from the energy-dependent stopping power of tungsten.

Figure 2 is a ³He NDP spectrum for nano-cavity tungsten samples implanted with 10^{20 3}He/m² at 850 °C (•). Another nano-cavity tungsten sample (▲) was heated to 2000 °C 100 times for 10 s each, cooling down to 850 °C between the implantation steps. This is in contrast to heating continuously to 2000 °C for a time equivalent to 100 steps of 10 s. By taking this approach it was ensured that changes happening in the nano-cavity microstructures are not affecting the helium retention in tungsten. The sample heated in steps lost 74% He. The results of the same dose implanted in one step but heated continually for time equivalent to 100 steps heating showed 85 % loss of ³He, which is comparable to the stepwise heating in this work.

The results show that nano-cavity W heated to 2000 °C for a longer time either in short cycles or one long heating drives most of the ³He out through the nano-cavities and reduces bubble formations and exfoliation, clearly improving the longevity of the first wall armor. The results are more promising than previous results obtained with single and poly crystalline tungsten (≈26% loss in both cases). The next series of experiments with nano-cavity microstructure tungsten will examine the upper limit of ³He fluence introduced in a single cycle at which the material starts retaining ³He before degradation of the surface occurs.

• Measured helium mass and depth distribution in tungsten film    
• Identified potential new material for construction of first wall fusion reactors