A. René Raffray UCSD With Contribution from S. O’Dell PPI HAPL Meeting

November 8-9, 2005

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“Engineering Analysis” of He Retention & Release Experiments to Determine Desirable Engineered W

Armor Microstructure

A. René RaffrayUCSD

With Contribution from S. O’DellPPI

HAPL MeetingUniversity of Rochester's Laboratory for

Laser EnergeticsRochester, NY

November 7-8, 2005

November 8-9, 2005

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He Implantation and Behavior in W Armor Quite Complex, Consisting of a Number

of Mechanisms

Bulk diffusion

ImplantedHe atom

Trapped He

Trapping Detrapping

Desorption

BULK W PORE or VOID

Porous W (~10-100 m)Fully dense W (~ 1 mm)Ferritic steel (~ 3 mm)

Coolant

Photon and ion threat from IFE microexplosion

1. M. S. Abd El Keriem, D. P. van der Werf and F. Pleiter, "Helium-vacancy interactions in tungsten," Physical review B, Vol. 47, No. 22, 14771-14777, June 1993.

2. W. D. Wilson and R. A. Johnson, in Interatomic Potentials and Simulation of Lattice Defects, edited by P. C. Gehlen, J. R. Beeler and R. I. Jaffee (Plenum New York, 1972), p375.

3. A. Wagner and D. N. Seidman, Phys. Rev. Letter 42, 515 (1979)

• Helium atoms in a metal may occupy either substitutional or interstitial sites. As interstitials, they are very mobile, but they will be trapped at lattice vacancies, impurities, and vacancy-impurity

complexes. • The following activation energies were estimated for different He processes in tungsten [1,2]:

- Helium formation energy: 5.47 eV

- Helium migration energy: 0.24 eV- He vacancy binding energy: 4.15 eV- He vacancy dissociation energy: 4.39 eV

- From [3], D (m2/s) = D0 exp (-EDif/kT); D0 = 4.7 x 10-7 m2/s and EDif = 0.28 eV

• Due to their high heat of solution, inert-gas atoms are essentially insoluble in most solids. • This can then lead to gas-atom precipitation, bubble formation and ultimately to destruction of the material.

November 8-9, 2005

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IFE Relevant Experimental data on He Implantation and Release in W

From UNC/ORNL experimental results described in past presentations from L. Snead, et al., e.g. at the March 2005 HAPL meeting or

at the US/Japan Workshop on Laser IFE, General Atomics, San Diego, CA, March 2005

He retention in polycrystalline and single crystal W

samples as a function of He dose per cycle

for different number of pulses based on He implantation and

temperature anneals

November 8-9, 2005

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“Engineering” Way of Interpreting Results

• Detailed modeling of all mechanisms useful and should be pursued

• However, many unknown parameters

• Effective diffusion analysis conducted to characterize activation energy associated with controlling mechanism in He migration and trapping in W

• Parametric study of experimental results to estimate effective diffusion activation energy to reproduce He retention for each experimental anneal case

Effective diffusion

x = 0

Symmetry: dC/dx=0

Fast desorption: C=0

x =

Experimental case; 1000 Shots; Eeff,dif=3.6eV; He Retention=0.33

0.0E+00

5.0E+23

1.0E+24

1.5E+24

2.0E+24

2.5E+24

0 2000 4000 6000 8000 10000 12000

time (s)

Average He concentration (atoms/m3)

He concentration in polycrystalline W as a function

of time for the case with 1000 shots and 0.33

retention in the end (Eeff,diff = 3.6 eV).

• An interesting observation from the results is that for all cases, quasi steady state has not yet been reached.

November 8-9, 2005

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Effective diffusion

activation energy required to reproduce the experimental

results for single crystal and

polycrystalline W

No. of

cycles

Dose per

cycle

(atoms/m2)

Approx. He

implanted

conc. per

cycle

(atoms/m3)

He retention

(normalized to

total amount of

implanted He)

Eeff,diff (eV)

Single

Crystal W

1 1019 6.67x1024 1 ~4.2-4.4

10 1018 6.67x1023 0.83 3.30

100 1017 6.67x1022 0.63 3.42

167 6x1016 4.0x1022 0.51 3.41

333 3x1016 2.0x1022 0.23 3.33

1000 1016 6.67x1021 0.0001 2.40

Polycrys.

W

1 1019 6.67x1024 1 ~4.4-4.8

10 1018 6.67x1023 1 ~4.4-4.8

100 1017 6.67x1022 0.73 3.53

167 6x1016 4.0x1022 0.69 3.57

333 3x1016 2.0x1022 0.48 3.51

1000 1016 6.67x1021 0.33 3.60

November 8-9, 2005

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Effective Diffusion Activation Energy (Eeff,diff) as a Function of Dose per He Implantation

(The curve fit has been drawn to suggest a possible variation of the activation energy with the He dose or

concentration)Hypothesis: • In general, trapping increases with He irradiation dose which creates sites through dpa's and formation of vacancies (followed by an anneal of the unoccupied trapped sites during the ensuing temperature transient). • At very low dose, only a few trapping sites are activated by the irradiation and the helium transport should be governed by bulk diffusion (with an activation energy of ~0.24-0.28eV).

