Magnetic Fusion Power Plants

1. Magnetic Fusion Power Plants Farrokh Najmabadi MFE-IFE Workshop Sept 14-16, 1998 Princeton Plasma Physics Laboratory

2. Fusion should demonstrate that it can be a safe, clean, & economically attractive option • Gain Public acceptance: • Use low-activation and low toxicity material and care in design. • Have operational reliability and high availability: • Ease of maintenance, design margins, and extensive R&D. • Have an economically competitive life-cycle cost of electricity: • Low recirculating power; • High power density; • High thermal conversion efficiency.

3. Utility Input R &D Needs Development Plan Design Options Mission and Goals Present Data base and Designs Evaluation Based on Customer Attributes Attractiveness Characterization of Critical Issues Feasibility Assessment Assessment Based on Attractiveness & Feasibility Redesign

4. For superconducting tokamaks,It is b/e (i.e.,bR0/a) that is important, not b • Fusion power density, P ~ b2BT4 =(b/e)2(eBT2)2 • Almost Constant for BT fixed at the TF coil MHD Figure of Merit eBT2 e = a/R

5. bA/S ( Plasma b) bp /A ( Bootstrap current fraction) Tokamak Research Has Been Influenced by the Advanced Design Program Current focus of tokamak research “Conventional” high-b tokamaks (Pulsed operation) 2nd Stability high-b tokamaks (Too much bootstrap) Advanced tokamak (Balanced bootstrap) PU: Pulsed Operation SS: 2nd Stability FS: 1st Stability, steady-state RS: Reversed-shear

6. Our Vision of Tokamaks Has Improved Drastically in the Last Decade

7. Key Performance Parameters of ARIES-RS

8. ARIES-RS is a conceptual 1000MWe power plant based on a Reversed-Shear tokamak plasma

9. The ARIES-RS Replacement Sectors are Integrated as a Single Unit for High Availability Key Features • No in-vessel maintenance operations • Strong poloidal ring supporting gravity and EM loads. • First-wall zone and divertor plates attached to structural ring. • No rewelding of elements located within radiation zone • All plumbing connections in the port are outside the vacuum vessel.

10. The ARIES-RS Blanket and Shield Are Segmented to Maximize Component Lifetime Outer blanket detail • Blanket and shield consists of 4 radial segments. • First wall segment, attached to the structural ring, is replaced every 2.5 FPY. • Blanket/reflector segment is replaced after 7.5 FPY. • Both shield segments are lifetime components: * High-grade heat is extracted from the high-temperature shield; * Ferritic steel is used selectively as structure and shield filler material.

11. The divertor is part of the replacement module, and consists of 3 plates, coolant and vacuum manifolds, and the strongback support structure The divertor structures fulfill several essential functions: 1) Mechanical attachment of the plates; 2) Shielding of the magnets; 3) Coolant routing paths for the plates and inboard blanket; 4) “superheating” of the coolant; 5) Contribution to the breeding ratio, since Li coolant is used.

12. Three RF Launchers Are Needed for Current Profile Control in ARIES-RS • The ICRF fast wave launcher uses a folded waveguide cavity with capacitive diaphragms and coax feed • Folded waveguides offer a compact and robust structure and can be built out of low-activation material with thin copper coating.

13. The ARIES-RS Utilizes An Efficient Superconducting Magnet Design TF Coil Design • 4 grades of superconductor using Nb3Sn and NbTi; • Structural Plates with grooves for winding only the conductor. TF Strcuture • Caps and straps support loads without inter-coil structure; • TF cross section is flattened from constant-tension shape to ease PF design.

14. Alternative Confinement Systems • No current-drive (low recirculating power): • Stellarators (SPPS): recent advances bring the size in-line with advanced tokamaks. Needs coils and components with complicated geometry. • No superconducting TF coils • Spherical tokamaks (ARIES-ST): Potential for high performance and small size devices for fusion research but requires high beta and perfect bootstrap alignment. Center-post is a challenge. • RFP (TITAN): Simple magnets and potential for high performance. Steady-state operation requires resolution of the conflict between current-drive and confinement.

15. Stellarator Power Plant Study focused the US Stellarator Activity on Compact Stellarators • Modular MHH configuration represented a factor of two improvement on previous stellarator configuration with attractive features for power plants. • Many critical physics and technology areas were identified.

16. Spherical Tokamak Option Fusion development devices (e.g., neutron sources): • Modest size machines can produce significant power; • Planned experiments should establish the physics basis. Power Plants: • Recirculating power fraction (mainly Joule losses in the center-post) is the driving force: Maximize plasma beta and minimize the distance between plasma and center-post.

17. The ARIES-ST Study Has Identified Key Directions for Spherical Tokamak Research • Substantial progress is made towards optimization of ST equilibria with >95% bootstrap fraction: • b = 54%, k = 3; • A feasible center-post design has been developed; • Several methods for start-up has been identified; • Current-drive options are limited; • 1000-MWe ST power plants are comparable in size and cost to advanced tokamak power plants.

18. Reversed-Field Pinches • High engineering beta as the toroidal field in the plasma is mainly produced by the currents flowing in the plasma. • TITAN Design: Major Radius 3.9 m Minor Radius 0.6 m Neutron wall loading 18 MW/m2 Poloidal b 0.22 Toroidal field at plasma surface -0.4 T Plasma current 18 MA

19. Reversed-Field Pinches • Pulsed RFP power plants are not attractive (large formation/startup voltseconds, high loop voltage). • Steady-state RFPs require efficient current-drive systems (bootstrap current is small). • Helicity injection (e.g., oscillating fields current drive) is an option but can cause increased transport. • Requires toroidal divertors, impact on dynamo is unknown. • Requires a conducting shell for stability.

20. Advanced Technologies: High-Temperature Superconductors • YBCO • Highly textured tapes. Short tapes is produced • High current density even at liquid nitrogen temperature as long as B is parallel to the surface of the tape. • BSSCO (2212-2223 varieties) • Wires and tapes have been manufactures (100’s m) • Easier to manufacture than YBCO but they less impressive performance. • Much higher current density and critical field capability compared to Nb3Sn at 4.2K

21. Advanced Technologies: High-Temperature Superconductors • Physics Implications: • Operation at higher fields (limited by magnet structures and wall loading) • Smaller size, plasma current and current drive requirements. • Engineering Implications: • Operation at higher temperatures simplifies cryogenics (specially is operation at liquid nitrogen temperature is possible) • Decreased sensitivity to nuclear heating of cryogenic environment.

22. Conclusions • Customer requirements establish design requirements and attractive features for a competitive commercial product. • Progress in the last decade is impressive and indicates that fusion can achieve its potential as a safe, clean, and economically attractive power source. • Additional requirements for fusion research: • A reduced cost development path • Lower capital investment in plants. • For fusion energy objectives, our program must address clearly the relationship between developing an attractive fusion product, the cost of an energy R&D pathway, the changing market place, and quality of environment issues such as global climate change.