Accommodation of Prompt Alpha-Particle Loss in Compact Stellarators

1. Accommodation of Prompt Alpha-Particle Loss in Compact Stellarators • R. Raffray, T. K. Mau, F. Najmabadi, and the ARIES Team University of California, San Diego PSI-17 Hefei, China May 21-26, 2006

2. Alpha Loss is an Important Issue in MFE, Governed by the Magnetic Field Ripple •  loss can impact tokamaks, where size and aspect ratio constraints can result in coil placement inducing significant ripple, and more importantly stellarators, where the inherent non-axisymmetry of the magnetic geometry causes an appreciable magnetic field ripple along the flux surfaces inside the plasma. • In this case, a non-negligible fraction of the alpha particles will either be born or “kicked” into orbits that are trapped in these ripples.• Depending on the magnetic topology, a fraction of these particles are promptly lost from the plasma and hit the PFC’s at energies equal to or close to their born value of 3.5 MeV. • The resultant footprints on the LCMS show that the bulk of these particles are lost in a region below the mid-plane on the outboard edge of the plasma cross-section. On the other hand, the more thermalized particles tend to diffuse from the plasma uniformly on the LCMS.

3. Several Classes of Stellarators Were Considered As Part of the ARIES-CS Study, Including NCSX-Based Configurations Example In-Core ARIES-CS Configuration with Port-Based Maintenance Coil Configuration and Plasma Shape for Example NCSX-Like 3-Field Period Concept

4. Example Spectrum of Particle Loss from ARIES-CS Compact Stellarator •  loss not only represents a loss of heating power in the core, but impacts the PFC design requirement and lifetime.

5. Accommodating Alpha Particle Flux on PFC • High heat flux could be accommodated by designing special divertor-like modules. • e.g. for typical ARIES-CS power plant parameters: - Pfusion = 2350 MW - Max. neutron wall load = 5 MW/m2 - FW Surface Area = 572 m2 - Assumed  loss fraction = 0.1 - Assumed  footprint = 0.05 - Ave. q’’ on alpha modules = 2.2 MW/m2 - Max. q’’ <10 MW/m2 for peaking factor < 4.5 - Divertor itself must be designed to accommodate both the divertor heat load and any incident  loss power. • Impact of -particle flux on armor lifetime (erosion) is an additional concern.

6. Divertor-Like Modules Can Accommodate Heat Flux of Up to 10 MW/m2 • T-tube divertor concept developed for ARIES-CS, able to accommodate ~10 MW/m2, utilizing a He-cooled jet configuration with W-alloy as structural material and a thin W armor. - Inlet and outlet He temperatures ~600°C and ~700°C - Maximum W alloy temperature < 1300°C - Total stress intensity < 370 MPa • Issues requiring further R&D effort include fabrication methods, W alloy development and thermo-fluid performance verification.

7. ~15 mm ~100 mm W armor W alloy inner cartridge W alloy outer tube (~ 1 mm thick) Graded transition between W and ODS FS He coolant inlet and outlet T-tubes (W alloy) Module Body (FS) ARIES-CS Divertor T-Tube Concept • T-tubes can be assembled to form target plate of required size.

8. Alpha Particle Flux Impact on Armor Lifetime • For these high  energies, sputtering is less of a concern and armor lifetime would be governed by some other mechanism, such as exfolation, resulting from accumulation of He atoms in the armor. • The implantation depth for ~ 1 MeV He in W is ~1.5 m. - Pfusion = 2350 MW - FW Surface Area = 572 m2 - Assumed  loss fraction = 0.1 - Assumed  footprint = 0.05 -  flux on PFC is ~3 x 1018 ions/m2-s - Eff. Vol. generat. rate ~2 x 1024 ions/m3-s • Sputtering yield of W as a function of He energy at different incidence angles • (from W. Eckstein, “Sputtering, reflection and range values for plasma edge codes,” IPP-Report, Max-Planck-Institut fûr Plasmaphysik, Garching bei Munchen, Germany, IPP 9/117, March 1998.)

