1. Towards a self-consistent assessment of He effects in W armored IFE first walls
Brian D. Wirth*,
with valuable conversations and input from
HAPL MWG
Nuclear Engineering Department
University of California, Berkeley
Presented at
HAPL Ion Transport and Surface-Thermomechanics
in W and SiC Armor Workshop
15 May 2006 *

2. Challenge
•  He insoluble, binds (interacts) with any impurity or extended defect in the microstructure - governed by strain, rather than chemical effects
•  Microstructure constantly evolving as a consequence of both IFE threat spectrum (ion irradiation, thermal pulses, chemistry changes, and dpa)
• Microstructure is constantly evolving, e.g., dislocation density, grain size, radiation defects, impurity concentration, precipitation (He & tungsten carbides?) - thereby providing both trapping (suppressed diffusivity) or fast migration paths (dislocations/grain boundaries)
• Objective is to map available experimental/computational techniques onto the IFE threat spectrum/temperature history *
* A.R. Raffray et al., Fusion Engineering & Design 81 (2006) 1627.

3. Experimental techniques
•  Thermal Helium Desorption Spectroscopy - He ion implantation followed by controlled thermal annealing in high vacuum with quadropole mass spectrometer - (Casper, van Veen & co-workers (DELFT), Kornelsen & Van Gorkum, UCB) - provides a series of He release peaks versus temperature - determination of underlying mechanisms is a challenge, although activation energies of release peaks easily determined by measurements with different heating rates
• Nuclear Reaction Analysis/Neutron Depth Profiling - enables measurement of the He depth profile (covered by Prof. Parikh) - good technique for comparing with spatial dependent modeling
• Positron annihilation spectroscopy - positron lifetime measures the time between positron implantation and detecting the annihilation photon provides information on vacancy/vacancy cluster microstructure, coincident Doppler broadening measures the momentum of the electron-positron pair at annihilation and provides information on chemical environment at annihilation site (sensitivity to noble gas unknown)
• TEM/in-situ microscopy - provides insight into bubble mitigated exfoliation and dislocation, grain boundary, cavity/bubble microstructure

4. Computational modeling techniques
•  Atomistic modeling (molecular dynamics) - provides short time dynamics of He - microstructure interaction - confirms rapid He migration as an interstitial, strong binding of He with vacancies and substitutional impurities, self-interstitial recombination & kickout mechanism with substitutional He, and trapping with extended defects
• Kinetic Monte Carlo - provides spatially dependent, atomic-scale defect/impurity diffusion - based on jump/binding energies of the species.
• Kinetic rate theory/cluster dynamics - He concentration and bubble profiles

5. What is known about He in Tungsten
•  Low migration energy for interstitial He - (Em = 0.24 ± 0.04 eV) motion between octahedral & tetrahedral interstitial positions
• Strong binding of He with impurity and micrsotructure traps:
- He - He (interstitial), Eb ~ 1.0 eV (atomistic modeling, Henriksson(2005)), Edis = Eb + Em ~ 1.3 eV
- He - solutes, Eb ~ eV (In, Ag, Xe, Ar, Ne, THDS. Kolk (1985))
- He - dislocation, Edis ~ 2 eV (positron annihilation, Subrahmanyam (1994))
- He - vacancy, Eb = 4.2 ± 0.2 eV, Edis = 4.5 ± 0.4 eV (THDS, atomistic modeling)
- He migration along dislocation, Em ~ 0.35 eV (De Hosson (1976))
- He - grain boundary binding & migration - unknown (Kurtz et al. studied He in grain boundary and dislocation cores in Fe)
- He2V - Edis = 3.2 ± 0.2 eV
- He3V - Edis = 2.8 ± 0.3 eV (THDS, atomistic modeling)
- He4V - Edis = 2.6 ± 0.3 eV
- He - cavity/bubble, He - carbide precipitate: trap efficiency, binding energy?
• Tungsten point defect properties:
- Self-interstitial migration, Em ~ 0.05 eV
- Vacancy formation energy, Ef = 3.6 ± 0.2 eV
- Vacancy migration energy, Em = 1.8 ± 0.2 eV, di-vacancy migration similar
- Di-vacancy binding energy, Eb ~ 0.7 eV

6. What is known about He in Tungsten
* Landolt-Bornstein, Volume 25, Atomic Defects in Metals

7. Needs
•  Parametric studies combining modeling techniques (flavors of rate theory, atomistics, kinetic Monte Carlo) - focus on understanding He/trap microstructure dependence on ion flux/energy spectrum & time at temperature
• Additional experiments
- He ion implantation & thermal anneals, with measurement of He release and He concentration profile by Neutron Depth Profiling
- Fundamental He - defect interactions: He implantation, THDS, atomistic modeling?
- Pulsed IEC experiments: higher temperatures, lower ion currents (dose rates)?

8. Citations
J.Th.M. de Hosson, A.W. Sleeswyk, L.M. Caspers, W. van Hengten, and A. van Veen, Solid State Commun. 18 (1976) 479.
K.O.E. Henriksson, K. Nordlund, A. Krasheninnikov, and J. Keinonen, App. Phys. Lett 87 (2005)
G.J. van der Kolk, A. van Veen, L.M. Caspers, and J. Th.M. de Hosson, J. Nuc Mat 127 (1985) 56.
V.S. Subrahmanyam, P.M.G. Nambissan and P. Sec, Solid State Comm 89 (1994) 523.
A. van Veen, Mat Sci Forum (1987) 3.
Landolt-Bornstein, Group III, Volume 25