1. Helium Retention Studies in Tungsten
S. Gilliam a, S. Gidcumb a, N. Parikh a, J. Hunn b, L. Snead b, R. Downing c
a University of North Carolina at Chapel Hill, Chapel Hill, NC , USA
b Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN , USA
c National Institute of Standards and Technology, Gaithersburg, MD , USA
HAPL Meeting, ORNL, March 22, 2006
Ion Beam Materials Analysis and Modifications Group
University of North Carolina at Chapel Hill

2. IFE Reaction Chamber Tungsten He DT pellet
Helium threat spectrum most damaging of all ionic debris
First wall temperature ~850°C with periodic spikes to ~2500°C

3. Energy spectrum of slow debris ions in the IFE reactor
# of ions vs. energy (keV)
UCSD ARIES program on fusion energy technology
J. Perkins,

4. Energy spectrum of fast ions in the IFE reactor
# of ions vs. ion energy (keV)
UCSD ARIES program on fusion energy technology
J. Perkins,

5. A summation of the fast and slow He ions

6. Previous studies with monoenergetic helium
SEM of blistered tungsten
Implanted helium is trapped and accumulates to form stable bubbles Bubbles grow until the pressure blisters the surface
1.3 MeV 3He implanted at 850°C to a dose of 2 x 1021 He/m2 followed by a flash anneal at 2000°C

7. Neutron Depth Profiling
NDP
Neutron Depth Profiling
Si detectors
Technique: Neutron Depth Profiling (NDP) measures elemental concentration profiles up to a few micrometers in depth for elements that emit a charged particle following neutron capture. (R.G. Downing, et al., NIST J. Res. 98 (1993)109.)
Elements Analyzed: boron, lithium, helium, nitrogen and several additional light elements with less sensitivity.
Sample Environment: In an evacuated chamber, samples are irradiated with a beam of low energy neutrons. A small percentage of the emitted reaction particles are analyzed by surface barrier detectors to determine their number and individual energies.
Principles: The emission intensity is compared to a known standard to quantitatively determine the elemental concentration. The emitted particles lose energy at a predicable rate as they pass through the film; the total energy loss correlates to the depth of the reacting nucleus.
Advantage: NDP is non-destructive. NDP analysis allows repeatedly determinations of the sample volume following different treatments.
Neutron beam flux at sample: ~7.5x108 n/cm2-s
Beam area: from a few mm2 to ~110 mm2
Neutron monitor
Neutron q
Sample
beam
NDP Experimental Arrangement
1e22
1e20
1e18
1e16
1e14
1e12
NDP of boron in silicon
Depth range: 15 nm – 3.8 µm
FTIR
XRF
RBS
NDP
Detection limit (at/cm3)
TOF-SIMS
TXRF
Dynamic SIMS
1000 Å 1µm 10 µm 100 µm 1 mm 1 cm
Sample Dimension

8. Neutron Depth Profiling Neutron Reactions of Merit

9. 3He(d, p)4He nuclear reaction analysis
Tungsten Target
780 keV deuterons
3He profile
depth = 1.7 m
~13 MeV protons, ~2 MeV alphas, backscattered deuterons
12.6 m Mylar foil
Preamplifier
Amplifier
MCA
1500 m depletion depth
detector at 155 with respect
to the incident beam direction
Used proton yield from the reaction to compare helium retention

10. Less retention with cyclic implantation and annealing
Implanted He/m2 at 850°C followed by a flash anneal at 2000°C
Same total dose was implanted in 1, 10, 100, and 1000 cycles of implantation and annealing
Relative 3He retention for single crystal and polycrystalline tungsten with a total dose of 1019 He/m2. Percentage of retained 3He compared to implanting and annealing in a single cycle.

11. Current project objective
More accurately mimic the IFE reactor conditions to study effects of helium irradiation on the first wall.
Produce the IFE helium threat spectrum and implant tungsten samples

12. How do we produce a helium threat spectrum?
Degrade the monoenergetic beam by transmission through a thin Al foil
Tilting a single foil provides a range of degraded energies by varying the path length d through the foil material where  = 0° is normal incidence
Foil
Tungsten
E0 He beam
E = E0 – Efoil t
Transmitted energy is approximated as a Gaussian centered at Ei = (E0 – Efoil) and broadened by the energy straggling through the foil 

13. Approximating the threat spectrum
Helium threat spectrum is approximated as a function f(E)
Approximate f(E) as a linear combination of the Gaussian degraded energies where f(Ej) is a point on the profile, wij is a weighting coefficient, Gi is the ith Gaussian contribution to the jth point on the profile f(E)

14. Computing the solution
Many of the matrix elements will be zero because Gaussians far away from Ei won’t contribute to the point f(Ei)
Weighting coefficient matrix elements correlate the Gaussians to each other
Diagonalize W to find the weight for each individual Gaussian function so that the linear combination approximates the desired energy spectrum f(E)
Weighting coefficients determine the dose to implant and each Gaussian has an associated tilt angle
Assuming a constant beam current, then dose  time Therefore, we have tilt angle  vs. time
Apply a polynomial fit to this  vs. t plot and use the time derivatives (i.e. angular velocity and acceleration) to program the tilt position motor

15. Scattering Chamber / Electronics
2.5 MV Van de Graaff
Scattering Chamber / Electronics
Control Panel / Bending Magnet
Energy degrader foil and sample holder