9. He Implantation and Behavior in W Armor Quite Complex, Consisting of a Number of Mechanisms PORE or VOID BULK W Implanted He atom Desorption Bulk diffusion Trapping Detrapping Trapped He • Due to high heat of solution, inert- gas atoms essentially insoluble in most solids. • This can then lead to gas-atom precipitation, bubble formation and ultimately to destruction of the material. • He atoms in a metal may occupy either substitutional or interstitial sites. As interstitials, they are very mobile, but they will be trapped at lattice vacancies, impurities, and vacancy-impurity complexes. • The following activation energies were estimated for different He processes in tungsten [1,2]: - Helium formation energy: 5.47 eV - Helium migration energy: 0.24 eV - He vacancy binding energy: 4.15 eV - He vacancy dissociation energy: 4.39 eV - From [3], D (m2/s) = D0 exp (-EDif/kT); D0 = 4.7 x 10-7 m2/s and EDif = 0.28 eV 1. M. S. Abd El Keriem, D. P. van der Werf and F. Pleiter, "Helium-vacancy interactions in tungsten," Physical review B, Vol. 47, No. 22, 14771-14777, June 1993. 2. W. D. Wilson and R. A. Johnson, in Interatomic Potentials and Simulation of Lattice Defects, edited by P. C. Gehlen, J. R. Beeler and R. I. Jaffee (Plenum New York, 1972), p375. 3. A. Wagner and D. N. Seidman, Phys. Rev. Letter 42, 515 (1979)

10. Recent Experimental Data on Cyclic He Implantation and Release in W(from S. B. Gilliam, S. M. Gidcumb, N. R. Parikh, D. G. Forsythe, B. K. Patnaik, J. D. Hunn, L. L. Snead and G. P. Lamaze, J. Nucl. Mat. 347 (2005) 289.) • Results indicate that He retention decreases drastically when a given He dose is spread over an increasing number of pulses, each one followed by W annealing to 2000°C, to the extent that there would be no He retention below a certain He dose per pulse.

11. Fast desorption: C=0 Symmetry: dC/dx=0 Effective diffusion x = 0 x =  Effective Diffusion Analysis of Experimental Results for He Implantation and Release in W (from S. B. Gilliam, S. M. Gidcumb, N. R. Parikh, D. G. Forsythe, B. K. Patnaik, J. D. Hunn, L. L. Snead and G. P. Lamaze, J. Nucl. Mat. 347 (2005) 289.) Simple effective diffusion model • Set effective activation energy to reproduce final He retention level in experiments for given number of cycles, He dose implantation and temperature anneals • Activation energy derived from analysis would not be that of bulk diffusion but of the rate- controlling mechanism, much probably some form of trapping/detrapping mechanisms. - D0 = 4.7 x 10-7 m2/s -  = 1.5 m (for ~ 1 MeV He in W)

12. Effective Diffusion Activation Energy (Eeff,diff) as a Function of Dose per He Implantation from Modeling of Experimental Results (The curve fit has been drawn to suggest a possible variation of the activation energy with the He dose or concentration)

13. Possible Explanation of Effective Activation Energy Results • In general, trapping would increase with He irradiation dose, which creates defects and vacancies (followed by an anneal of the unoccupied trapped sites during the ensuing temp. transient). • At very low dose, He migration in W should be governed by bulk diffusion (Eeff,dif ~0.24-0.28eV) • As the dose per cycle increases, trapping sites are formed or activated and Eeff,dif increases. It seems that there is a near- threshold of He dose at which Eeff,dif increases rapidly to ~3.3-3.6 eV and stays there over a dose range of ~2 orders of magnitude. • Above this range, Eeff,dif increases rapidly to ~ 4.2-4.8 eV, indicating an increase in trapping perhaps due to He build up in vacancies (the vacancy dissociation energy is ~4.4 eV) • Overall, the dependence of activation energy with dose is about the same for SC and polycrystalline W except that it is shifted to lower doses for the latter case.

