1. Ion Beam Materials Analysis and Modifications Group University of North Carolina at Chapel Hill Helium Threat Spectrum Implantation in Tungsten S. Gilliam a, J. Holden a, S. George a, N. Parikh a J. Hunn b, L. Snead b R. Downing c a University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA b Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6138, USA c National Institute of Standards and Technology, Gaithersburg, MD 20899-3460, USA

2. IFE Reaction Chamber DT pellet He Tungsten Helium flux and temperature conditions Helium flux: 5 x 10 18 He/cm 2 /day Implantation pulse width: ~2  sec Helium spectrum: 50 – 500 keV Wall temperature: 600 – 800°C with 2  sec spikes to ~2400°C Rep rate: 5 Hz

3. # of ions vs. energy (keV) Slow ion debris in NRL Direct Drive IFE reactor UCSD ARIES program on fusion energy technology J. Perkins, http://aries.ucsd.edu/aries/wdocs/aries-ife/spectra/

4. # of ions vs. ion energy (keV) Fast ion debris in NRL Direct Drive IFE reactor UCSD ARIES program on fusion energy technology J. Perkins, http://aries.ucsd.edu/aries/wdocs/aries-ife/spectra/

5. Total helium threat spectrum Bulk of helium spectrum 100 – 500 keV

6. 2.5 MV Van de GraaffScattering Chamber / Electronics Control Panel / Bending MagnetEnergy degrader foil and sample holder UNC-Chapel Hill Accelerator Laboratory Facility

7. Previous studies with monoenergetic helium 1.3 MeV 3 He implanted at 850°C with flash heating to 2000°C between implant steps or at the end of a single step implant In multiple step implantations the total dose was divided by the no. of steps. In each step the partial dose was implanted, then the sample was flash heated to 2000°C, and the cycle repeated. Helium retention was measured and compared by nuclear reaction analysis (NRA) and neutron depth profiling (NDP) Determined that retention was reduced when the same helium dose was implanted in many implant/heating cycles; more reduction observed in single crystal tungsten

8. 12.6  m Mylar foil PreamplifierAmplifierMCA ~13 MeV protons, ~2 MeV alphas, backscattered deuterons Tungsten Target 3 He profile depth = 1.7  m 1500  m depletion depth detector at 155  with respect to the incident beam direction 3 He(d, p) 4 He nuclear reaction analysis Used proton yield from the reaction to compare helium retention 780 keV deuterons

9. 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 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

10. Technique: Neutron Depth Profiling (NDP) measures elemental concentration profiles up to a few micrometers in depth for elements that emit a charged particle following neutron capture. ( R.G. Downing, et al., NIST J. Res. 98 (1993)109.) Elements Analyzed: boron, lithium, helium, nitrogen and several additional light elements with less sensitivity. Sample Environment: In an evacuated chamber, samples are irradiated with a beam of low energy neutrons. A small percentage of the emitted reaction particles are analyzed by surface barrier detectors to determine their number and individual energies. Principles: The emission intensity is compared to a known standard to quantitatively determine the elemental concentration. The emitted particles lose energy at a predicable rate as they pass through the film; the total energy loss correlates to the depth of the reacting nucleus. Advantage: NDP is non-destructive. NDP analysis allows repeatedly determinations of the sample volume following different treatments. Neutron beam flux at sample: ~7.5x10 8 n/cm 2 -s Beam area: from a few mm 2 to ~110 mm 2 Neutron Sample  beam NDP Experimental Arrangement NDP NDP of boron in silicon Depth range: 15 nm – 3.8 µm Sample Dimension TXRF NDP XRF RBS Detection limit (at/cm 3 ) TOF-SIMS Dynamic SIMS FTIR 1000 Å 1µm 10 µm 100 µm 1 mm 1 cm 1e22 1e20 1e18 1e16 1e14 1e12 Neutron monitor

11. Neutron Depth Profiling Neutron Reactions of Merit

12. NDP analysis of monoenergetic helium implantation NDP uses 3 He(n, p)T reaction to determine the helium depth profile Reaction produces 191 keV tritons and 572 keV protons Number of protons is proportional to helium concentration Detected proton energy converted to depth scale by energy loss Comparison to a standard provides absolute measure of depth profile Single crystal W implanted with monoenergetic 1.3 MeV 3 He at 850°C and flash heated to 2000°C to a dose of 10 20 He/m 2

