1. IFSA, Kyoto, Japan, September 2001 1 Dry Chamber Wall Thermo-Mechanical Behavior and Lifetime under IFE Cyclic Energy Deposition A. R. Raffray 1, D. Haynes 2, R. R. Peterson 2, M. S. Tillack 1, X. Wang 1, M. Zaghloul 1 1 University of California, San Diego, 460 EBU-II, La Jolla, CA 92093-0417, USA 2 University of Wisconsin, Fusion Technology Institute, 1500 Engineering Drive, Madison, WI 53706-1687, USA 2 nd International Conference on Inertial Fusion Science and Applications Kyoto, Japan September 17-21, 2001

2. IFSA, Kyoto, Japan, September 2001 2 Lifetime is a Key Dry Chamber Wall Issue Past studies, such as SOMBRERO [1], indicated the need for a protective gas at a significant pressure (e.g. Xe at ~0.5 torr) to prevent unacceptable wall erosion for a carbon chamber wall even for direct-drive targets. This creates a formidable challenge for such a design since the presence of a gas would have to also accommodate target and laser requirements. Recent studies indicated that only minimal target temperature increase (~1 K) can be tolerated during injection to maintain the required target uniformity for a symmetrical burn. High speed target injection (~100’s m/s) through a background gas could result in higher target temperature deviation due to convection and friction effects [2]. The presence of a background gas could also lead to laser breakdown depending on the gas density [1].

3. IFSA, Kyoto, Japan, September 2001 3 More Accurate Analyses Were Carried Out to Assess Armor Lifetime and Design Window for Recently Computed Direct and Indirect Drive Target Spectra Until recently, no reasonable design window seemed to exist satisfying the conflicting chamber gas constraints from wall protection on one hand and from the latest target and laser considerations on the other. A recent effort as part of the ARIES-IFE program has provided a more detailed assessment of dry chamber wall [6] based on ion and photon spectra from new direct-drive target from NRL [3,4] and indirect drive target from LLNL [4,5] Detailed analyses using very fine meshes were performed for both the energy deposition from the photons and ions and the thermal behavior of the wall (ANSYS and BUCKY). Photon and ion time of flight effects were included as well as armor melting (for metallic armor) and evaporation. C and W flat walls and a C fibrous carpet as an example of an engineered surface were considered in the analyses

4. IFSA, Kyoto, Japan, September 2001 4 X-ray and Charged Particles Spectra NRL Direct-Drive Target [3,4] 1. X-ray (2.14 MJ) 2. Debris ions (24.9 MJ) 3. Fast burn ions (18.1 MJ) (from J. Perkins, LLNL) 3 1 2

5. IFSA, Kyoto, Japan, September 2001 5 X-ray and Charged Particles Spectra HI Indirect-Drive Target [4,5] 1. X-ray (115 MJ) 2. Debris ions (18.1 MJ) 3. Fast burn ions (8.43 MJ) (from J. Perkins, LLNL) 1 2 3

6. IFSA, Kyoto, Japan, September 2001 6 Photon and Ion Attenuation in Carbon and Tungsten for Direct Drive Target Spectra Without Protective Chamber Gas

7. IFSA, Kyoto, Japan, September 2001 7 Photon and Ion Attenuation in Carbon and Tungsten for Indirect Drive Target Spectra for an Example Case Without a Protective Chamber Gas

8. IFSA, Kyoto, Japan, September 2001 8 Photon and Ion Time of Flight Effects Were Taken into Account Example Photon Temporal Distribution [1] Debris Ions Time 20ns 0.2  s 1s1s 2.5  s Fast Ions Photons Energy Deposition Schematic of Temporal Distribution for Photons and Ions Based on Direct Drive Spectrum and 6.5 m Chamber without a Protective Gas

9. IFSA, Kyoto, Japan, September 2001 9 Sublimation is a Temperature-Dependent Process Increasing Markedly at the Sublimation Point Carbon Latent heat of evaporation = 5.99 x10 7 J/kg Sublimation point ~ 3367 °C Tungsten Latent heat of evaporation = 4.8 x10 6 J/kg Melting point ~ 3410 °C Use evaporation heat flux as a f(T) as surface boundary conditions to include evaporation/sublimation effect in ANSYS calculations

10. IFSA, Kyoto, Japan, September 2001 10 Temperature-Dependent Properties for Carbon and Tungsten Were Used C thermal conductivity as a function of temperature for 1 dpa case (see figure) C specific heat = 1900 J/kg-K W thermal conductivity and specific heat as a function of temperature from ITER material handbook Calculated thermal conductivity of neutron irradiated MKC-1PH CFC (L. L. Snead, T. D. Burchell, Carbon Extended Abstracts, 774-775, 1995)

