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Stellarator magnets

Stellarator magnets. L. Bromberg J.H. Schultz MIT Plasma Science and Fusion Center ARIES meeting March 8-9, 2004. Organization of talk. Magnet design with dual temperature superconductor MgB 2 and NbTi Costing algorithm Future work. Medium Temperature Superconductor.

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Stellarator magnets

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  1. Stellarator magnets L. Bromberg J.H. Schultz MIT Plasma Science and Fusion Center ARIES meeting March 8-9, 2004

  2. Organization of talk • Magnet design with dual temperature superconductor • MgB2 and NbTi • Costing algorithm • Future work

  3. Medium Temperature Superconductor

  4. Preliminary designMgB2 Needed for determination of radiation effects on magnet due to B in the SC

  5. Algorithm for low Tc coil

  6. Manufacturing • Inserted from the plasma side into large structures. • If wound. substantial amount of strain capability required for assembly • Nb2Sn requires low strain • MgB2 has similar low strain capability • (both require react and wind) • Stuck with either NbTi (at low temperature at high field) or high temperature superconductor.

  7. Multiple temperature operation? • By using either MgB2 or BSSCO, it is possible to grade the magnet for cost reduction • Use NbTi at 4 K for the low temperature region • The low field region is further removed from the plasma as is better shielded.

  8. Method for performing dual temperature - Option 1 • Radiation captured at the higher temperature • Need to transfer loads through low temperature region (one insulation region required)

  9. Method for performing dual temperature - Option 2 • Substantially decreased thermal loads • Need to transfer outward loads from high temperature region through low temperature region before getting to structure (if bucking): two insulation regions.

  10. Dual temperature • Impact mainly on cost. • Tradeoff: • Much cheaper superconductor if using dual temperature • But • More complicated arrangement • Need for 4 K refrigerator • Final design will investigate both

  11. Design/Costing algorithms • Prepared simple spread sheet model for designing coils and costing • Coil designs (conductor) • Simple coil (one type of conductor) and coils with multiple types of conductors • Coil structure • Coil cost • Spread sheet transferred to Jim Lyon

  12. Using design by J. Lyon(presented in Dec. meeting) • Coil aspect ratio small • There may be sufficient space between coils without having to change the winding pack shape

  13. Coil structure • Assumes that all space between coils is filled with structure • Regions for access ports need to be reinforced elsewhere • Conservative assumption • Subtraction of relatively large numbers for estimating the volume • Good for system code. Will be improved once design space is narrowed down.

  14. Calculation of external structure • Using simple beam theory (good for system analysis) • A = B02 R02/2 m0 • S1 = A (R2 ln(R2/R1) - R2 +R1)/(R1 (R2-R1)) • S2 = A (R2 - R1 - R1 ln(R2/R1))/(R2 (R2-R1)) • t1 = S1/ sm ; t2 = S2/ sm • R3 = (S1 R12 + S2 R22)/(A ln(R2/R1)) • a = 3/8 S2 R2 (R2-R3)/sb / R3) • R2: outer external structure radius • R1: inner external structure radius

  15. Design criteria for external support • Use SS-316 because of the large amounts of materials required could have substantial cost penalty • Cross section of support determined by bending • Maximum stress by lowest of 1.5 x greater of (2/3 sy or 1/3 su) • Use around 800 MPa (for SS-316) • SS-316: (1400 MPa yield, 1800 MPa ultimate)

  16. Bucking cylinder and total structure cost • Large structure with tight tolerances. • $50/kg may be an underestimate • Can we use rapid prototyping methods for manufacturing of these large structures?

  17. Total coil costing • Cost of magnet dominated by winding assembly • Cost of BSCCO would be larger than that of Nb3Sn

  18. Magnet design • Very aggressive IFE magnet design ran into a lot of resistance (from the reviewers of Fusion Eng Design article) • I propose that the baseline be wound magnet • BSCCO and NbTi, if cost warrants • Else, just BSCCO • Use aggressive design as option

  19. Work to be done • System design tools have been developed • Detailed calculations need to be performed for a design point, in order to assure the appropriateness of the algorithms • Calculate stresses using actual currents and geometries • Structural considerations • Deformations and strains of the structure and superconductor • Superconductor characteristics • Power supplies • Cryogenic loads and cooling • Need one design point to calculate some details • NCSX (??) • HSR

  20. Resources • Can PPPL provide assistance with the calculations of the loads? • Stresses? • P. Titus has been performing calculations for NCSX, but only earthquake analysis

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