A PROPOSED TF JOINT DESIGN FOR THE NSTX CENTERSTACK UPGRADE

1. A PROPOSED TF JOINT DESIGN FOR THE NSTX CENTERSTACK UPGRADE Robert D Woolley 22 January 2009

2. NSTX CSUG CONCERNS • Reliability of TF Joints in CSUG • Cost & time to implement CSUG • Costs are dominated by PPPL labor • Costs are driven by design complexity • Reliability of other CSUG items • Structural supports (e.g., vacuum vessel) • Interturn insulation shear in TF central bundle • PF coils and their supports

3. PRESENT NSTX CENTERSTACK DESIGN • 2 TF layers in centerstack • TF Flags on 2 elevations • TF Flags bolted into centerstack conductors • TF Flags supported by flag boxes and annular hub rings • Conducting flexes • Spline locks rotation to umbrella, vacuum vessel • Lid not directly involved

4. TF OUTER LEG TERMINATIONS WITH CENTERSTACK REMOVED • Each of 12 TF Outer Legs includes 3 conductors • Each center terminal is above 2 side terminals, to align with 2 TF layers in centerstack

5. NSTX TF JOINT DESIGNS HAVE BEEN A SOURCE OF TROUBLES • Arc and fire occurred after a TF joint of the first design opened while powered. (Cost=1 yr & $) • TF joints of the second design then failed due to inadequate potting quality (Cost=0.5 yr & $) • Third try still behaves strangely above 0.5 Tesla (Cost=No higher field operations to 0.6 Tesla) • Pitting of joint contact surfaces continues, perhaps due to localized current concentrations & arcing • A fundamental issue is support of the radials.

6. TF JOINT APPARENT RESISTANCE • P VS R MEASUREMENTS (2003) FOR SILVERED CU JOINTS LED TO THE FOLLOWING PLOT • THIS WAS CHECKED BY A MARCH 04 TEST USING A SPARE TF FLAG & DIFFERENT BOLT TENSIONS.

7. TF JOINT APPARENT RESISTANCE(2) • •EACH OF THE 72 TF FLAG JOINTS HAS TWO VOLTAGE PROBES, ON LEFT AND RIGHT SIDES RESPECTIVELY. EACH PROBE IS IN A GROOVE HALFWAY UP ITS JOINT SIDE. • PROBE MEASURE VOLTAGE DROP BETWEEN ADJACENT TF TURN FACE AND FLAG POINT LOCATED A “SETBACK” DISTANCE AWAY. •EACH RECORDED VOLTAGE SIGNAL IS SAMPLED 2000 TIMES PER SECOND, WITH 8192 SUCCESSIVE SAMPLES ARCHIVED FOR THAT SHOT.

8. TF JOINT APPARENT RESISTANCE(3) • FOR EXAMPLE, THE ADJACENT MATLAB PLOTS DISPLAY VOLTAGES VS. TIME FOR ALL 72 JOINTS DURING PLASMA SHOT NUMBER 112987, AND ON THE SAME TIME BASE SHOWS COIL AND PLASMA CURRENTS. • The APPARENT RESISTANCE OF A TF JOINT IS THE TIME-VARYING RATIO OF THE MEASURED VOLTAGE TO THE TF CURRENT

9. TF JOINT APPARENT RESISTANCE(4) • Additional tests on a spare TF flag (By H. Schneider) showed that applying vertical or horizontal pressure changed these apparent resistances. Test measurements were fitted and corelated with an ANSYS model. • The resulting “Rosetta” curve and associated ANSYS model allowed the structural situation to be inferred from voltage measurements.

10. TF JOINT APPARENT RESISTANCE(5) • Pressure Profiles vs. Net Moment (0-5E4 inch-lbs) 2004 Probe data showed many TF joints were in trouble 2004 disassembly then showed Flag Box potting was deficient

11. PITTED TF JOINTS • Before operation joint faces were silvered • After operation silver was eroded in regions and underlying copper was locally pitted • Pitting has continued even after fixing potting in 2005

12. REASONS FOR THE PREVIOUS NSTX DESIGNS’ TROUBLES (IN MY OPINION) • TF flag stiffness transmits to joints torques resulting from lateral & vertical flag forces • Two competing load paths: TF flag stiffness vs flag box potting compound • Joint lift-off occurs at full field, but occurs at lower field if potting is bad • The design's right-angle turn in current direction concentrates current at joint corner • Current concentration jumps to new location when lift-off occurs (thus arcing?) • Magnetic forces do not clamp joints closed, they only rock joints laterally or vertically • Contact surfaces are cut by bolts which concentrate current in the heightened contact pressure zones surrounding bolt holes while also reducing overall net contact area

13. TF FLAG CURRENT TURNS CORNER • Note that current streamlines bunch more at corner than these current direction arrows

14. TF JOINT DESIGN CONCEPT SHOULD BE REPLACED FOR THE CSUG • The new NSTX operations will increase toroidal field, plasma current, poloidal field, and pulse duration, so forces and temperature rises will increase at many locations. • These force and temperature increases could challenge the original design concept.

