EVOLUTION OF VISIONS FOR TOKAMAK FUSION POWER PLANTS Farrokh Najmabadi University of California, San Diego, La Jolla, CA, United States of America IEEE 19th Symposium on Fusion Engineering (SOFE 19) January 22-25, 2002 Atlantic City, NJ You can download a copy of the paper and the presentation from the ARIES Web Site: ARIES Web Site: http://aries.ucsd.edu/ARIES/

The ARIES Team Has Examined Several Magnetic Fusion Concept as Power Plants in the Past 14 Years TITAN reversed-field pinch (1988) ARIES-I first-stability tokamak (1990) ARIES-III D-3He-fueled tokamak (1991) ARIES-II and -IV second-stability tokamaks (1992) Pulsar pulsed-plasma tokamak (1993) SPPS stellarator (1994) Starlite study (1995) (goals & technical requirements for power plants & Demo) ARIES-RS reversed-shear tokamak (1996) ARIES-ST spherical torus (1999) Fusion neutron source study (2000) ARIES-AT advanced technology and advanced tokamak (2000) ARIES-IFE assessment of IFE chambers (current research)

ARIES Research Aims at a Balance Between Attractiveness & Feasibility Top –Level Requirements for Commercial Fusion Power Have an economically competitive life-cycle cost of electricity: Low recirculating power; High power density; High thermal conversion efficiency; Less-expensive systems. Gain Public acceptance by having excellent safety and environmental characteristics: Use low-activation and low toxicity materials and care in design. Have operational reliability and high availability: Ease of maintenance, design margins, and extensive R&D. Reasonable Extension of Present Data base Physics: Solid Theoretical grounds and/or experimental basis. Technology: Demonstrated at least in small samples.

Power Plant Physics Needs And Directions for Burning Plasma Experiments

For the same physics and technology basis, steady-state devices outperform pulsed tokamaks Physics needs of pulsed and steady-state first stability devices are the same (except non-inductive current-drive physics). ARIES-I’ Pulsar Non-inductive drive Expensive & inefficient PF System Very expensive but efficient Current-drive system High Low Recirculating Power High Bootstrap, High A, Low I Optimum Plasma Regime Yes, 65-%-75% bootstrap fraction bN~ 3.3, b ~ 1.9% No, 30%-40% bootstrap fraction bN~ 3, b ~ 2.1% Current profile Control Higher (B ~ 16 T on coil) Lower because of interaction with PF (B ~ 14 T on coil) Toroidal-Field Strength Medium Low Power Density Medium (~ 7 m major radius) High (~ 9 m major radius) Size and Cost

Directions for Improvement Increase Power Density What we pay for,VFPC r D Power density, 1/Vp r > D High-Field Magnets ARIES-I with 19 T at the coil (cryogenic). Advanced SSTR-2 with 21 T at the coil (HTS). Little Gain Big Win r ~ D r < D Improvement “saturates” at ~5 MW/m2 peak wall loading (for a 1GWe plant). A steady-state, first stability device with Nb3Sn technology has a power density about 1/3 of this goal. High bootstrap, High b 2nd Stability: ARIES-II/IV Reverse-shear: ARIES-RS, ARIES-AT, A-SSRT2 Decrease Recirculating Power Fraction Improvement “saturates” about Q ~ 40. A steady-state, first stability device with Nb3Sn Tech. has a recirculating fraction about 1/2 of this goal.

