Optimization of the ARIES-CS Compact Stellarator Reactor Parameters - PowerPoint PPT Presentation

Optimization of the ARIES-CS Compact Stellarator Reactor Parameters J. F. Lyon, ORNL for the ARIES Group 15th International Stellarator Workshop Madrid October 3, 2005

Topics • ARIES Reactor Optimization Approach • Configuration Properties • Coil, Blanket/Shield Models • Systems Optimization Code • Typical Case Results • Parameter Variations and Scaling

ARIES-Compact Stellarator Program Has Three Phases FY 2003/2004: Exploration of FY 2003/2004: Exploration of Plasma/Coil Configuration and Plasma/Coil Configuration and Engineering Options Engineering Options 1. Develop physics requirements and 1. Develop physics requirements and FY 2004/2005: Exploration of FY 2004/2005: Exploration of modules (power balance, stability, a modules (power balance, stability, a Configuration Design Space Configuration Design Space confinement, divertor, etc.) confinement, divertor, etc.) 1. Physics: aspect ratio, number of 1. Physics: aspect ratio, number of 2. Develop engineering requirements periods, rotational transform 2. Develop engineering requirements periods, rotational transform and constraints. profile, β , α losses, etc. and constraints. profile, β , α losses, etc. 2. Engineering: configuration 3. Explore attractive coil topologies. 2. Engineering: configuration 3. Explore attractive coil topologies. optimization, management of optimization, management of space between plasma and coils, space between plasma and coils, etc. etc. 3. Focus on two configurations and Present status 3. Focus on two configurations and choose one for detailed design. choose one for detailed design. FY 2006: Detailed system design FY 2006: Detailed system design and optimization and optimization

Goal: Stellarator Reactors Similar in Size to Tokamak Reactors • Need a factor of 2-4 reduction compact stellarators 14 HSR-5 Plasma Aspect Ratio < R >/< a > 12 FFHR-1 HSR-4 10 SPPS 8 MHR-S 6 Stellarator Reactors Compact Stellarator 4 Reactors ARIES ARIES AT 2 RS Tokamak Reactors Circle area ~ plasma area 0 0 4 8 12 16 20 24 Average Major Radius < R > (m)

Parameter Optimization Integrates Plasma/Coil Geometry and Reactor Constraints Plasma & Coil Geometry Reactor Constraints • Shape of last closed flux surface • Blanket and shield thickness and < R axis >/< a plasma >, β limit • B max,coil vs j coil for superconductor • Shape of modular coils and • Acceptable wall power loading B max,coil / B axis vs coil cross section, • Access for assembly/disassembly < R coil >/< R axis >, ∆ min /< R axis > • Component costs/volume • Alpha-particle loss fraction Parameter Determination • < R axis >, < a plasma >, < B axis > • B max,coil , coil cross section, gaps • n e,I,Z (r), T e,i (r), < β >, P fusion , P rad , etc. • Operating point, path to ignition • Cost of components, operating cost cost of electricity Requires non-linear constrained optimization

Stellarator Properties Affect Reactor Optimization • Inherently steady state ⇒ no current drive power ignited plasma, small recirculating power • No plasma disruptions ⇒ higher density operation than in tokamaks, set by radiation losses ⇒ confinement time increases with density ⇒ < β β > appears to be limited by equilibrium rather than by β β stability • Modular coils determine plasma shape, hence plasma properties ⇒ higher-order field components needed at plasma surface decay rapidly with distance from the coils – B max /< B axis > is a function of the plasma-coil distance

Configuration Optimization Approach NCSX scale-up Coils Physics 1) Increase plasma-coil separation 1) Confinement of α α α particle α 2) Simpler coils 2) Integrity of equilibrium flux surfaces Critical to 1 st wall heat load and divertor High leverage in sizing Reduce consideration on MHD stability in light of W 7-AS and LHD results New classes of QA configurations How good/robust can How low should the one “design” the flux plasma aspect be? surfaces? MHH2 SNS 1) Develop very low aspect ratio geometry 1) Nearly flat rotational transforms 2) Detailed coil design optimization 2) Excellent flux surface quality Friday: L-P Ku, “New Classes of Quasi-axisymmetric Configurations”

First Class of Quasi-Axisymmetric Configurations Studied NCSX-like configurations NCSX-like configurations � Good QA, low effective ripple (<1%), α energy loss ≤ 7% � Good QA, low effective ripple (<1%), α energy loss ≤ 7% � Stable to MHD modes at < β > ≥ 4% � Stable to MHD modes at < β > ≥ 4% � Coils can be designed with aspect ratio ≤ 6 and are able to � Coils can be designed with aspect ratio ≤ 6 and are able to yield plasmas that capture all essential physics properties yield plasmas that capture all essential physics properties � Resonance perturbation can be minimized � Resonance perturbation can be minimized Footprints of escaping α α ’s on LCFS α α Energy loss ~12% in model calculation Heat load maybe localized and high (~a few MW/m 2 )

