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.

Vacuum Vessel

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

Parameters

Material Inconel 625

Weight 8300 lbs

Outer radius ~ 2 m

Height ~ 1.7 m

Wall thickness ~ 1 cm

Contour tolerance +/- 5 mm

Inside surf area ~ 37 m2

Enclosed volume ~ 6 m3

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 ~ 30-40 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

VacuumVessel Helical

Coils

StructuralShell

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

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 20000713- 8

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/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?

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/cm2

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/cm2 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

Beforecompaction

Aftercompaction

What is minimum bend radius?

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

2 options now being studied

Saddle coils plus 1/R (TF)

background coilsand 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

Parameter

Major radius, m 1.70

No. of modular coils 21

Coil to plasma distance, cm 21.7

Coil to coil distance, cm 15.0

Nominal cross section, cm 11 x 20

Current / Coil, kA 810

No. of Turns / Coil 40

Structure thickness, cm 1.6

Epoxy fill, cm 0.3

Winding pack dimensions, cm 4 x 16.2

Turn insulation thickness, mm 0.8

Ground wrap, mm 0.8

Turn dimensions, mm 14.4 x 17.6

Hole diameter, mm 5

Cable packing fraction, % 75

Current density in Cu, kA/cm2 11.5

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 ProcessParameter Analysis planned or in progress

Access Access will be evaluated against requirements

Design margin Compare calculated response to allowables for range of operating parameters

Cost (initial and oper.) Algorithms being developed to evaluate all costs

Risk (e.g., fab. errors cause bad surfaces)

Risk will be evaluated based on experience, discussions with vendors

Maintainability Feasibility and ease of maintenance activities (eg, tile replacement) will be evaluated

Fabrication and assembly schedule

Flow charts of fab and assy steps will be developed and durations will be estimated

Other differentiating parameters TBD

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