1. High Average Power Laser Program Workshop University of Wisconsin, Madison October 22-23, 2008 Pulsed E-beam Thermofatigue System Chad E. Duty Materials Science & Technology Division Oak Ridge National Laboratory Helium Retention in Nano-Porous Tungsten Nalin Parik, R. Parker, J. Gladden University of North Carolina, Chapel Hill R. Downing, L. Cao National Institute of Standards and Technology Gaithersburg, MD With a special appearance by the ghost of Jeff Latkowski

2. Pulser Unit High Voltage Shield Electron Gun & Chamber Vacuum System & Controls Pulsed E-beam Thermofatigue System (PETS) Calculations courtesy of: Ion : J. Blanchard Electron : F. Hegeler Peak Voltage = 70 kV (variable) Peak Current = 74 Amps Pulse Width = 0.5 to 1.5 µsec (variable) Pulse Rise/Fall Time = 800 ns Pulse Frequency = Single shot to 100 Hz Duration = > 10 million shots Usable Beam Waist = 0.565 cm (1 cm 2 area) Current Density Variation (at UBW) ≈ 3:1

3. FY-08 Goals Design, fabricate, install heated sample stage. Demonstrate high-cycle, high vacuum operation on heated tungsten sample. Prepare for use with radioactive samples (FY-10 and beyond.)

4. Sample Holder / Cooling Design Functions -- Hold sample securely -- Permit optical access (from both top & sides) -- Versatile clamping design (allow for various sample dims) -- Cool sample (provide heat sink for e-beam energy) -- Heat sample (low temp thermal processing / aging) -- Electrically ground sample (prevents sample from charging) -- Measure temperature (either fast TC or melt blocks) Useable Beam Waist ( Ø = 1.13 cm) Tungsten Target (1-2mm thick) Molybdenum Clamping Disks (1-2mm thick ea) Molybdenum Cap ( λ = 138 W/mK ) Quick Response Thermocouple (Under Development) Ceramic Resistive Heating Element (12A, 250W) Coaxial Stainless Steel Cooling Tube ( λ = 26.3 W/mK ) Thin Fins (Tube Alignment) Coaxial Water Flow (0.5 gpm) High Conductivity Wire to Ground ( 7 Gauge) Ø = 1.25”

5. Sample Holder / Cooling Design E-beam chamber supplied by HeatWave Inc. Path to ground for electrons Ceramic Insulator for 8” Conflat (prevents e - from returning to copper anode ) Thermocouple / Electrical Feedthrough Rated 1250W at 1.5 gpm Use oscilloscope to measure voltage drop across a 1  resistor (1 V ≈ 1 A through e-beam) Resistive Heater & Thermocouple Leads Sample Holder (previous slide) ~8”

6. Finite Element Thermal Analysis Radial Symmetry Steady State Resistive Heater (250 W) Coolant Flow T inf = 10-20dC h off = 50 W/mK h on = 5,000 W/mK Radiation T inf = 20dC  = 0.2 (free surfaces) Conduction W = 174 W/mK Mo = 138 W/mK Steel = 26 W/mK Insul = 1 W/mK Developed preliminary finite element model of sample holder in ABAQUS. E-beam Heating Options Equal Inside = 738 W/cm 2 Outside = 738 W/cm 2 2:1 Split Inside = 428 W/cm 2 Outside = 856 W/cm 2 Example Case E-beam = 2:1 Split Heater = 250 W Coolant = 5,000 W/mK

7. Steady State Heat Transfer Due to radial symmetry, natural tendency is for center of sample to get hotter. Splitting beam produced more uniform surface temperature than using the resistive heater. Predict 2.6:1 beam distribution will not be problematic (steady state). E-beam Equal (738:738) E-beam Split (428:856) Heater Off (0 W) Heater On (250 W)

8. Components for Holder are Bench Assembled

9. (lack of) Progress since last HAPL Meeting Prior to previous HAPL meeting a series of vacuum leaks were plugged and issues with power conditioning resolved. Held 10 -8 Torr Vacuum for 5 months After resolving issues with radiological operation (75 mR/hr operation,) we moved onto long duration test of ebeam. ----> immediately and still frustrated by operational issues.

10. Interference / Pressure Spike? Baseline pressure = 1x10 -7 Torr Pulse at 1 Hz, 30 kV Pressure pulse to 4x10 -7 Torr Pulse in sync with 1 Hz e-beam Occasional spike of 7x10 -7 Torr (trip) Shield cables & ground system Verify system pressure pulse on separate gauge Conclusion: Pressure pulse is real If so, should “bake out” overnight

11. E-beam “Bake Out” Pulse overnight at 15% power (12 kV) Pressure pulse decreased by 0.6x10-7 Torr Increased trip point from 40% to 70% power Pulse overnight at 50% power (38 kV) Pressure pulse decreased by 0.3x10-7 Torr Increased trip point from 70% to 100% power Power (kV) Pressure Pulse (x10 -7 ) Pressure Pulse Change During Overnight Bake Out Radiation Measurement 100% power (74 kV) 1 Hz pulse rate 1 μs pulse width ~75 mR/h at contact

