1. 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:

2. 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)

3. 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.

4. Power Plant Physics Needs And Directions for Burning Plasma Experiments

5. 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

6. 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.

7. 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)

8. 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

9. 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

10. 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) ~ A better confinement has to be degraded!
Understanding (and manipulating) local transport is critical to optimizing plasma profiles.

11. Fusion Technologies Have a Dramatic Impact of Attractiveness of Fusion

12. 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.

13. 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%

14. ARIES-AT Fusion Core

15. 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%.

16. 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

17. 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.

18. 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.

19. 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

20. ARIES-AT Also Uses A Full-Sector Maintenance Scheme

21. 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

22. 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

23. 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.