Analysis of carbon-bearing materials for use as first wall armor in the HAPL chamber

1. Analysis of carbon-bearing materials for use as first wall armor in the HAPL chamber T.A. Heltemes and G.A. Moses Fusion Technology Institute, University of Wisconsin — Madison 18th High Average Power Laser Program Workshop Santa Fe, NM, April 8–9, 2008

2. BUCKY surface temperature plots for 10.5 m bare chamber with 0.5 mtorr helium buffer gas Silicon Carbide(Unirradiated) Pyrolytic Graphite(Unirradiated) Chambers simulated with the LLNL 365 MJ target x-ray and ion threat spectra

3. BUCKY surface temperature plots for 10.5 m bare chamber with 11.6 mtorr helium buffer gas Silicon Carbide(Unirradiated) Pyrolytic Graphite(Unirradiated) Chambers simulated with the LLNL 365 MJ target x-ray and ion threat spectra

4. Helium ion ranges in tungsten and graphite were calculated • Helium ions primary penetration ranges • tungsten • 0–3 µm • 5–7 µm • carbon • 8–20 µm • 40–50 µm • 100–200 µm • Median of helium ion penetration • tungsten: ~2 µm • carbon: ~8 µm • Mode of helium ion penetration • tungsten: ~1 µm • carbon: ~8 µm • Maximum helium ion penetration depth • tungsten: 56 µm • carbon: 3.2 mm

5. ESLI pyrolytic carbon fiber wall concept

6. Features of an engineered graphite wall • Effective surface area multiplication of ~330 • Equivalent radius of 190.3 m • R/R0 of 18.125 • Thermal transients appear to be nearly suppressed, but we must be careful because a 1-D code cannot model this 2-D surface very well • Ablation of the tips of the fibers is possible depending on graphite planar orientation • Thermal conduction down the fiber is not accurately modeled

7. The challenge of transient thermal analysis of the ESLI pyrolytic carbon fiber wall concept • Incident x-rays and ions impinge with a variable intensity depending on impact location on the fiber surface • The thermal conductivity of pyrolytic graphite is highly anisotropic • The thickness of the fiber changes as well, creating a location-specific conduction channel size (the central region of the fiber) • ANSYS calculations will need to be performed to determine a more accurate temperature profile

8. How does the equivalent radius scheme affect armor lifetime? • Onset of damage is assumed to be 1017 He+ ions/cm2 • Increasing the helium buffer gas pressure from 0.5 mtorr to 11.6 mtorr will result in the absorption of all helium ions with KE0 <= 271 keV, increasing the chamber time to threshold by 24.8% • Standard HAPL target with 10.5 m conventional tungsten chamber armor • 0.5 mtorr He chamber buffer gas • Shots to reach threshold: 8,651 • Time to threshold at 5 Hz: 29 minutes • 11.6 mtorr He chamber buffer gas • Shots to reach threshold: 10,796 • Time to threshold at 5 Hz: 36 minutes • Standard HAPL target with 10.5 m engineered carbon fiber chamber armor • 0.5 mtorr He chamber buffer gas • Shots to reach threshold: 2,841,854 • Time to threshold at 5 Hz: 158 hours (~6.5 days) • 11.6 mtorr He chamber buffer gas • Shots to reach threshold: 3,546,634 • Time to threshold at 5 Hz: 197 hours (~8.2 days)

9. Comments and Future Work • Questions that need to be addressed • Thermal conductivity effects of sputtered carbon deposits • Thermal transport and dust damage issues due to broken fibers • Ensuring proper orientation of graphite planes in carbon fibers • Ablation of fiber tips • Ion damage severity as a function of implantation energy to more accurately assess wall lifetimes for proposed chamber armor configurations • Future Work • Refine carbon fiber wall thermal calculations • Explore heating and damage issues in carbon nanotube composites