12. Chamber Leaks & Water Vapor “Condition” the chamber by running e-beam After 4 days of running, pressure unstable Beam expanding to hit chamber walls Excessive heat on end plates / gaskets Surfaces oxidizing & thermally cycling Water vapor and leaks due to thermal expansion need to be dealt with. No sample Leaks

13. Future Directions Stabilize pressure during pulse Design / install radiation shielding Design / install sample interlock Maintain high vacuum (~10 -8 T) No need to recondition cathode Install sample holder / target Adequately cool sample Increase radiation levels Start processing samples Radiation Shield

14. Engineered Tungsten Armor Development Vacuum Plasma Spray (VPS) forming techniques are being used to produce engineered tungsten armor. The engineered tungsten is comprised of a primary tungsten undercoat and a nanoporous tungsten topcoat. Nanometer tungsten feedstock powder is being used to produce the nanoporous tungsten topcoat. The resulting nanoporous topcoat allows helium migration to the surface preventing premature failure. Low Activation Ferritic Steel Primary W Layer Nanoporous W Topcoat Schematic showing the VPSing of the engineered W armor. SEM image showing nanometer W feedstock powder produced by thermal plasma processing. Analysis has shown the average particle size is less than 100nm. This is one of two nanometer W feedstock materials used to produce the nanoporous topcoat.

15. Retention of Monoenergetic Helium Relative 3 He retention for single crystal and polycrystalline tungsten with a total dose of 10 19 He/m 2. Percentage of retained 3 He compared to implanting and annealing in a single cycle. Implanted 10 19 3 He/m 2 (1.3 MeV) at 850°C followed by a flash anneal at 2000°C Same total dose was implanted in 1, 10, 100, and 1000 cycles of implantation and flash heating 2.5 MV Van de Graaff accelerator Implant 3 He Threat Spectrum by Degrading Energy of Mono energetic beam at various temperature and flash heating at 2000 0 C. 2.5 MV Van de Graaff accelerator Implant 3 He Threat Spectrum by Degrading Energy of Mono energetic beam at various temperature and flash heating at 2000 0 C. 20 MW Nuclear Reactor- Cold neutron source Measure helium retention by neutron depth profiling (NDP) technique (NIST, Gaithersburg, MD) 20 MW Nuclear Reactor- Cold neutron source Measure helium retention by neutron depth profiling (NDP) technique (NIST, Gaithersburg, MD) 1 i/a10 i/a 100 i/a 1000 i/a

16. He retention comparisons for 1e10 20 3 He/m 2 Nano-Cavity W(<100nm Particles)

17. DoseSCWPoly-Wnano-porous W w/HfC 1e19 1 step*98% Done, NA 1e19 = 100 x (1e17 impl + anneal)65%75%Done, NA 1e19 = 500 x (2e16 impl + anneal)21%43%Done, NA 1e20 1 step72%99%15% 1e20 = 100 x (1e18 impl + anneal)74%84%26% 1e20 = 500 x (2e17 impl + anneal)81%15% Comparison of He Retention with W- micro-Structure for Single and multi-Step Implants and Anneal * Flash Heated to 2000 0 C for 5 s between the Steps

18. Results of Surface Blistering/Exfoliation Study Samples Implanted at 850 0 C at High Dose of He and then Heated to 2000 0 C for 10 s. Single implant/anneal Comparison between Poly-W vs. nano-cavity W.

19. Exfoliation- Poly-W vs. Nano-Cavity W Poly-W with 2 x 10 21 He/m 2 in 1 step Ploly-W with 1 x 10 22 He/m 2 in 1 step nano-W unimplanted nano-W with 5e21 He/m 2 in 1 step nano-W unimplanted nano-W with 5e21 He/m 2 in 1 step (>100 hrs)

20. 5e21 He/m 2 in Nano-Cavity W

21. Surface Exfoliation Results of Poly-W vs. Nano-porous W Iwakiri 2000 5000 nm Poly-W with 2 x 10 21 He/m 2 showed blistering Poly-W with 1 x 10 22 He/m 2 showed exfoliation Nano-cavity W with 5x 10 21 He/m 2 did not show Surface blistering or exfoliation

22. Nanoporous W He Retention Remarks Helium retention in tungsten is a strong function of the amount of helium implanted prior to annealing. For HAPL-relevant implant/anneal conditions polycrystalline materials exhibit more retention than single crystal. This indicates that microsctructure and perhaps impurities may be important factor in helium retention. Nanoporous tungsten appears to have significantly less retention than both polycrystalline and single crystal tungsten. This result is supported by the lack of blistering / exfoliation in the as-implanted surface, supporting reduced “ path length ” argument for mitigating helium retention. These results should be considered preliminary. Further study into the release kinetics, as well as the structure of the implanted surface is required, though results are encouraging for this material system. –--> need for thermal desorption analysis –--> need to reproduce cyclic implantation/anneal study for nanoporous materials –--> need to move to more relevant annealing condition (degraded helium deposition and laser annealing with in-situ thermal desorption.)