11. IFSA, Kyoto, Japan, September 2001 11 Example Temperature History for Carbon Flat Wall Under Energy Deposition from Direct-Drive Target Spectra Without Protective Chamber Gas Coolant temperature = 500°C Chamber radius = 6.5 m Maximum temperature = 1530 °C Sublimation loss per year = 3x10 -13 m (availability=0.85) Coolant at 500°C3-mm thick Carbon Chamber Wall Energy Front Evaporation heat flux B.C at incident wall Convection B.C. at coolant wall: h= 10 kW/m 2 -K

12. IFSA, Kyoto, Japan, September 2001 12 Summary of Thermal and Sublimation Loss Results for Carbon Flat Wall Under Direct Drive Target Spectra Without Protective Chamber Gas Coolant Temp. Energy Deposition Maximum Temp. Sublimation Loss Sublimation Loss (°C) Multiplier (°C) per Shot (m) per Year (m) * 5001 1530 1.75x10 -21 3.31x10 -13 8001 1787 1.19x10 -18 2.25x10 -10 10001 1972 5.3x10 -17 1.0x10 -8 5002 2474 6.96x10 -14 1.32x10 -5 5003 3429 4.09x10 -10 7.73x10 -2 * Shot frequency = 6; Plant availability = 0.85 Encouraging results even for conservative case without chamber gas: sublimation only takes off when energy deposition is increased by a factor of 2-3 Margin for setting design parameters such as coolant temperature, chamber wall radius, and target yield, and foraccounting for uncertainties

13. IFSA, Kyoto, Japan, September 2001 13 Example Temperature History for Tungsten Flat Wall Under Energy Deposition from Direct-Drive Target Spectra Without Protective Chamber Gas Coolant temperature = 500°C Chamber radius = 6.5 m Maximum temperature = 1438 °C Coolant at 500°C 3-mm thick W Chamber Wall Energy Front Evaporation heat flux B.C at incident wall Convection B.C. at coolant wall: h= 10 kW/m 2 -K Key issue for tungsten is to avoid reaching the melting point = 3410°C W compared to C: Much shallower energy deposition from photons Somewhat deeper energy deposition from ions

14. IFSA, Kyoto, Japan, September 2001 14 Summary of Thermal Results for Tungsten Flat Wall for Direct-Drive Target Spectra Without Protective Chamber Gas Coolant Temp. Energy Deposition Maximum Temp. (°C) Multiplier (°C) 500 1 1438 800 1 1710 1000 1 1972 500 2 2390 500 3 3207 500 5 5300 Encouraging results even for conservative case without chamber gas: melting point (3410°C) is not reached even when energy deposition is increased by a factor of 3 Margin for setting design parameters and accounting for uncertainties

15. IFSA, Kyoto, Japan, September 2001 15 Consider Engineered Surface Configuration for Improved Thermal Performance Porous Media -Fiber diameter ~ diffusion characteristic length for 1  s -Increase incident surface area per unit cell seeing energy deposition ESLI Fiber-Infiltrated Substrate (http://www.esli.com) Large fiber L/d ratio ~100 L A incident   ncident  fiber =  incident sin 

16. IFSA, Kyoto, Japan, September 2001 16 Example Thermal Analysis for Fiber Case Under Energy Deposition from NRL Direct-Drive Spectra Without Protective Chamber Gas Incidence angle = 30° Porosity = 0.9 Effective fiber separation = 54  m Sublimation effect not included Convection B.C. at coolant wall: h= 10 kW/m 2 -K Single Carbon Fiber 10  m Coolant at 500°C 1 mm Temperature Distribution in Fiber Tip at 2.5  s Max. Temp. = 1318°C

17. IFSA, Kyoto, Japan, September 2001 17 Summary of Thermal Results for Carbon Fibrous Wall Under Energy Deposition from NRL Direct-Drive Spectra Without Protective Chamber Gas Porosity Fiber Effective Incidence Maximum Temp. Separation (  m) Angle (°)(°C) 0.829.6 5654 0.829.6 301317 0.829.6 451624 0.954 30 1318 C flat wall as comparison: 1530 Initial results indicate that for shallow angle of incidence the fiber configuration perform better than a flat plate and would provide more margin (confirmed by recent experimental results) Statistical treatment of incidence angle and fiber separation would give a better understanding Coolant temperature = 500 °C Energy deposition multiplier = 1