Reverse Shear Plasmas Lead to Attractive Tokamak power Plants Second Stability Regime Requires wall stabilization (Resistive-wall modes) Poor match between bootstrap and equilibrium current profile at high b. ARIES-II/IV: Optimum profiles give same b as first-stability but with increased bootstrap fraction Reverse Shear Regime Requires wall stabilization (Resistive-wall modes) Excellent match between bootstrap & equilibrium current profile at high b. ARIES-RS (medium extrapolation): bN= 4.8, b=5%, Pcd=81 MW (achieves ~5 MW/m2 peak wall loading.) ARIES-AT (aggressive extrapolation): bN= 5.4, b=9%, Pcd=36 MW (high b is used to reduce peak field at magnet)

Evolution of ARIES Designs 1st Stability, Nb3Sn Tech. ARIES-I’ Major radius (m) 8.0 b (bN) 2% (2.9) Peak field (T) 16 Avg. Wall Load (MW/m2) 1.5 Current-driver power (MW) 237 Recirculating Power Fraction 0.29 Thermal efficiency 0.46 Cost of Electricity (c/kWh) 10 High-Field Option ARIES-I 6.75 2% (3.0) 19 2.5 202 0.28 0.49 8.2 ARIES-RS 5.5 5% (4.8) 16 4 81 0.17 0.46 7.5 Reverse Shear Option ARIES-AT 5.2 9.2% (5.4) 11.5 3.3 36 0.14 0.59 5 Approaching COE insensitive of current drive Approaching COE insensitive of power density

Estimated Cost of Electricity (c/kWh) Our Vision of Magnetic Fusion Power Systems Has Improved Dramatically in the Last Decade, and Is Directly Tied to Advances in Fusion Science & Technology Major radius (m) Estimated Cost of Electricity (c/kWh) Approaching COE insensitive of power density High Thermal Efficiency High b is used to lower magnetic field

What About Confinement Time? Confinement Experiments Global confinement time is critical as Plasma stored energy = Heating power / Confinement time Burning Plasma Experiments Improved global confinement means we can build a smaller (fusion power) machine. After machine is operational, confinement better than needed for ignition has to be degraded! Power Plant Power plant size is set by economy of scale of fusion power density. All ARIES designs require confinement performance similar to present experiments: H(89P) ~2-3. A better confinement has to be degraded! Understanding (and manipulating) local transport is critical to optimizing plasma profiles.

Fusion Technologies Have a Dramatic Impact of Attractiveness of Fusion

SiC composites are attractive structural material for fusion ARIES-I Introduced SiC Composites as A High-Performance Structural Material for Fusion SiC composites are attractive structural material for fusion Excellent safety & environmental characteristics (very low activation and very low afterheat). High performance due to high strength at high temperatures (>1000oC). Large world-wide program in SiC: New SiC composite fibers with proper stoichiometry and small O content. New manufacturing techniques based on polymer infiltration or CVI result in much improved performance and cheaper components. Recent results show composite thermal conductivity (under irradiation) close to 15 W/mK which was used for ARIES-I.

Improved Blanket Technology Continuity of ARIES research has led to the progressive refinement of research ARIES-I: SiC composite with solid breeders Advanced Rankine cycle Many issues with solid breeders; Rankine cycle efficiency saturated at high temperature Starlite & ARIES-RS: Li-cooled vanadium Insulating coating Improved Blanket Technology Max. coolant temperature limited by maximum structure temperature ARIES-ST: Dual-cooled ferritic steel with SiC inserts Advanced Brayton Cycle at  650 oC High efficiency with Brayton cycle at high temperature ARIES-AT: LiPb-cooled SiC composite Advanced Brayton cycle with h = 59%

ARIES-AT Fusion Core

ARIES-AT2: SiC Composite Blankets Outboard blanket & first wall Simple, low pressure design with SiC structure and LiPb coolant and breeder. Simple manufacturing technique. Very low afterheat. Class C waste by a wide margin. LiPb-cooled SiC composite divertor is capable of 5 MW/m2 of heat load. Innovative design leads to high LiPb outlet temperature (~1,100oC) while keeping SiC structure temperature below 1,000oC leading to a high thermal efficiency of ~ 60%.