13. Helium threat spectrum implantation project More accurately mimic the IFE reactor conditions to study effects of helium irradiation on the first wall Produce the IFE helium threat spectrum and implant tungsten samples Use aluminum foils to degrade the monoenergetic He beam Use LabVIEW to control energy degrader foils, dosimetry, and sample temperature Analyze implanted samples by NDP to determine helium depth profiles and compare retention levels under various experimental conditions

14. How do we produce a helium threat spectrum? E 0 He beam FoilTungsten E = E 0 –  E foil t Degrade the monoenergetic beam by transmission through a thin Al foil Tilting the foil provides a range of degraded energies by varying the path length d through the foil where  = 0° is normal incidence 36 effective foil thicknesses (degraded energies) were used 1.7 MeV 3 He beam through 1.5 and 3.0 micron Al foils tilted 0 – 60° Degraded energies: 1400 – 100 keV Al stopping power: ~300 keV/micron

15. Degraded Energy Measurements Detector placed at 10° from beam direction to eliminate swamping the detector SRIM 2003 generated energy profiles used in our threat spectrum approximation

16. Approximating the threat spectrum Each foil thickness produces a Gaussian degraded energy profile Approximated threat spectrum with a linear combination (weighted sum) of the 36 degraded energy profiles Normalized by the sum of the weights so that any total He dose can be divided appropriately among the 36 degraded energies

17. Threat spectrum implantation conditions 1.7 MeV 3 He beam transmitted through 1.5 and 3.0 micron Al foils each tilted 0 – 60° to generate the range of energies necessary Implantation at 850°C with flash heating to 2000°C between implant steps or at the end of a single step implant. (Temp. measured by infrared thermometer.) Total helium dose is divided by the no. of steps Partial dose is implanted as a threat profile with the sample at 850°C Sample heating 850°C  2000°C  850°C Next implant begins LabVIEW automates foil tilt motions to implant correct dose at each position and controls sample temperature via power controller and infrared thermometer

18. Computer LabVIEW DAQ Card Power Controller Cup Control Infrared Thermometer Sample Controls implant dose Reads sample temperature Controls sample temperature Current Integrator (Faraday Cup) Current Integrator (BPM ) Stepper Motors Foil Holder Instrumentation Flow Chart

19. Recent single crystal W implantations Sample ID He Dose (m -2 ) No. of steps Dose/Step (m -2 ) 2000°C Heating Retention (m -2 ) S 6810 19 1 none2.5 x 10 18 S 6910 19 1 3 min. 1 3.0 x 10 18 S 7410 19 1010 18 each step S 7010 20 1 None3.6 x 10 19 S 7110 20 1 3 min. 1 2.6 x 10 19 S 7310 20 1010 19 each step3.4 x 10 19 S 7210 20 10010 18 each step3.9 x 10 19 1 Heated to 2000°C for 3 min. immediately after implanting Discrepancy between implanted dose and overall retention by NDP analysis 1) Beam spread through degrader foil during implant 2) NDP analysis (scaling due to analyzed area) NDP determined peak concentration at ~0.8 micron but we expected 0.35 micron corresponding to the 200 keV peak in the threat spectrum

20. Sample ID He Dose (m -2 ) No. of steps Dose/Step (m -2 ) 2000°C Heating Retention (m -2 ) S 6810 19 1 none2.5 x 10 18 S 6910 19 1 3 min.3.0 x 10 18

21. Sample ID He Dose (m -2 ) No. of steps Dose/Step (m -2 ) 2000°C Heating Retention (m -2 ) S 7010 20 1 none3.6 x 10 19 S 7110 20 1 3 min.2.6 x 10 19

22. Sample ID He Dose (m -2 ) No. of steps Dose/Step (m -2 ) 2000°C Heating Retention (m -2 ) S 7010 20 1 none3.6 x 10 19 S 7210 20 1010 19 each step3.9 x 10 19 S 7310 20 10010 18 each step3.4 x 10 19