18. IFSA, Kyoto, Japan, September 2001 18 1. Several Erosion Mechanisms Must Be Considered for the Armor in Particular for C [8] Carbon erosion could lead to tritium co- deposition, raising both tritium inventory and lifetime issues for IFE with a carbon wall. Redeposition/co-deposition requires cold surfaces which would exist in the beam penetration lines and pumping ducts. (For H/C=1, 60 g T per 1  m C for R=6.5 m) Macroscopic erosion might be a more important lifetime issue than sputtering and sublimation for IFE operating conditions for high energy ions (>>1 keV) (From the ARIES Tritium Town Meeting, March 6–7, 2001, Livermore, [9]) Must Consider Alternate Options for Armor (e.g. refractory metals such as Tungsten) 2. Tritium Co-Deposition is a Major Concern for Carbon Because of Cold Surfaces (Penetration Lines)

19. IFSA, Kyoto, Japan, September 2001 19 Analysis for Indirect Drive Spectra Using BUCKY [7] The threat spectrum produced by the indirect drive target differs significantly from that of direct drive targets. The massive hohlraum converts capsule ion and x-ray debris into relatively soft and more potentially damaging x-rays.In order for the chamber dry wall armor to survive, some buffer gas MUST be present to absorb the prompt x-rays, re-radiating their energy over a longer period of time.To determine the minimum amount of Xe necessary to prevent significant first wall vaporization a series of BUCKY [7] radiative hydrodynamic simulations were conducted.

20. IFSA, Kyoto, Japan, September 2001 20 Example Results from BUCKY for Indirect Drive Spectra for a 6.5 m Radius C Chamber Wall Assuming 1 monolayer loss per shot as a measure of acceptable lifetime, the results show that for pre-shot armor temperatures less than ~1000°C, the amount of Xe necessary to protect such a chamber wall is significantly less than the 0.5 torr previously estimated in the SOMBRERO study.Unlike the direct-drive laser case, the presence of Xe actually aids in some heavy ion driver beam transport mechanisms, and target heating and injection complications from the presence of the Xe gas are much less important for the massive hohlraum- protected indirect-drive target than for the bare cryogenic direct-drive laser target. Pre-Shot Wall Temperature as a Function of Xenon Pressure for Loss of 1 Monolayer of Graphite Per Shot

21. IFSA, Kyoto, Japan, September 2001 21 Conclusions: Cautious Optimism for IFE Dry Chamber Wall Without Protective Gas Very encouraging results were obtained from IFE dry wall analyses based on recent direct drive and indirect drive target spectra. The analysis included the key effect of photon and ion time of flight on the energy deposition. The results showed that for the direct drive target, a design window exist for C and W armor lifetime even for a case without a protective gas which will greatly help to accommodate target thermal control and laser breakdown requirements while providing some margin for optimization of design parameters. The presence of a protective gas such as xenon is much more important for the indirect-drive spectra but at a much lower pressure than previously considered. Several erosion mechanisms exist for C and tritium codepostion on cold surfaces is a key concern. Alternate armor material candidates must be considered (e.g. W or other refractory metals). Initial analysis indicates also the benefit of using armor with engineered surface such as a C fibrous carpet. However, other issues must be addressed such as the possibility of coating the fiber with a refractory to avoid the C problems (in this case), manufacturing and long term integrity.

22. IFSA, Kyoto, Japan, September 2001 22 References [1]W. R. Meier, et al., OSIRIS and SOMBRERO Inertial Fusion Power Plant Designs: Final Report, WJSA-92-01 (DOE/ER/54100-1), March 1992. [2]D. T. Goodin et al., Nuclear Fusion 41 (5) May 2001. [3]S. E. Bodner, et al., Physics of Plasmas 7(6), June 2000, pp. 2298-2301. [4]http://aries.ucsd.edu/ARIES/WDOCS/ARIES-IFE/SPECTRA/ [5]D. A. Callahan-Miller and M. Tabak, Nuclear Fusion, Vol. 39, No. 7, July 1999. [6]A. R. Raffray, R. R. Peterson, et al., Assessment of IFE dry chamber walls, to appear in Fusion Engin. & Design. [7]J.J. MacFarlane, G.A. Moses, R.R. Peterson, BUCKY-1 - A 1-D Radiation Hydrodynamics Code for Simulating Inertial Confinement Fusion High Energy Density Plasmas, UWFDM-984, Univ. of Wisconsin, August1995. [8]J. Roth, et al., Erosion of graphite due to particle impact,” Nuclear Fusion, 1991. [9]http://aries.ucsd.edu/LIB/MEETINGS/0103-ARIES-TTM/http://aries.ucsd.edu/LIB/MEETINGS/0103-ARIES-TTM/