The effect of displacement damage on deuterium retention in plasma-exposed tungsten

1. The effect of displacement damageon deuterium retention in plasma-exposed tungsten W.R.Wampler, Sandia National Laboratories, Albuquerque, NM R. Doerner University of California, San Diego CA Guang-Nan Luo, Institute of Plasma Physics, Hefei, China 9th International Workshop on Hydrogen Isotopes in Fusion Reactor Materials Salamanca, Spain June 2-3, 2008 • Questions: • How much deuterium is retained in tungsten from exposure to plasma? • Does displacement damage increase D retention? • How much displacement damage in ITER & how will this impact T retention?

2. Retention of deuterium in tungsten exposedto PISCES plasma with displacement damage • Tungsten samples were irradiated with 12 MeV Silicon ions to simulate displacement damage by fusion neutrons to depth of ~2 microns. • Three damage levels, 0.01, 0.1 and 1 dpa, * from Si ion fluences of 5E12, 5E13, 5E14/cm2. • Implanted Si is unlikely to change D retention,~104 displacements/Si, max Si concentration <100 appm(~spec. concentration of Si as received). • Expose to PISCES D plasma at T=45, 200, 500 °C ** to fluence of 1022 (and 1021 D/cm2) • Measure D concentration vs depth to 3 μm by D(3He,p)αNRAin damaged and undamaged regions. • Two types of tungsten:- Plansee 99.97%- Vacuum Plasma Sprayed (VPS) from ASIPP, * ITER lifetime neutron fluence (0.3 MWa/m2) is estimated to produce ~ 0.1 dpa. dpa from Si ion irradiation was calculated using SRIM 2006. ** Vacancies in tungsten anneal at 500°C but not at 200°C. [Eleveld and Van Veen, JNM 212-215 (1994) 1421]

3. Retention of D in tungstenwith displacement damage • T~40°C • Plansee • - D mainly <0.5 µm • - Little increase with damage • - ~2x1016 D/cm2 • VPS • D retention similar to Planseeup to 0.1 dpa, • ~2x greater at 1 dpa

4. Retention of D in tungstenwith displacement damage • T~200°C, 1022 D/cm2 • Plansee • Below 0.1 dpa, ~2x more D retainedthan at 40°C but nowextends to ~2.5 µm. • 3x increase from damage. • VPS • D retention similar to Plansee

5. D retention is insensitive to D fluence Plansee at T~200°C D retention decreased ~ 0.73x by 10x lower D fluence.

6. D retention is much lower at 500 °C • Plansee • D retention much lower thanat 40 & 200°C. • D retention increased at 1dpaand follows damage profile • VPS • D retention ~5x higher than Planseebut with similar depth profile. • Vacancies are mobile at 500°C, • D traps are now probably less numerous but thermally more stable vacancy clusters.

7. Blisters on Plansee W exposed at 200°C. Shadowed from / Exposed to plasma 100µm Few or no blisters seen on VPS W or on Plansee W exposed at 40 or 500°C

8. Summary of D retention in damaged tungsten Areal density D retained (1016/cm2) within damaged region (< 3μm) Concentration of D retained (appm) at damage peak (2μm)

9. Hs Ht Deuterium trapping - Energetics Trapping: D bound to vacancy Precipitation: formation of gas from solution Enthalpies: HM = 0.39 eV migration Hs = 1.04 eV solution Frauenfelder JVST 6 (1969) 388 Hv = 1.43 eV dissociation from trap Ht = Hv – HM = 1.04 eV binding to trap (vacancy) Eleveld and Van Veen, JNM 191-194 (1992) 433 vacuum surface solution vacancy Equilibrium condition Chemical potential and L0 ~0.01 D/W/atm1/2 Ideal gas assuming Ss= St solution traps Traps are strong relative to solution but weak relative to gas phase. Precipitation is favored over trapping. For traps 50% occupied (σ = 0.5) the equilibrium gas pressure is Pt~104 atm, almost independent of T since Hs~ Ht

10. Deuterium trapping from plasma - Kinetics Depth of implantation xp~3 nm for 100eV D in W Depth of trapsxt~2 µm for 12 MeV Si in W D concentration D Plasma Φp 100 eV Φt flux to traps Φout Depth High Ps means D is likelyto precipitate (blisters)before it reaches the traps when T<<500°C. With strong traps and fast surface recombination, flux of D into traps:Φt= Φp xp/xt = (2x1018 /cm2s) (3 nm/2000 nm) = 3x1015 /cm2s Time to fill traps = Nt/ Φt ~ 30 seconds for Nt=1017/cm2 However, concentration c and corresponding chemical potential or equilibrium gas pressure Ps depend strongly on temperature: Φout= D(T) c/xp and Ps=[c/L(T)]2 (for ideal gas)

11. Conclusions • D retention in plasma-exposed tungsten is mainly near the surface for T<200C,at ~3x1016 D/cm2 for damage below 0.1 dpa,corresponding to ~1 milligram/m2 of tritium. • D retention is similar in Plansee and VPS tungsten. • Displacement damage increases D retention,but the effect is modest and significant only above 40C and ~0.1 dpa.The concentration of retained D is much lower than the concentration of displacements produced. • D retention within 3 microns is much smaller at 500C than at 40 or 200C,(<10 appm up to 0.1 dpa in Plansee W), due to annealing of damage. • High chemical potential of D injected from plasma leads to gas precipitation. Plansee tungsten exposed at 200C has D blisters from precipitation of D2 gas. • Bubbles or blisters near the surface will intercept mobile D preventing it from reaching greater depths. However, blisters will also impede heat transport, potentially increasing W erosion in ITER by melting of blister caps.

12. Extra slides

13. TDS RT 8.23E15 D/cm2 200°C 8.96E15 500°C 26.3E15 Retention of deuterium in VPS tungsten exposed to PISCES plasma (without displacement damage) • Vacuum plasma sprayed (VPS) tungsten from ASIPP was exposed to deuterium plasma in PISCES. • Three 1” diameter disks were exposed at temperatures of 50, 200 & 500 °C to a fluence of 1022 D/cm2. • D retention was measured by nuclear reaction analysis (NRA) and thermal desorption spectrometry (TDS). Conclusions: • D retention is below 1 milligram/m2. • RT & 200 °C samples: Areal densities of D measured by TDS & NRA are similar. Most of the D is within 3 μm. • 500 °C sample: NRA sees less D but TDS sees more D than in the other two samples. Most of the D is deeper than 3 μm. • The lower temperature TDS peak in the RT & 200 °C samples must be from the near-surface D.The higher temperature TDS peak in the 500 °C sample must be from D at depths greater than 3 μm.