Second Class of Quasi-Axisymmetric Configuration Studied MHH2 MHH2 � Low plasma aspect ratio ( A p ~ 3.6) in 2 field periods � Low plasma aspect ratio ( A p ~ 3.6) in 2 field periods � Good QA, low effective ripple (<0.8%), α energy loss ≤ 5% . � Good QA, low effective ripple (<0.8%), α energy loss ≤ 5% . � Stable to MHD modes at < β > ≥ 4% � Stable to MHD modes at < β > ≥ 4% 16 simpler coils

Stellarator Geometry Is Characterized by Ratios Center of Coil • Distances, areas, volumes scale photo- Winding Surface graphically for fixed plasma and coil Major Radius R 0 configuration • Plasma Surface Plasma aspect ratio A p = < R axis >/< a > – plasma (and wall) surface areas ∝ Ave. Radius < a > ∝ < R > 2 ∝ ∝ (costs ∝ ∝ areas for fixed thickness parts) ∝ ∝ ∆ ∆ ∆ ∆ B 0 p wall ∝ ∝ 1/wall area, often sets < R axis > min ∝ ∝ Minimum Distance – surface area ∝ ∆ 2 / A p ∝ A ∆ ∝ ∝ ∆ ∆ ∆ between Plasma ∆ ∆ ∆ – plasma volume ∝ ∝ < R > 3 ∝ ∝ Edge and Center • Coil-plasma distance: A ∆ ∆ = < R axis >/ ∆ ∆ ∆ ∆ of Coil Winding ∆ ∆ – can also set < R axis > min = A ∆ Surface ∆ (D + ct/2) ∆ ∆ where D is the space needed for scrapeoff, first wall, blanket, shield, coil case, and assembly gaps • ct = coil ∝ < R axis > 2 , coil-coil Coil volume (cost) ∝ ∝ ∝ thickness Plasma spacing ∝ ∝ < R axis > ∝ ∝ ∆ ∆ ∆ ∆ Coil B max

Selected Two Main Plasma and Coil Configurations to Study Key Configuration Properties NCSX MHH2 Plasma aspect ratio A p = < R >/< a > 4.55 2.66 Wall (plasma) surface area/< R > 2 11.78 18.55 Minimum pl-coil dist. ratio A ∆ = < R >/ ∆ ∆ ∆ min 5.89 5.55 ∆ ∆ ∆ ∆ Minimum coil-coil dist. ratio < R >/(c-c) 10.03 10.33 Total coil length/< R > 89.3 91.0 NCSX B max /< B axis >, 0.3 m x 0.3 m coil pack 2.63 2.69 • B max /< B axis > varies rapidly with coil distance from plasma and coil pack dimensions • Only quasi-axisymmetric type of compact stellarators studied: 7 variants of NCSX and 4 of MHH2 MHH2

B max on the Coils Is an Important Parameter 8 15 7 square coil pack Current Density cross section ( k = 1) 6 2 ) (10-kA/mm Nb 3 Sn 10 axis > 5 B max /< B NbTiTa 4 Conductor Cost MHH2-8 5 ($/kA-m) 3 MHH2-16 2 NCSX cases 1 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 4 6 8 10 12 14 16 18 1/2 , m d = (cross section) B max (T) • Larger plasma-coil spacings lead to more convoluted coils and higher B max /< B axis >; constrains value of < B axis > if B max is limited • Coil current density and cost depend on B max ; Nb 3 Sn examined first

Coil Complexity Also Dictates Choice of Superconducting Material � Strains required during winding process are large � Strains required during winding process are large � NbTi-like (at 4K) ⇒ B < ~7-8 T � NbTi-like (at 4K) ⇒ B < ~7-8 T � NbTi-like (at 2K) ⇒ B < 9 T, problem with temperature margin � NbTi-like (at 2K) ⇒ B < 9 T, problem with temperature margin � Nb 3 Sn or MgB 2 ⇒ B < 16 T, Wind & React: � Nb 3 Sn or MgB 2 ⇒ B < 16 T, Wind & React: � Need to maintain structural integrity � Need to maintain structural integrity during heat treatment (700 o C for a few during heat treatment (700 o C for a few hundred hours) hundred hours) � Inorganic insulators � Inorganic insulators � Inorganic insulation is assembled with � Inorganic insulation is assembled with magnet prior to winding and thus able to magnet prior to winding and thus able to withstand the Nb 3 Sn heat treatment withstand the Nb 3 Sn heat treatment process process – Two groups (one in the US, the other in – Two groups (one in the US, the other in Europe) have developed glass-tape that Europe) have developed glass-tape that can withstand the process can withstand the process A. Puigsegur et al., Development Of An Innovative Insulation For Nb3Sn Wind And React Coils

Minimum Coil-Plasma Distance Can Be Reduced By Using a Shield-Only Zone