15. PROPOSED CONCEPT FOR TF JOINT & FLAG REPLACEMENT • CURRENT SHAPING GENERATES E-M FORCES PRESSING JOINTS CLOSED • FLEXIBLE CONDUCTING TF STRAP REDUCES JOINT TORQUES AND ALLOWS THERMAL EXPANSION • SINGLE-LAYER TF IN CENTERSTACK • ELECTRICAL JOINT ALSO SERVES AS SHEAR JEY • OOP SUPPORT BY LEANING ON STATIONARY INTERTURN STRUCTURE, + CS JOINT RESTRAINT

16. TF FLEX-STRAP SHAPE • MANY THIN FLEXIBLE CONDUCTORS • CONSTANT-TENSION CURVE SHAPES SEPERATED BY THIN GAPS • ELEVATION CHANGES OVER CT CURVES • Different Possible Ways • High Inner may maximize clamping force

17. THE NEW DESIGN SHOULD HAVE THE FOLLOWING FEATURES (1) • Flexible conductors should be used since they only transmit tensile forces without moments • OOP forces on flexes would be supported via nearby structural surface contact • Inplane forces on flexes would be supported via their constant-tension curve shapes

18. THE NEW DESIGN SHOULD HAVE THE FOLLOWING FEATURES (2) • Current should have parallel (vertical) current flow on both sides, as lap-joints

19. THE NEW DESIGN SHOULD HAVE THE FOLLOWING FEATURES (3) • For TF joints the vertical lap-joint configuration automatically results in magnetic forces clamping the joints closed and thus increasing contact pressure

20. THE NEW DESIGN SHOULD HAVE THE FOLLOWING FEATURES (4) • TF joints will not be cut by any bolts. Instead, either a radial clamp can press from an adjacent ring, or alternatively, bolts can be used in non-conducting regions above or below joints.

21. SINGLE-LAYER TF IN CENTERSTACK • 36 TF centerstack conductors form a single layer, with 36-fold rotational symmetry in the toroidal direction. • Existing TF outer leg terminations require additions: • Existing terminals for each end of the 12 three-conductor TF outer legs accommodate the present 2-layer version of the TF central bundle. Thus 2/3 of the existing TF outer leg terminals are at elevations closer to the plasma while 1/3 are at elevations farther from the plasma. • Vertical copper bars will be attached to the 2/3 of outer leg terminals closer to the plasma to extend them to the same elevation as the 1/3 of terminals farther from the plasma.

22. SINGLE-LAYER TF IN CENTERSTACK (2) • Max radius 0.1925 m • Each conductor subtends 10 degrees • Turn width is 33.5 mm (= 1.32”)

23. COPPER BARS FOR TF OUTERLEG TERMINALS • Bars bolted to lower TF outerleg terminals extend to match elevation of upper terminal • Bars may also need to spread sideways • 36 equal spaces • Enough space to insert bolts

24. THE PROPOSED TF DESIGN INCLUDES THE FOLLOWING ASSEMBLIES: • (72) TF Radial assemblies shaped to direct internal TF current in an optimized pattern. Each assembly includes the following copper components: • A Boltplate, to be bolted to an TF outer leg terminal • A VerticalBar, with a TF joint surface on the lower part of its inner vertical face, and with shearkey features registering its fit with a TF turn • Flexible cables connecting Boltplate with VerticalBar • (72) Lid-mounted OOP radial rib supports • (2) Bucket-ring Supports (if needed) • Including screw-adjusted inclined-plane clamps

25. CT FLEX CURVES CLOSE TO OH FIELD • This reduces OOP forces near centerstack where support is difficult • Compromised to avoid lid and to maintain EM clamping force on joint • Flexes could run closer to OH field lines if lid raised

26. ANSYS TF MODEL • 36-fold rotational symmetry • Multiphysics includes conduction, magnetics, structural, electromechanical contact • Includes TF Central Conductor, insulation, shearkey, joint contact, straps • Outer leg not realistic • PF not yet modeled • No Vacuum Vessel

27. TOROIDAL FIELD

28. TF CURRENT DENSITY (JS) • JS not best current est.

29. TF CURRENT DIRECTION (JS)

30. VM STRESS ON TURN MIDPLANE • Max VM stress < 70 MPa • (SMX value set by point restraint at model’s bottom)

31. VM STRESS, CENTERSTACK REMOVED • The concentration near the VerticalBar’s bottom is a clamping force artifact • VM stresses are modest

32. JOINT CONTACT PRESSURE • No sharp spatial variations

33. FLEXES SHOULD BE BRAIDED, NOT SOLID • Water-jet cut is only flexible in vertical plane. • Misalignments require horizontal flexibility • OOP Support of flexes by Lid-Mounted Radial Ribs requires horizontal flexibility • Flat braided cable isi commercially available in many sizes, including up to 500 kcm. Total needed is 6.4 mcm, so as few as 13 cables could suffice.

34. ADDITIONAL OOP SUPPORT • An additional OOP support may be needed. • The VerticalBar may need OOP support tying it to the TF central bundle of conductors. • The TF central bundle may need OOP support if insulation shear is excessive without it. • These can both be provided if needed via a bucket-ring structure clamped to the TF centerstack.

35. SUMMARY • For the CSUG with its new higher field operations, the scheme for TF radials and joints should be replaced with a better one. • A flexible radial structure with CT curve shape will eliminate competing multiple load paths. • A lap-joint configuration will provide EM clamping pressure and more uniform current