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

1. “Engineering Analysis” of He Retention & Release Experiments to Determine Desirable Engineered W Armor Microstructure A. René Raffray UCSD With Contribution from S. O’Dell PPI HAPL Meeting University of Rochester's Laboratory for Laser Energetics Rochester, NY November 7-8, 2005

2. He Implantation and Behavior in W Armor Quite Complex, Consisting of a Number of Mechanisms PORE or VOID BULK W Implanted He atom Desorption Bulk diffusion Trapping Detrapping Photon and ion threat from IFE microexplosion Trapped He Porous W (~10-100 m) Fully dense W (~ 1 mm) Ferritic steel (~ 3 mm) Coolant • 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. • 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 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)

3. IFE Relevant Experimental data on He Implantation and Release in W 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 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

4. “Engineering” Way of Interpreting Results Fast desorption: C=0 Symmetry: dC/dx=0 Effective diffusion x = 0 x =  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. • 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

5. Effective diffusion activation energy required to reproduce the experimental results for single crystal and polycrystalline W

6. 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). IS THIS REASONABLE? • 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). IFE Case ~5 x 1016 atoms/m2

7. Simulation of IFE W Armor Case 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 ~1.5mm) Eeff,diff =3.52 eV for polycrystalline W caseResults for Different Porous Microstructure Dimensions are Shown W Armor Temperature History for 350 MJ Target and 10.75 m Chamber Radius

8. 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).

9. 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.