Presented by Brad Nelson Tom Brown, Mike Cole, Paul Fogarty, Paul Goranson, Phil Heitzenroeder,

1.  Stellarator Core Engineering Presented by Brad Nelson Tom Brown, Mike Cole, Paul Fogarty, Paul Goranson, Phil Heitzenroeder, Wayne Reiersen, Dave Williamson, and others NCSX PAC Meeting August 2, 2000

2. Presentation Outline • Review of original option, “saddle coils in PBX-M” • Vacuum vessel and PFCs • Coil / structure design • Access for heating and diagnostics • Development of winding design criteria • Design solutions for “out-of-PBX” options • modular options • Saddles with new 1/R coils • Plans for design development for CDR and PVR.

3. Vessel is stand-alone structure bake-able to 350C Vacuum Vessel

4. Vessel fabrication options • Press-forming is preferred option • Only 3 segment shapes required • Explosive forming a possible variant • Brake-bending (ala W7-AS) possible but requires too much welding

5. PFC requirements • Carbon-based material, bake-able to 350C • Neutral beam armor • Protect VV from NBI shine-through and during beam calibration • Peak power density ~ 800 W/cm^2 • Limiters / Divertor • Task force developing options for September • Limiters may be adequate for initial operation • Vessel coverage • Coatings (eg boronization) may be acceptable initially • Full coverage may be required ultimately • Avg. heat flux ~ 30-40 W/cm^2 for 12 MW heating • Geometry: PFCs fit within 25 mm envelope inside VV

6. NBI armor • Assume graphite tiles attached to VV, inertial cooling • Temperature < 700 C, well below 1200 C limit

7. PFC coverage • Plan staged approach, with limiters and coatings for initial operation • Preferred approach is full graphite tile coverage, but: • Total coverage of wall with conformal tiles is costly • major cost driver, equal to VV cost • large number of tile shapes • Options under study • coatings on low heat flux regions • simplified tile design • conformal blankets

8. Saddle coils wound on shell • Cast Nickel-Al-Bronze structure, 48 segments, ~0.5-in wide x 3.5-in deep slots Structural Shell Vacuum Vessel Helical Coils Core Assembly Typical Coil Cross-Section

9. Vessel sector Outboard shell subassembly Inboard shell subassembly Shell / VV assembly

10. Leads and thermal insulation Thermal insulation sprayed over shell (7 cm polyurethane with butyl rubber vapor barrier) Saddle coils wound on shell coaxial lead for each coil installed

11. What are the issues? • Access for Heating and Diagnostics • Allowable winding parameters : Design Criteria • Current density • Bend radius • Shell groove dimensions

12. Table 1 - Summary of Diagnostic Port Needs on NCSX Preliminary Assessment Access requirements • Heating Systems • 4 NBI injectors: adapt existing PBX-M systems • 6 MW ICH (4 antennas) • Diagnostics • Ports with correct size and view for initial set of diag. (83 listed) • Spare ports for upgrades (20) • Fueling/pumping • Man access

13. Access for NBI • Workable solutions found • Beams tangent at 1.5 m • Slight interference between beam boxes and with TF coils will require modifications • No symmetry with 3 field periods and 20 TF coils

14. Access for RF heating • Launcher envelope is 400x440x240 mm • 4 launchers will fit within the large ports

15. Access solutions identified for nearly all diagnostics Access ports must avoid saddle coils, shell parting lines, and external coils Access for diagnostics MJC 20000713- 8

16. Vacuum pumping on NBI duct

17. Winding design criteria • Current density is the primary issue • LN2 cooling required for J up to ~20 kA/cm2 • Room temperature cooling possible for J < 10 kA/cm2 • Current density is limited by a number of factors • material temperature limits • thermal stress due to temperature rise • power consumption • cooldown and pulse repetition rate • fatigue, other effects • The issues, then, are: • how hot does the conductor get during a pulse? • how hot can the conductor get before it reaches a limit?

18. Temp. depends on J in copper

19. Outboard Flange Top/Bottom Interface Coil Slot Bolt Pocket How hot is too hot? • Thermal stress was considered primary limit • Thermal stress in shell depends on winding spacing, bolt spacing and stiffness of winding • Stiffness of winding is the primary factor, “stiff” conductor would limit temp rise to < 20K current density to < 10 kA/cm2

20. 4-in dia Plunger Conductor Sample 13-in R&D shows cable is soft • Tests show cable soft enough to allow 250K rise • Current density target set at <20 kA/cm2 in copper considering all factors

21. What is minimum bend radius? • The cable conductor is made from very fine (36 Gage) wire • Even after compaction, this cable is very flexible, and can be readily wound on a radius of 1.5 times the conductor thickness • Recommend bend radius of 3 times the thickness to avoid excessive key-stoning and bunching

22. Design criteria summary • “In-PBX” studies provided insight for design saddle coil windings • Current density limit is 22 kA/cm2 in copper • Bend radius limit is 3 x conductor width • Winding groove > 13 mm for machining • Winding spacing depends on current in winding (ie, loads on ligament) • Criteria used to develop / optimize new options

23. 2 options now being studied Modular coils Plus weak 1/R background coils and PF/OH set Saddle coils plus 1/R (TF) background coils and PF/OH set

24. Coil option studies • Design concepts being developed for both the saddle and modular coil options • Vacuum vessel and PFCs • Winding (including conductor, leads, etc.) • Structure (including field/force/stress analysis) • Integration with background PF/TF coils • Design evaluation process has begun

25. 3 period modular coils

26. Modular coil winding pack

27. Winding forces are toward web • EM Force (lb/in) for Coil #1 in Coil Local Coordinate System B A C Lateral force direction is away from structure

28. Structure concepts • I-beam casting supports two winding • packs per coil • Coil radial forces reacted by inner • cylinder - tabs part of casting • Vertical tabs extend to surface at • +/- 1-m for coil-to-coil support • Additional shear structure required Alternate Concepts

29. Coil Structure, Windings, and Side Plates Assembly Sequence based on QOS Study Modular coil fab and assy. Figures are for QOS design

30. Modular coil issues • Coil winding trajectory • Bend radius • twist • Access for heating and diagnostics • Current density • Thermal insulation of modular coils difficult

31. Minimum bend radius • Bend radii should be on the order of 10 cm • Initial coil cases only 4 cm, but optimization has produced ~9 cm minimum radius • W7-AS and HSX coils have 11.5 and 8.1 cm minimum bend radii • Twist must also be addressed

32. Design Evaluation Process

33. Plans for PVR, CDR • Evaluate and select best option for chosen plasma (Saddles + 1/R, Modular + weak TF coils,) • Optimize geometry • Iterate winding aspect ratio, twist, bend radius, etc. • Configure ports • Continue R&D (primarily CDR) • Small coil winding test • Small structural casting test • Vacuum vessel pressing

34. Summary • The stellarator core design effort has included several different magnetic configurations, but only two are now candidates • Saddle coils plus background TF and PF coils (includes “In-PBX” option) • Modular coil option with “weak” 1/R coils and PF coils • Concepts for vacuum vessel, saddle coils, and modular coils have been developed • Access for heating and diagnostics evaluated in detail for C82, tools in place for detailed look at other specific configurations • Plans in place to reach the PVR and CDR, most R&D is pending selection of configuration option