# Presentation: 452597

## NCSXStellarator CorePre-conceptual Design Progress and Status

- Mike Cole, Paul Goranson, Brad Nelson, Dave Williamson
- NCSX PAC Meeting
- PPPL
- June 3-4, 1999

## Presentation Outline

- Requirements overview  	*What must the stellarator core do?*
- Stellarator core configuration  	*What is the design concept?*
- Evaluation  			*Will it work?*
- Issues  			*What are we worried about?*
- R&D  			*How do we plan to resolve the issues?*

## Baseline parameters for core design studies____________________________________________

- Pre-assembled, “drop in “ stellarator core, consisting of vacuum vessel, saddle coils, and saddle coil structure
- Option 2C - Re-use PBX TF, PF coils and structure
- C10 plasma with SAD18.5_16 coil set, LN cooled
- 26 coils per period
- 84 kA/coil at 2 Tesla field on axis
- 400 m total winding length
- 16 x 70 mm winding size, ~ 17,500 A/ cm ^2
- Vacuum vessel is stand-alone, structural vessel bake-able to 350C

## NCSX in the PBX Facility____________________________________________

## Proposed Stellarator Core Concept

-  Plasma-Shaped Vacuum Vessel
-  Inner Structural Shell (“apple core”)
-  Outer Structural Shell
-  Saddle Coils Wound on Outside of Shell
-  Fiberglass cryostat

## Saddle Coil Design Basis / Requirements

**C10A Plasma**

**SAD185-16 Coilset**
- Dist from Plasma to Coil	18-cm
- Longest Coil Length (1-turn)	16.7-m
- Total Length of Conductor	4080-m
- Total Weight of 78 Coils	6000-lb

## Proposed Concept

- General Arrangement

- Vacuum Vessel

- Inner Coil Structure

- Outer Coil Structure

## Proposed Concept

- Design Features

- Element
- Material
- (Shell)
- Poloidal
- Segmentation
- Toroidal
- Segmentation
- Assembly
- Coil Construction
- & Leads
- Slot Closure
- Cooling/
- Insulation

- Options
-  Stainless Steel
-  Aluminum
-  Bronze
-  Mid-plane
-  Top/Bottom
-  24 (Wedges)
-  120 (Plates/Wedges)
-  Bolting
-  Flexible Sq Conductor
-  Welded Cover Plate
-  Cooling Line Plug
-  Compacted Fill/Epoxy
-  LN Cooling
-  PBX Vessel / Cryostat

- Preference
- Nickel Aluminum
- Bronze (NAB)
- Top/Bottom
- 24 Wedge Segments
- Bolting
- Flexible Sq Conductor
- Cooling Line Plug
- LN Cooling Line +
- Fiberglass Enclosure

## Proposed Concept - Inboard shell

- Wedge Attachment
- (1-in dia bolts, 3-in spacing)

- 24 Wedge Segments

- Inboard Structure

## Proposed Concept- Outboard shell

- 24 Wedge Segments

- Outboard Structure

## Fewer segments lowers cost

- - Segmentation reduced from earlier designs to 8 segments per period
- - Fewer segments dramatically decreases:
-         - Number of casting patterns (cost)
-         - Machining set-ups and and time (cost)
-         - Number and accuracy of assembly fixtures (cost)

## Proposed Concept

- Coil Leads

- - Two pancake windings in a full-radius groove
- - Pancake-to-pancake transition occurs in a pocket
-   in the bottom of the groove
- - Layer-to-layer transitions are adjacent
- - Leads transition to coaxial configuration

- Coaxial
- Leads

- Full Radius

- Pocket

## Proposed Concept

- Coil Leads

- Coils above
- mid-plane

- Coils below
- mid-plane

- Mid-plane
- accessible

## Proposed Concept

- Coil Leads

## Performance Evaluation

- Helical Coils
- Structure

- - 16 x 70-mm winding pack, ~17.5-kA/cm2 at 2T
- - Approx 1-s ESW pulse length, 80-130K temperature rise
- - Cool-down in <10-min
- - 8 segments/period give ~20-ms longest time constant
- - Maximum coil EM load at 2T is ~1900-lb/in
- - EM loads calculated using ANSYS are in good agreement (~10%)
- - Maximum bolt load due to EM is 52-kN (shear), 34-kN (axial)
- - Combined EM/thermal bolt loading TBD, may need additional space

