Ion Beam Materials Analysis and Modifications Group

University of North Carolina at Chapel Hill

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 27599-3255, USAb Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6138, USA

c National Institute of Standards and Technology, Gaithersburg, MD 20899-3460, USA

HAPL Meeting, ORNL, March 22, 2006

IFE Reaction Chamber

DT pellet

He

Tungsten

• Helium threat spectrum most damaging of all ionic debris

• First wall temperature ~850°C with periodic spikes to ~2500°C

# of ions vs. energy (keV)

Energy spectrum of slow debris ions in the IFE reactor

UCSD ARIES program on fusion energy technologyJ. Perkins, http://aries.ucsd.edu/aries/wdocs/aries-ife/spectra/

# of ions vs. ion energy (keV)

Energy spectrum of fast ions in the IFE reactor

UCSD ARIES program on fusion energy technologyJ. Perkins, http://aries.ucsd.edu/aries/wdocs/aries-ife/spectra/

IFE Helium Threat Spectrum

0 200 400 600 800 1000 1200 1400

Energy (keV)

Rel

ativ

e H

e D

ose

A summation of the fast and slow He ions

• Implanted helium is trapped and accumulates to form stable bubblesBubbles 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

SEM of blistered tungsten

Previous studies with monoenergetic helium

• 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

Sample

beam

NDP Experimental Arrangement

NDNDPP

NDP of boron in siliconNDP of boron in siliconDepth range: 15 nm – 3.8 µmDepth range: 15 nm – 3.8 µm

Sample Dimension

TXRF

NDP

XRFRBS

Det

ectio

n lim

it (a

t/cm

3 )

TOF-SIMS

Dynamic SIMS

FTIR

1000 Å 1µm 10 µm 100 µm 1 mm 1 cm

1e22

1e20

1e18

1e16

1e14

1e12

Neutron monitor

At. Cross section Abundance Recoil Mass Particle BranchNo. Element Reaction (barns)‡ (or at/mCi) T1/2 Energy (keV) Energy (keV) Ratio

3 He 3He(n,p)3H 5331 1.37E-06 - 191.291 572.4656 Li 6Li(n,a)3H 940.3 0.075 - 2727.877 2055.51510 B* 10B(n,a)7Li 3837 0.199 - 1013.126 1775.868 6.28%10 B* 10B(n,a)7Li 3837 0.199 - 839.635 1471.763 93.70%14 N 14N(n,p)14C 1.83 0.99634 - 42.02 583.85117 O 17O(n,a)14C 0.24(?) 3.80E-04 - 404.064 1413.63233 S 33S(n,a)30Si 0.1686 0.0075 - 411.534 3081.80435 Cl 35Cl(n,p)35S 0.489 0.7577 - 17.233 597.93140 K 40K(n,p)40Ar 4.4 (0.39 α) 1.17E-04 1.277 E9 y 56.259 2230.7817 Be 7Be(n,p)7Li ~ 48000 2.45E+14 53.12 d 206.523 1437.71822 Na† 22Na(n,p)22Ne 27800 4.38E+15 2.6019 y 158.831 3465.797 0.84%22 Na† 22Na(n,p)22Ne 27800 4.38E+15 2.6019 y 102.979 2247.069 99.10%59 Ni 59Ni(n,a)56Fe ~ 12.3 1.28E+20 76000 y 340.316 4755.791

210 Po 210Po->a->206Pb --- 6.38E+14 138.376 d 103.077 5304.381* A 477.595 (3) keV (IAEA) g emission occurs in 93.9 % of the 10B reactions† A 1274.58 keV g emission occurs in XX % of the 22Na reactions

Neutron Depth Profiling Neutron Reactions of Merit

Tungsten Target780 keV deuterons

12.6 m Mylar foil

Preamplifier Amplifier MCA

~13 MeV protons, ~2 MeV alphas, backscattered deuterons

3He profiledepth = 1.7 m

1500 m depletion depthdetector at 155 with respect to the incident beam direction

3He(d, p)4He nuclear reaction analysis

• Used proton yield from the reaction to compare helium retention

Less retention with cyclic implantation and annealing

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1E+16 1E+17 1E+18 1E+19Dose/Cycle (He/m2)

Rel

ativ

e 3 H

e re

tent

ion

Single Crystal

Polycrystalline

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.