0

1

2

3

4

5

6

1.0E+15 1.0E+16 1.0E+17 1.0E+18 1.0E+19 1.0E+20

Dose per He implantation (ions/m2)

Effective Diffusion Activation Energy ( eV)

Single crystalPolycrystalline

• As the dose per cycle increases, an increasing number of trapping sites are formed or activated and Eeff,diff increases. • It seems that there is a near- threshold of He dose at which Eeff,diff increases rapidly to ~3.3-3.6 eV and stays at this value over a significant dose range.• Above this range, Eeff,diff increases rapidly to ~ 4.2-4.8 eV, indicating an increase in trapping perhaps due to He build up in vacancies (the vacancy dissociation energy is ~4.4 eV).

IS THIS REASONABLE?

IFE Case ~5 x 1016 atoms/m2

November 8-9, 2005

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200

600

1000

1400

1800

2200

2600

3000

0.0E+0 1.0E-5 2.0E-5 3.0E-5 4.0E-5 5.0E-5

Time ( s)

1-mm W + 2.5 mm FS

W Density = 19350 kg/m

3

Coolant Temp. = 580°C

h =10 kW/m

2

-K

350 MJ DD Target Spectra

R=10.75 m

Surface

1 μ m

5 μ m

10 μ m

100 μ m

Simulation of IFE W Armor Case

W Armor Temperature History for 350 MJ Target

and 10.75 m Chamber Radius

1E+23

1E+24

1E+25

1E+26

1E+27

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Time (s)

For comparison, atomic density of W = 6.22 x 1028

atoms/m3

Characteristic porous microstructure dimension (nm):

1000

100

50

10

History of He Concentration in W Armor assuming a per-shot

implanted He concentration of ~3.2x1022 atoms/m3 (assuming

~6.8x1019 He ions per shot for a 350 MJ target in a ~10.5 m chamber with an average implantation depth

of ~1.5m)

Eeff,diff =3.52 eV for polycrystalline W case

Results for Different Porous Microstructure Dimensions are

Shown

November 8-9, 2005

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Example Results from Modeling He Retention in Porous W IFE Armor

• He concentration for SC W (Eeff,diff=3.38 eV) ~55-60% that of PC W (Eeff,diff=3.52 eV)

• Key question: what at.% of He in W is acceptable for acceptable armor lifetime?

- Previously, G. Lucas suggested ~15 at.% as critical concentration for a blister to exfoliate

- This suggests that porous W microstructure dimension could be between 0.1 and 1 mm

- However, given uncertainties in modeling, it seems prudent to maintain a porous W microstructure ~ 50-100nm until shown

otherwise by prototypical experimental results (Recommendation to PPI)

• As indicative of cases with lower He ion doses, example results for Eeff,diff=2.4 eV also shown (for ~<1016 ions/m2 per shot ).

- He retention much reduced, indicating benefit of operation at ion doses below the assumed

threshold shown earlier- This threshold is ~1016 ions/m2

based on these initial experimental results. Future effort is

needed to confirm and better understand the material form dependence of this threshold (a factor of five increase would bring it very close to the current IFE case).

Microstructure

dimension (nm)

Maximum He

concentration

(atoms/m3)

He retention concentration

(normalized to atomic

concentration of W = 6.22

x 1028 atoms/m3)

Single Crystal W

Eeff,diff=3.38 eV

10 ~2.6x1024 ~4.2x10-5

50 ~6.5x1025 (estimated) ~1.1 x 10-3

100 ~2.6x1026 (estimated) ~4.2x10-3

1000 ~2.6x1028 (estimated) ~0.42

Polycrystalline W

Eeff,diff=3.52 eV

10 ~4.8x1024 ~7.7x10-5

50 ~1.2x1026(estimated) ~1.9 x 10-3

100 ~4.8x1026(estimated) ~7.7x10-3

1000 ~4.8x1028(estimated) ~0.77

Example low ion flux

case

Eeff,diff=2.4 eV

10 ~4x1022 ~6x10-7

50 ~9x1023 ~1.5x10-5

100 ~4x1024 ~6x10-5

1000 ~4x1026 (estimated) ~6x10-3

November 8-9, 2005

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PPI’s Progress in Manufacturing Porous W with Nano Microstructure

• Plasma technology can produce tungsten nanometer powders. - When tungsten precursors are injected into the plasma flame, the materials are heated, melted, vaporized and the chemical reaction is induced in the vapor phase. The vapor phase is quenched

rapidly to solid phase yielding the ultra pure nanosized W powder - Nano tungsten powders have been successfully produced by plasma technique and the product is ultra pure with an average particle size of 20-30nm. Production rates of > 10 kg/hr are feasible.

• Process applicable to molybdenum, rhenium, tungsten carbide, molybdenum carbide and other materials.

• The next step is to utilize such a powder in the Vacuum Plasma Spray process to manufacture porous W (~10-20% porosity) with characteristic microstructure dimension of ~50 nm .

TEM images of tungsten nanopowder, p/n# S05-15.