## Performance Evaluation

- Coil Cool-Down After Pulse

## What is max field on saddle coil conductor?

- BMOD Nodal Fringe Plot

-  Maximum Field at Coil Surface is 5.6-T
-  Bmax/B0 = 2.8  (2 Tesla toroidal field at 1.45m)
-  C10, SAD185-16 coil set

## What are the forces on the saddle coils?

- View Inside Looking Out

- Force Due To Toroidal Field

## Saddle Coil #1 Force/Deflection

## Do we believe the forces?

-  EM Forces calculated using two independent methods
-  ANSYS and MAGFOR agree within 10%

- Force Vectors (ANSYS)

- Force Vectors (MAGFOR)

## What do the saddle coil forces do to the shell?

-  C10-2T Load Case (Max Load = 1963-lb/in)
-  Net Force / Period = -767-kN (radial)
-  Maximum Displacement = 0.6-mm

- (m)

## Global Shell Stresses are reasonable

-  Max General Stress = 65-Mpa for 4 cm shell
-  Peak Stress at Support Points
-  Grooves not included

- 65-MPa

## The smooth shell works, but what about all the grooves?

## Ligaments between windings must react lateral forces

-  Conductor Modulus = 1-Gpa
-  Plasma Config C10-2T, Coil Set SAD185-16, Cut Thru One Half-Period

- C10

## Issue:  Current Density and Pulse Length

- Current density determines pulse length for adiabatic heating
- Current density is a function of number of turns, packing fraction in braided conductor, insulation thickness, allowance for tolerance, ligament width, etc.
- Baseline design (C10)
- Number of turns per saddle coil = 10
- Current density in copper  **~17.5 kA/cm^2 at 2 T**
- Pulse length for 80 to 130 K rise = ~**1 second** (equiv square wave)
- Power at end of pulse ~45 MW

## Current Density depends on winding details

## Temperature rise in conductor limits pulse length

## Thermal stress limits the temperature rise in conductor 

- Winding stiffness determines thermal stress, will be addressed by R&D

## Issue:  Bad Geometry of Adjacent Windings

- Saddle Coil #1
- (Guiding Coil)

- ~11-deg

- Coil #2
- (Adjusted)

- Coil #2
- (Original)

- - Curvature effects can be minimized by making coils parallel in critical regions
- - Parallel cross sections may provide more space for windings in critical regions
- - Program using Pro/E geometry has been demonstrated

## Proposed Stellarator Core Concept

-  **Plasma-Shaped Vacuum Vessel**
-  Inner Structural Shell (“apple core”)
-  Outer Structural Shell
-  Saddle Coils Wound on Outside of Shell
-  Fiberglass cryostat

## Vacuum Vessel Configuration

- Parameters

-  Inner radius = 38-in (0.97-m)
-  Outer radius = 78-in (1.98-m)
-  Height = 68-in (1.73-m)
-  Wall Thickness = 0.375-in
-  Contour Tolerance = +/-5-mm
-  Inside Surface Area = 58k-in2
-  Enclosed Volume = 218-ft3
-  Weight w/o PFCs = 8300-lb

-  Liner is a stand-alone, structural vessel that can be baked to 350-C

## Vacuum Vessel Design Choices

_**Material**_

_**Fabrication technique**_

-  316L or 304L stainless steel
- lowest material cost option
-  Inconel 600 or 625
- Low permeability, higher electrical resistivity

-  Break bending and welding
-  Explosive forming, casting, other

- Status - Break bending chosen for baseline design

- Status - Inconel 600 chosen for baseline design

- Options -

- Options -

## Vacuum Vessel Design Choices

- W 7 - AS/X Fabrication

## Vacuum Vessel Design Choices

_**Assembly / Design Integration**_

_**Heating / Cooling**_

-  One Piece
- Best procurement option
-  Three Pieces
- Coil structure design flexibility