Innovative Design Results in a LiPb Outlet Temperature of 1,100oC While Keeping SiC Temperature Below 1,000oC • Two-pass PbLi flow, first pass to cool SiCf/SiC box second pass to superheat PbLi PbLi Inlet Temp. = 764 °C Bottom Top Max. SiC/PbLi Interf. Temp. = 994 °C Max. SiC/SiC Temp. = 996°C PbLi Outlet Temp. = 1100 °C

Advanced Brayton Cycle Parameters Based on Present or Near Term Technology Evolved with Expert Input from General Atomics* Recuperator Intercooler 1 Intercooler 2 Compressor 1 Compressor 2 Compressor 3 Heat Rejection HX W net Turbine Blanket Intermediate 5' 1 2 2' 3 8 9 4 7' 9' 10 6 T S 5 7 Divertor LiPb Coolant He Divertor 11 Key improvement is the development of cheap, high-efficiency recuperators.

Multi-Dimensional Neutronics Analysis was Performed to Calculate TBR, activities, & Heat Generation Profiles Very low activation and afterheat Lead to excellent safety and environmental characteristics. All components qualify for Class-C disposal under NRC and Fetter Limits. 90% of components qualify for Class-A waste. On-line removal of Po and Hg from LiPb coolant greatly improves the safety aspect of the system and is relatively straight forward.

Use of High-Temperature Superconductors Simplifies the Magnet Systems HTS does not offer significant superconducting property advantages over low temperature superconductors due to the low field and low overall current density in ARIES-AT Inconel strip YBCO Superconductor Strip Packs (20 layers each) 8.5 430 mm CeO2 + YSZ insulating coating (on slot & between YBCO layers) HTS does offer operational advantages: Higher temperature operation (even 77K), or dry magnets Wide tapes deposited directly on the structure (less chance of energy dissipating events) Reduced magnet protection concerns And potential significant cost advantages Because of ease of fabrication using advanced manufacturing techniques

ARIES-AT Also Uses A Full-Sector Maintenance Scheme

Continuity of ARIES Research Has Led to the Progressive Refinement of Plasma Optimization Pulsar (pulsed-tokamak): Trade-off of b with bootstrap Expensive PF system, under-performing TF For the same physics & technology basis, steady-state operation is better Improved Physics ARIES-I (first-stability steady-state): Trade-off of b with bootstrap High-field magnets to compensate for low b Need high b equilibria with aligned bootstrap ARIES-RS (reverse shear): Improvement in b and current-drive power Approaching COE insensitive of current drive Better bootstrap alignment More detailed physics ARIES-AT (aggressive reverse shear): Approaching COE insensitive of power density High b is used to reduce toroidal field

ARIES designs Correspond to Experimental Progress in a Burning Plasma Experiment Pulsar (pulsed-tokamak): Trade-off of b with bootstrap Expensive PF system, under-performing TF “Conventional” Pulsed plasma: Explore burn physics Improved Physics ARIES-I (first-stability steady-state): Trade-off of b with bootstrap High-field magnets to compensate for low b Demonstrate steady-state first-stability operation. ARIES-RS (reverse shear): Improvement in b and current-drive power Approaching COE insensitive of current drive Explore reversed-shear plasma Higher Q plasmas At steady state ARIES-AT (aggressive reverse shear): Approaching COE insensitive of power density High b is used to reduce toroidal field Explore envelopes of steady-state reversed-shear operation

Advanced Technologies Have a Dramatic Impact on Fusion Power Plant Attractiveness High-performance, very-low activation blanket: High thermal conversion efficiency; Smallest nuclear boundary. Impact Dramatic impact on cost and attractiveness of power plant: Reduces fusion plasma size; Reduces unit cost and enhanced public acceptance. High-temperature superconductors: High-field capability; Ease of operation. Simpler magnet systems Not utilized; Simple conductor, coil, & cryo-plant. Advanced Manufacturing Techniques Utilized for High Tc superconductors. Detailed analysis in support of: Manufacturing; Maintainability; Reliability & availability. High availability of 80-90% Sector maintenance leads to short schedule down time; Low-pressure design as well as engineering margins enhance reliability.