1. The effect of displacement damage on 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 9 th 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.  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/cm 2.  Implanted Si is unlikely to change D retention, ~10 4 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 10 22 (and 10 21 D/cm 2 )  Measure D concentration vs depth to 3 μm by D( 3 He,p)α NRA in damaged and undamaged regions.  Two types of tungsten: - Plansee 99.97% - Vacuum Plasma Sprayed (VPS) from ASIPP, * ITER lifetime neutron fluence (0.3 MWa/m 2 ) 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] Retention of deuterium in tungsten exposed to PISCES plasma with displacement damage

3. T~40°C  Plansee - D mainly <0.5 µm -Little increase with damage - ~2x10 16 D/cm 2 VPS -D retention similar to Plansee up to 0.1 dpa, -~2x greater at 1 dpa Retention of D in tungsten with displacement damage

4. T~200°C, 10 22 D/cm 2  Plansee Below 0.1 dpa, ~2x more D retained than at 40°C but now extends to ~2.5 µm. 3x increase from damage.  VPS D retention similar to Plansee Retention of D in tungsten with displacement damage

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

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

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

8. Plansee dpa41°C200°C500°C 01101000<10 0.0011601400<10 0.11502600<10 12105700100 VPS (ASIPP) dpa45°C225°C500°C 01.82.30.01 0.0011.82.30.03 0.12.33.40.06 13.96.80.34 VPS (ASIPP) dpa45°C200°C500°C 0130760<10 0.001130800<10 0.1210170042 15204300300 Concentration of D retained (appm) at damage peak (2μm) Areal density D retained (10 16 /cm 2 ) within damaged region (< 3μm) Plansee dpa41°C200°C500°C 01.63.30.0035 0.0011.73.40.007 0.11.84.60.009 12.18.90.08 Summary of D retention in damaged tungsten

9. Deuterium trapping - Energetics Trapping: D bound to vacancy Precipitation: formation of gas from solution Enthalpies: H M = 0.39 eV migration H s = 1.04 eV solution Frauenfelder JVST 6 (1969) 388 H v = 1.43 eV dissociation from trap H t = H v – H M = 1.04 eV binding to trap (vacancy) Eleveld and Van Veen, JNM 191-194 (1992) 433 Chemical potential Ideal gas solution traps assuming S s = S t andL 0 ~0.01 D/W/atm 1/2 For traps 50% occupied (σ = 0.5) the equilibrium gas pressure is P t ~10 4 atm, almost independent of T since H s ~ H t Traps are strong relative to solution but weak relative to gas phase. Precipitation is favored over trapping. HtHt HsHs vacuum surface solution vacancy Equilibrium condition

10. D Plasma Φ p 100 eV Depth of implantation x p ~3 nm for 100eV D in W Depth D concentration Depth of traps x t ~2 µm for 12 MeV Si in W Φ t flux to traps Φ out With strong traps and fast surface recombination, flux of D into traps: Φ t = Φ p x p /x t = (2x10 18 /cm 2 s) (3 nm/2000 nm) = 3x10 15 /cm 2 s Time to fill traps = N t / Φ t ~ 30 seconds for N t =10 17 /cm 2 However, concentration c and corresponding chemical potential or equilibrium gas pressure P s depend strongly on temperature: Φ out = D(T) c/x p andP s =[c/L(T)] 2 (for ideal gas) Deuterium trapping from plasma - Kinetics T(°C)c (appm)P s (atm) 40460010 33 2003510 17 5000.8510 5 High P s means D is likely to precipitate (blisters) before it reaches the traps when T<<500°C.

11. Conclusions  D retention in plasma-exposed tungsten is mainly near the surface for T<200C, at ~3x10 16 D/cm 2 for damage below 0.1 dpa, corresponding to ~1 milligram/m 2 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 D 2 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.  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 10 22 D/cm 2.  D retention was measured by nuclear reaction analysis (NRA) and thermal desorption spectrometry (TDS). Retention of deuterium in VPS tungsten exposed to PISCES plasma (without displacement damage) Conclusions:  D retention is below 1 milligram/m 2.  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. TDS RT 8.23E15 D/cm 2 200°C 8.96E15 500°C 26.3E15