14. Application of Modeling Results to Estimate He Retention in ARIES-CS W Armor Due to  Loss • For ARIES-CS, both He irradiation and temperature are steady-state - Choice of Eeff,diff is tricky as the experimental results were obtained for cyclic conditions with a finite He dose per implantation. • As a rough guide, the experimental temperature anneals were integrated and manipulated to estimate the Tuniform which would result in the same diffusive time scale as the 60 s shown for each cycle. - Effective Tuniform ~ 1590°C, - Corresponding maximum dose rate ~ 1.6x1017 ions/m2-s. - Increasing Tuniform would result in shorter effective time and larger max. dose rates (e.g. ~ 3 s and 3 x 1018 ions/m2-s for a ~1800°C temperature), and reducing it would have the opposite effect. • For ARIES-CS, the steady state  dose rate is ~ 3 x 1018 He ions/m2-s and the experimental results would be best applied for high temp. cases with Eeff,diff corresponding to the max. dose, ~ 4.8 eV.

15. 15% at. He in W Alpha particle flux Porous W (~10-100 m) Fully dense W (~ 1 mm) Structure (W alloy) Coolant Estimate of He Retention in W Armor Due to  Loss as a Function of Operating Temperature and Characteristic Diffusion Length • Simple effective diffusion analysis for Eeff,dif = 4.8 eV • Not clear what is the max. He conc. limit in W to avoid exfolation - perhaps ~ 0.15-0.2 at.%[4,5] • High W temp. and shorter diffusion dimensions help, perhaps a nano- structured porous W - e.g. 50-100 nm at >~1800°C 4. G. Lucas, personal communication. 5. S. B. Gilliam, S. M. Gidcumb, N. R. Parikh, D. G. Forsythe, B. K. Patnaik, J. D. Hunn, L. L. Snead and G. P. Lamaze, J. Nucl. Mat. 347 (2005) 289.

16. Effect of Changing the Activation Energy of Governing He Migration Process on Estimated He Retention • A reduction in Eeff,diff would have a major effect, e.g. for a diffusion distance of 100 nm and a temperature of 1800°C, the steady state He to W inventory ratio, IHe/W: - IHe/W ~ 0.1 for Eeff,diff =4.8 eV - IHe/W ~ 4x10-5 for Eeff,diff =3.4 eV - IHe/W ~ 8x10-13 for Eeff,diff =0.24 eV. • An interesting question is whether at a such high W operating temperature some annealing of the defects might further help helium release. • Further R&D requiired to understand and better characterize integrated effect of governing He migration processes under prototypical conditions

17. Plasma Processes Incorporated is Working on Manufacturing Porous W with Nano Microstructure Which Could Enhance He Release • After W precursors are injected into the plasma flame, the vapor phase is quenched rapidly to solid phase yielding nano-sized W powder • Nano tungsten powders have been successfully produced by plasma technique and the product is ultra pure with an ave. particle size of 20-30 nm. Production rates of > 10 kg/hr are feasible. • Process applicable to molybdenum, rhenium, tungsten carbide, molybdenum carbide and other materials. • The next step is to utilize such a powder in the Vacuum Plasma Spray process to manufacture porous W (~10-20% porosity) with characteristic microstructure dimension of ~50 nm . TEM images of tungsten nanopowder, p/n# S05-15. (from S. O’Dell, PPI, personal communication)

18. Conclusions • • Alpha loss is a major issue in stellarators, impacting • the survival of the first wall. • • The armor lifetime under the alpha flux would depend on a number of parameters: • - a-particle energy spectrum • - armor material choice, configuration and temperature • - activation energies of processes governing He behavior in armor • • Use of a nano-sized porous W armor and high • operating temperature would help to enhance the • release of implanted He. • • Further R&D required to make sure that a credible solution exists for a CS.