1. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 1 Update on Helium Retention Behavior in Tungsten D. Forsythe, 1 S. Gidcumb, 1 S. Gilliam, 1 N. Hashimoto 2, J. D. Hunn, 2 G. Lamaze, 3 N. Parikh, 1 S. J. Zinkle 2, L. Snead 2 1 Dept. of Physics and Astronomy, UNC-Chapel Hill, Chapel Hill, NC 2 Oak Ridge National Laboratory, Oak Ridge, TN 3 National Institute of Standards and Technology, Gaithersburg, MD

2. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 2 As-rolled Powder Metallurgy W

3. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 3 Powder Met W annealed at 1000°C for 1 hr

4. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 4 Powder Met W annealed at 1200°C for 1 hr Planimetric procedure (ASTM Designation: E112-96) Number of Grains, N A (/mm 2 ) = 11911 Average Grain Area, A = 1/ N A = 84 (  m 2 ) Average Diameter, d = √(1/ N A ) = 9.2 (  m) ASTM Grain Size #, G = (3.321928 log 10 N A ) - 2.954 = 10.6

5. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 5 Planimetric procedure (ASTM Designation: E112-96) Number of Grains, N A (/mm 2 ) = 8336 Average Grain Area, A = 1/ N A = 119 (  m 2 ) Average Diameter, d = √(1/ N A ) = 11.0 (  m) ASTM Grain Size #, G = (3.321928 log 10 N A ) - 2.954 = 10.1 Powder Met W annealed at 1300°C for 1 hr

6. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 6 Summary of Powder metallurgy W annealing results Grain number, N A (mm -2 ) Ave. Grain Area, A (  m 2 ) Ave. Grain diameter, d (  m) ASTM Grain Size #, G Annealed at 1200°C for 1 hr 119118410.6 Annealed at 1200°C for 2 hrs 622216112.79.6 Annealed at 1200°C for 5 hrs 408824515.69.0 Annealed at 1300°C for 1 hr 833611911.010.1

7. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 7 Recrystallization in Powder Metallurgy W

8. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 8 At room temp. growth of He bubbles beneath the surface causes blistering at ~3 x 10 21 /m 2 and surface exfoliation at ~10 22 /m 2. For IFE power plant, MeV He dose >>> 10 22 /m 2. MeV Helium First Wall Armor vacancy 0 1 2 3 4 5 6 7 8 9 10 Time of microseconds

9. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 9 AFM of blistering  Topographical AFM image of surface blisters on polycrystalline tungsten  Blister caps are ~1.9  m tall comparable to helium implant depth

10. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 10 Direction of Research Over Past Year Complete study of stepwise dose annealing. Automate system for very large dose(>10 19 n/m 2 ) and higher (>2000°C.) Total Dose  1E193E195E191E20 Dose/Step  100% (1) 1E1885% (10) 1E1765% (100)70% (300)*70% (500)77% (1000) 1E165% (1000) Polycrystalline W Single Crystal W

11. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 11

12. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 12 Where We are Going  It is now clear that : - Helium retention is a function of material and a combination of implanted dose and annealing temperature - For IFE-relevant levels of implanted helium and peak annealing temperatures we are near a limit below which helium may not accumulate The direction we are moving : - More refined experiments designed to give a) More precise measurement of low level accumulation b) Better understanding of the kinetics - More detailed and experimentally coupled modeling.

13. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 13 Neutron Depth Profiling  3 He(n, p)T used to obtain absolute helium depth profile  Used to profile monoenergetic 1.3 MeV 3 He implanted in tungsten  Ratio of areal densities determined by NDP agreed with ratio of proton yields resulting from NRA Single crystal W implanted with monoenergetic 1.3 MeV 3 He at 850°C and flash-annealed at 2000°C to a dose of 10 20 He/m 2

14. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 14 Producing IFE helium ion spectrum 1.6 MeV 3 He degraded by 1.37  m C foil, backscattered from Au film

15. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 15 Variable energy helium implantation  1.6 MeV 3 He beam degraded by carbon foils  Foil thickness: 1.37, 2.00, 2.55, 3.55  m  Approx. 10 different tilt angles (~0 – 40°) for each foil  43 degraded energy profiles weighted appropriately  Implanted two single crystal samples with 10 20 He/m 2 at room temp.  One sample flash annealed to 2000°C after implant  Both samples to be analyzed by NDP

16. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 16 Cavity distribution in He-implanted and annealed W Single crystal W implanted with10 19 He/m 2 followed by annealing at 2000°C Polycrystalline W implanted with10 19 He/m 2 followed by annealing at 2000°C * Single step annealing (2 sec.) resulted in the formation of a large number of tiny cavities. * No visible cavities were observed in the 1000 step annealed (33 min.) single crystal W * The presence of grain boundaries led to significant cavity formation and greater cavity growth than in single crystal tungsten. * Annealing in 1000 steps resulted in no visible cavity formation even though the NRA results found polycrystalline tungsten had more He retention than single crystal tungsten.

17. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 17 Observed Area Specimen 1m1m Over Focus Image 100nm Under Focus Image 100nm Cavity Distribution of Helium-implanted Single Crystal W Implanted at RT to 2 x 10 17 m -2 and annealed at 2000°C for 5 sec. and repeated this 50 times for a total dose of 1 x 10 19 m -2

18. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 18 Thermal Desorption Spectroscopy  Implant single crystal and polycrystalline tungsten with 3 He  Mass spectrometer monitors 3 He partial pressure while sample temperature is ramped from room temperature to 2400°C  Goal is to determine differences in helium trapping/detrapping mechanisms for single crystal and polycrystalline tungsten under different implantation conditions

19. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 19 TDS data for a single crystal W sample implanted with 5 x 10 20 He/m 2 at 850°C. The temperature was ramped from room temperature to 2400°C at ~2°C/s. Well defined desorption peaks were observed at 620, 730, and 900°C. The “plateau” between 1000 and 1200 s occurred while the sample was held at 2400°C (temperature ramp stopped due to furnace limitations). 620˚C 730˚C 900˚C

20. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 20 Summary At IFE relevant conditions, variables affecting retention and eventual spalling include: - amount of helium implanted for each fusion event 10 16 ions/m 2 (~ IFE) packet, 2000°C has limited retention - annealing temperature following event currently limited to 2000°C due to specimen fatigue issues in ion beam chamber (specimen holder redesign needed) - microstructure as expected, helium retention at grain boundaries is an important factor Issues: - current experiment limited in total dose and annealing temperature - more IFE-relevant irradiations should include: shorter pulse, higher temperature annealing (requires laser) - need to define defect energies by using recently developed TDS system