STUDY OF HELIUM RETENTION IN NANO-CAVITY TUNGSTEN AS A FIRST WALL IN A FUSION CHAMBER USING NEUTRON DEPTH PROFILING

Lei Cao ^{1, 2}, Nalin Parikh³, and R. Gregory Downing¹

1. Chemical Science & Technology Laboratory, National Institute of  Standards and Technology, Gaithersburg, MD 20899, USA

2. Department of Material and Engineering, University of Maryland, College Park, MD 20742, USA

3. University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA

         A long-standing challenge to the inertial fusion energy (IFE) reactor is the longevity of material used for the chamber’s inner wall, called the first wall. This wall is exposed to high temperature helium-3 produced by the DT fusion reaction ²H+³H->⁴He + n. Tungsten is a favorable material for the first wall but blistering caused by accumulated helium and repeated surface exfoliation are immediate threats to its durability. Nano-cavity tungsten has been proposed as first wall armor to reduce bubble formations based upon the assumption that nano cavities provide channels for helium gas exit the material.

         This study simulated the helium exposure of tungsten using a ³He+ beam generated from a Van de Graaff accelerator. The implanted beam is similar to the actual threat, which has an energy spectrum between 50 to 500 keV and a dose rate of 5 x 10²² He/m²/day. The tungsten sample was prepared by vacuum plasma spray (VPS) and then implanted with ³He+ for a total dose of 10^{20 3}He/m² at 850 °C followed by various heating cycles. Neutron Depth Profiling (NDP), located at the NIST Center for Neutron Research, was then used to determine the ³He concentration as a function of depth by detecting charge particles emitted after ³He(n,p)T reaction. The 572 keV protons were used in this research to establish the depth scale. The NDP results show that nano-cavity tungsten heated to 2000 °C for a longer time either in short cycles or one long heating has reduced ³He retention significantly, which verifies the driving mechanism of ³He by nano-cavities. As a result, the bubble formations and exfoliation are inhibited and the longevity of the first wall armor is expected to improve.

CATEGORY: Chemistry

Mentors Name: R. Gregory Downing

Division: Analytical Chemistry Division

Laboratory: CSTL

Room: B173, Building: 235, Mail stop: 8395

Tel: 301-975-8862

Fax: 301-208-9279

Email:Raymond.cao@nist.gov

Is your mentor a Sigma Xi Member?     Yes