-  Water/Steam
-  DowTherm

- Status - **Dowtherm system like NSTX chosen for baseline**

- Status - **Three pieces chosen as baseline**

- Options -

- Options -

## Vacuum Vessel Parameters/Performance 12 MW operation

- AVERAGE PLASMA HEAT FLUX			32 w/cm2
- BAKE OUT TEMPERATURE				350 C
- OPERATING TEMPERATURE				> 38 C
- TEMPERATURE RISE DURING PULSE			21 C
- 3 sec flat top pulse
- 1.27 cm liner
- THERMAL LOSS THRU SHELL DURING BAKE OUT	10.4 kW
- 2.54 cm thermal blanket
- THERMAL LOSS THRU SHELL DURING OPERATION	9.9 kW
- 2.54 cm thermal blanket
- THERMAL GROWTH DURING BAKE OUT		1 cm radial

## Vacuum Vessel Issues will be addressed with R&D

- Weld distortion
-  Bad experience with ATF (but good with W7-AS)
-  Close control of welding procedures essential
-  Resolve during R&D phase
- Forming
-  Experience lacking, issues not identified
-  Resolve during R&D phase

## Proposed Stellarator Core Concept

-  Plasma-Shaped Vacuum Vessel
-  Inner Structural Shell (“apple core”)
-  Outer Structural Shell
-  Saddle Coils Wound on Outside of Shell
-  **Fiberglass cryostat**

## Proposed Cooling/Insulation/Cryostat Concept

- - Cryogenic boundary provided by fiberglass enclosure
- - Enclosure may conform to shell or be more generic in shape
- - Nitrogen gas flow within prevents frost, cools shell during bake-out
- - Flexible connection to vessel ports (100-350C) required

- LN2 Vapor

- Vent or
- Recirculate

## Coil Design Evolution:  C82 vs C10

## Coil Current Density:  C10 and a typical C82 case 

## Coil Design Evolution:  C82 vs C10

- C-10

- C82-FD121

## R&D for Stellarator Core

- Early R&D has greatest pay-off for design improvements and cost savings
- Plan is based on three phases of work for each subsystem
- Simple, in-house tests for specific issues (e.g. conductor stiffness test)
- Integrated prototype fabrication and tests by vendor (full scale winding)
- Follow-on production of final components by same vendor, if possible

## Saddle coil winding R&D

**Conductor Compression Test**

- Goal
-    Establish stress-strain behavior of cable in rigid slot
- Status
-    Drawings complete and in shop

- Compression
- Test Fixture

- Conductor
- Sample

## Saddle coil winding R&D

- 40

- 6

- Two conductor loops

- Nickel-Al-Bronze Block

**Winding/Thermal Cycle Test**

- Potting dam

- Goal
-   Investigate coil behavior during pulse thermal cycles, cool-down
- Status
-   Drawings out for estimates

## Issues:  What are we still worried about?

## We have selected a baseline design configuration and fabrication concept
LN cooled copper braid wound into grooves on segmented shell
Stand-alone, bakeable vacuum vessel made by break-bending

Baseline design developed around C10 coils is workable 
>1 second flat-top at 1 Tesla
2 Tesla operation is structurally ok

Coil set is still evolving
We have a handle on cost drivers, fabrication limits
Coil groups working to reduce number of coils and current density

Several issues remain that will be addressed by R&D

**We have selected a baseline design configuration and fabrication concept**
- LN cooled copper braid wound into grooves on segmented shell
- Stand-alone, bakeable vacuum vessel made by break-bending
**Baseline design developed around C10 coils is workable**
- >1 second flat-top at 1 Tesla
- 2 Tesla operation is structurally ok
**Coil set is still evolving**
- We have a handle on cost drivers, fabrication limits
- Coil groups working to reduce number of coils and current density
**Several issues remain that will be addressed by R&D**

## Slide 47

## Slide 48