23. Minimizing Experiment Time Total dose, beam current, switching foils, heating, and number of steps affect the time required to complete an experiment Initial Experiments (time per step) Switching foils: 50 sec Heating 850°C  2000°C  850°C: ~90 sec Implantation: varied greatly depending on dose and beam current (6 – 30 min) Kept beam currents low to ensure accurate dosimetry Initial worst case scenario: ~30 min/step Estimated time for 1000 steps: ~24 days Improvements Switching foils: 50 sec Heating 850°C  2000°C  850°C: ~15 sec Implantation: increasing beam current may permit ~2 min Accurate dosimetry should hold in the high intensity portion of threat spectrum Current best case scenario: ~3 min/step Estimated time for 1000 steps: ~50 hours

24. Another idea to speed things up Confine threat spectrum implant to the 100 – 500 keV region 1.0 MeV 3 He beam through a single 1.5 micron Al foils tilted 0 – 55° provides effective foil thickness ranging 1.5 to 2.6 microns Al stopping power: ~330 keV/micron Degraded energies: 500 – 100 keV Eliminates switching between foils saving 50 sec per step Estimated time for 1000 steps: ~30 hours

25. Experimental Concerns Resolve discrepancies in concentration and depth results expected vs. NDP results Small dose/step results in implantation time far less than the time required to tilt the foils May have to stop implantation while changing foil tilt to avoid overshooting implantation dose Reduce the separation distance between degrader foil and sample to reduce beam spreading which will result in a better defined implant area Confirm the foil thickness by RBS analysis

26. Future work Determine helium depth profile which should result from helium threat spectrum implantation and compare to NDP results Generate a set of threat spectrum implanted polycrystalline W samples to compare with single crystal Study helium threat spectrum retention characteristics of SiC and porous CVD tungsten. He desorption studies to evaluate helium trapping characteristics

27. Acknowledgement This research is supported under the US Department of Energy High Average Power Laser Program managed by the Naval Reactor Laboratory through subcontract with the Oak Ridge National Laboratory. Publications S. Gilliam, S. Gidcumb, D. Forsythe, N. Parikh, J. Hunn, L. Snead, G. Lamaze, Helium retention and surface blistering characteristics of tungsten with regard to first wall conditions in an inertial fusion energy reactor, Nuclear Instruments and Methods B, 241 (2005) 491-495. S. Gilliam, N. Parikh, S. Gidcumb, B. Patnaik, J. Hunn, L. Snead, G. Lamaze, Retention and surface blistering of helium irradiated tungsten as a first wall material, Journal of Nuclear Materials, 347 (2005) 289-297. Ion Beam Materials Analysis and Modifications Group University of North Carolina at Chapel Hill

28. Thermal Desorption Spectroscopy Wish to study thermal desorption of helium from tungsten and how it depends on implantation and flash heating parameters Study single crystal and polycrystalline W to determine differences in desorption characteristics Doses ranging from 10 16 to 5 x 10 20 He/m 2 implanted at RT or 850°C Residual gas analyzer (mass spectrometer) monitors He partial pressure while temperature is ramped from RT to ~2000°C Temp. ramping rate typically ~2°C/s

29. TDS study of helium implanted tungsten

30. Ramped sample temperature from RT to 2200°C Small pulses of desorbed He around 600 and 2000°C Significant He desorption above 2000°C correlates to surface blistering Higher partial pressure of 3 He detected due to higher dose of 3 He Time (s) Temp. (°C) RT 600 2000 2200 TDS Data: Poly W implanted with 5x10 20 3 He/m 2 at RT

31. Thermal Desorption System

32. Unimplanted polycrystalline tungsten sample ramped from RT to 2200°C Background partial pressure level of 3 He remained constant (~5x10 -12 Torr) Mass 2 is always present in mass spectrometery scans We have conducted TDS on 3 He and 4 He implanted W samples to determine if the tail of the mass 2 peak affects the mass 3 peak value So far we conclude that the mass 2 peak tail is not a great concern. TDS Data: unimplanted polycrystalline tungsten

33. TDS Data: Poly W implanted with 3x10 20 4 He/m 2 at RT Time (s) Temp. (°C) RT 600 2000 2200 Ramped sample temperature from RT to 2200°C Small pulses of desorbed He around 600°C Observed significant He desorption above 2000°C which correlates to simultaneous blistering of the sample surface Surface was blistered after completing the TDS experiment