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 1020 3He/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.
Start Date:October 1, 2003
Lead Organizational Unit:mml
R. Gregory Downing
Related Programs and Projects:
Parikh, R. N., Parker, R., Downing, R. G., Cao, R. L., “High Dose of Helium Implanted in Nano-Cavity Tungsten to Evaluate Threshold of Surface Blistering due to He Bubble Formation,” Transaction of America Nuclear Society Summer Meeting, 98, Anaheim, CA, (2008) 416-417.
Downing, R. G., Parker, R., Scelle, R., Parikh, N., “Helium Retention in Nano-Cavity Tungsten Implanted with Helium Threat Spectrum Mimicking IFE Reactor Conditions,” Trans. Am. Nucl. Soc. 97 (2007), 317-318.
S. B. Gilliam, S. M. Gidcumb, D. G. Forsythe, N. R. Parikh, J. D. Hunn, L. L. Snead, G. P. Lamaze, “Helium Retention and Surface Blistering Characteristics of Tungsten with Regards to First Wall Conditions in an Inertial Fusion Energy Reactor,” Nucl. Inst. and Meth. B 241 (2005) 491.
S. B. Gilliam, S. M. Gidcumb, N. R. Parikh, D. G. Forsythe, B. K. Patnaik, J. D. Hunn, L. L. Snead, G. P. Lamaze, “Retention and Surface Blistering of Helium Irradiated Tungsten as a First Wall Material,” J. Nucl. Mater. 347 (2005) 289-297.
R. Gregory Downing