# Presentation: 540719

**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

** **

**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

**Vacuum Vessel**

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

## Vessel fabrication options

**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

**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**

## Saddle coils wound on shell

-  Cast Nickel-Al-Bronze structure, 48 segments, ~0.5-in wide x 3.5-in deep slots

- Typical Coil Cross-Section

- Core Assembly

- Vacuum
- Vessel

- Helical
- Coils

- Structural
- Shell

**Saddle coils wound on shell**

## Shell / VV assembly

**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

**Access for RF heating**

**Launcher envelope is 400x440x240 mm**
**4 launchers will fit within the large ports**

## Access for diagnostics

- 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/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/cm2

**Thermal stress was considered primary limit**

## R&D shows cable is soft

**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

**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

**3 period modular coils**

## Modular coil winding pack

**Modular coil winding pack**

## Winding forces are toward web

- 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**

## 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

**Structure concepts**

## Modular coil fab and assy.

- 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**