• Implanted 1019 3He/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

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

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

E0 He beam

Foil Tungsten

E = E0 – Efoil

t

θ cost d

• Transmitted energy is approximated as a Gaussian centered at E i = (E0 – Efoil)

and broadened by the energy straggling through the foil

2

2i

2σ)E(E

e2πσ

1G(E)

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)

i

iijj )E(Gw)f(E

)f(E

)f(E

(E)G

(E)G

ww

wwww

n

1

n

1

nnn1

21

1n1211

IFE Helium Threat Spectrum

0 200 400 600 800 1000 1200 1400Energy (keV)

Rela

tive

He

Dos

e f(E)

approximation

Computing the solution

• Many of the matrix elements will be zero because Gaussians far away from E i 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 timeTherefore, 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

i

ii )E(Gw)f(E

Scattering Chamber / Electronics

Control Panel / Bending Magnet

2.5 MV Van de Graaff

Energy degrader foil and sample holder

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 keVAl 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.

SampleID

Foil thickness

(m)

Tilt angle (degrees)

Effectivethickness

t / cos (m)

M41 1.5 0 1.5

M42 41 2.0

M43 3.0 0 3.0

M44 31 3.5

M45 41 4.0

M46 4.5 0 4.5

M47 26 5.0

Efoil and from Neutron Depth Profiling

• NDP uses 3He(n, p)T reaction to measure the helium depth profileNumber of protons is proportional to helium concentrationDetected proton energy converted to depth scale by energy loss

• Projected range Rp and the longitudinal straggle Rp related to Efoil and

0.0E+004.0E+198.0E+191.2E+201.6E+202.0E+20

0 0.5 1 1.5 2Depth (microns)

Con

cent

ratio

n (3 H

e/m

3 )

Helium depth profile for tungsten implanted with 1.3 MeV 3He to a dose of 1020 He/m2

Tungsten

Helium

neutrons

protons

Rp

Rp

UNC 3He Implant into Tungsten Coupons(Neutron Depth Profiles with Background removed)

Channel No.800 1000 1200 1400 1600 1800 2000 2200 2400

Arb

itrar

y C

once

ntra

tion

Uni

ts

2e-4

4e-4

6e-4

8e-4

1e-3

M47

M46

M43M44

M45 NDP Reaction3He(n,p)3H

SampleSurface

UNC 3He Implant into Tungsten Coupons(Neutron Depth Profiles with Background removed)

Channel No.400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Arb

itrar

y C

once

ntra

tion

Uni

ts

2e-4

4e-4

6e-4

8e-4

1e-3

M47 M46 M45 M44 M43 M42M41

NDP Reaction3He(n,p)3H

SampleSurface

0

500

1000

1500

2000

2500

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Energy (MeV)

Bac

ksca

tter C

ount

s

1.5 m Al

D+ beam

Au

Al E1

E4

E1

E4

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

1.7 MeV deuterium backscattering spectrum for 1.5 m Al foil target with Au energy markers

System resolution is E1 = (EDet2 + EBeam

2)1/2 = 25 keV

Measured straggle of the transmitted beam is E42 = E2 + EDet

2 + EBeam2 = 46 keV

Energy straggling due to the degrader foil alone E = (E42 – E1

2)1/2 = 39 keV

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 foilseach 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.

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

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

TDS study of helium implanted 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.

TDS Data: unimplanted polycrystalline tungsten

0.00E+00

1.00E-11

2.00E-11

3.00E-11

4.00E-11

5.00E-11

6.00E-11

7.00E-11

8.00E-11

9.00E-11

1.00E-10

0 200 400 600 800 1000

Time (s)

3 He

Par

tial P

ress

ure

(Tor

r)

0.00E+00

5.00E-11

1.00E-10

1.50E-10

2.00E-10

2.50E-10

3.00E-10

3.50E-10

4.00E-10

4.50E-10

5.00E-10

0 200 400 600 800 1000 1200

4 He P

artia

l Pre

ssur

e (T

orr)

TDS Data: Poly W implanted with 3x1020 4He/m2 at RT

Time (s)

Temp. (°C)RT 600 2000 2200

• 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

• 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

0.00E+00

5.00E-10

1.00E-09

1.50E-09

2.00E-09

2.50E-09

3.00E-09

0 500 1000 1500

3 He

Part

ial P

ress

ure

(Tor

r)

Time (s)

Temp. (°C)RT 600 2000 2200

TDS Data: Poly W implanted with 5x1020 3He/m2 at RT

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

AcknowledgementThis 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) 491-495.

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) 289-297.