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 Stellarator Core Engineering

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.

“In-PBX” was original option Re-use PBX TF, PF coils and structure Add pre-assembled, “drop in “ stellarator core, consisting of –vacuum vessel, –saddle coils, –saddle coil structure –conformal cryostat Studies provided design information that can be transferred to other configurations

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

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

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 ~ W/cm^2 for 12 MW heating Geometry: PFCs fit within 25 mm envelope inside VV

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

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

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

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

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

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

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 Table 1 - Summary of Diagnostic Port Needs on NCSX Preliminary Assessment

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

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

MJC Access for diagnostics Access solutions identified for nearly all diagnostics Access ports must avoid saddle coils, shell parting lines, and external coils

Vacuum pumping on NBI duct

Winding design criteria Current density is the primary issue -LN2 cooling required for J up to ~20 kA/cm 2 -Room temperature cooling possible for J < 10 kA/cm 2 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?

Temp. depends on J in copper

How hot is too hot? –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/cm 2 Outboard Flange Bolt Pocket Top/Bottom Interface Coil Slot Thermal stress was considered primary limit

Tests show cable soft enough to allow 250K rise Conductor Sample 4-in dia 13-in Plunger R&D shows cable is soft Current density target set at <20 kA/cm 2 in copper considering all factors

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 What is minimum bend radius?

Design criteria summary “In-PBX” studies provided insight for design saddle coil windings –Current density limit is 22 kA/cm 2 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

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

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

3 period modular coils

Modular coil winding pack

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

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 Structure concepts

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

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

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

Design Evaluation Process

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

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