16. Energy degraded 3He implantation
1.8 MeV He beam transmitted through Al foils ranging 1.5 to 5.5 microns thick Degraded energies: 1400 – 100 keV Al stopping power: ~300 keV/micron
Compare theoretical and experimental values of Efoil and  through foils
Implanted tungsten samples with 1.8 MeV 3He degraded by various foil thicknesses listed below. Dose was 1 x 1020 He/m2 for each sample.
Sample ID
Foil thickness (m)
Tilt angle  (degrees)
Effective thickness t / cos  (m)
M41
1.5
M42 41
2.0
M43
3.0
M44 31
3.5
M45
4.0
M46
4.5
M47 26
5.0

17. Efoil and  from Neutron Depth Profiling
NDP uses 3He(n, p)T reaction to measure the helium depth profile Number of protons is proportional to helium concentration Detected proton energy converted to depth scale by energy loss
Projected range Rp and the longitudinal straggle Rp related to Efoil and 
Tungsten
neutrons
Rp
protons
Helium Rp
Helium depth profile for tungsten implanted with 1.3 MeV 3He to a dose of 1020 He/m2

20. Efoil and  from Rutherford backscattering
Backscattering used to measure energy straggling through foils for comparison to theoretical predictions such as the Bohr model
The key is a heavy energy marker such as Au on each side of the target foil
System resolution is E1 = (EDet2 + EBeam2)1/2 = 25 keV
Measured straggle of the transmitted beam is E42 = E2 + EDet2 + EBeam2 = 46 keV
Energy straggling due to the degrader foil alone E = (E42 – E12)1/2 = 39 keV
D+ beam
1.5 m Al
Al E1 E4
E E4 Au
1.7 MeV deuterium backscattering spectrum for 1.5 m Al foil target with Au energy markers

21. Current varied energy implantation
Polycrystalline W implanted at 850°C with 3He to a dose of 5 x 1019 He/m2
1.7 MeV 3He beam transmitted through 1.5 and 3.0 m Al foils each tilted 0 – 60° to create a broad continuous range of energies
A second sample is to be generated under similar conditions except with periodic heating to 2000°C during implantation.
NDP analysis will allow measurement and comparison of resulting helium depth profiles in each case.

22. Where we are now
Programming required for all calculations and foil tilt motion is near completion
After we successfully produce the IFE helium threat spectrum
1) Implant tungsten samples with the helium threat spectrum to study surface blistering and retention characteristics
2) Introduce implantation at 850°C and flash annealing at 2000°C as we did with monoenergetic helium implantation

23. Thermal Desorption Spectroscopy
Wish to study thermal desorption of helium from tungsten and how it depends on implantation and flash heating parameters
Study single crystal and polycrystalline W to determine differences in desorption characteristics
Doses ranging from 1016 to 5 x 1020 He/m2 implanted at RT or 850°C
Residual gas analyzer (mass spectrometer) monitors He partial pressure while temperature is ramped from RT to ~2000°C
Temp. ramping rate typically ~2°C/s

24. TDS study of helium implanted tungsten

26. TDS Data: unimplanted polycrystalline tungsten
Unimplanted polycrystalline tungsten sample ramped from RT to 2200°C
Background partial pressure level of 3He remained constant (~5x10-12 Torr)
Mass 2 is always present in mass spectrometery scans
We have conducted TDS on 3He and 4He implanted W samples to determine if the tail of the mass 2 peak affects the mass 3 peak value
So far we conclude that the mass 2 peak tail is not a great concern.

27. TDS Data: Poly W implanted with 3x1020 4He/m2 at RT
Time (s)
Temp. (°C) RT
Ramped sample temperature from RT to 2200°C
Small pulses of desorbed He around 600°C
Observed significant He desorption above 2000°C which correlates to simultaneous blistering of the sample surface
Surface was blistered after completing the TDS experiment

28. TDS Data: Poly W implanted with 5x1020 3He/m2 at RT
Time (s)
Temp. (°C) RT
Ramped sample temperature from RT to 2200°C
Small pulses of desorbed He around 600 and 2000°C
Significant He desorption above 2000°C correlates to surface blistering
Higher partial pressure of 3He detected due to higher dose of 3He

29. Future work
Study helium irradiation surface damage (blistering) and retention characteristics of SiC and high porosity sprayed tungsten.
Automate heating and data collection of TDS
Carry out 3He desorption studies of single crystal, polycrystalline, and sprayed W, and also SiC to evaluate helium trapping characteristics

30. Acknowledgement This research is supported under the US Department of Energy High Average Power Laser Program managed by the Naval Reactor Laboratory through subcontract with the Oak Ridge National Laboratory.
Publications
S. Gilliam, S. Gidcumb, D. Forsythe, N. Parikh, J. Hunn, L. Snead, G. Lamaze, Helium retention and surface blistering characteristics of tungsten with regard to first wall conditions in an inertial fusion energy reactor, Nuclear Instruments and Methods B, 241 (2005) S. Gilliam, N. Parikh, S. Gidcumb, B. Patnaik, J. Hunn, L. Snead, G. Lamaze, Retention and surface blistering of helium irradiated tungsten as a first wall material, Journal of Nuclear Materials, 347 (2005)