# Presentation: 816227

## Engineering Issues in the Design of the National Compact Stellarator Experiment

**W. Reiersen**
***on behalf of the NCSX Design Team***
**13****th**** International Stellarator Workshop**
**Canberra, Australia**
**September 24-28, 2001**

## NCSX – plowing new ground

**New concept - quasi-axisymmetric stellarator (QAS)**
**Goal: Combine the best features of tokamaks (compact, good confinement, high beta operation) and stellarators (disruption-proof, steady state)**
**New territory**
**Low aspect ratio (4.4)**
**Few periods (3)**
**Substantial plasma current (174kA)**
**But same old design issues **
**Design issues = things that keep you awake at night**
**Field errors at the top of the list**

## NCSX machine configuration

## NSCX assembly sequence

## Modular coils

**21 modular coils, 4 coil types**
**V=0 coils pulled out for NB access**
**Coil geometry generated by CoilOpt **
**Parametric representation of winding surface and modular coil trajectory**
**Good fit to the required B****norm**** on the plasma boundary**
**0.6% avg, 2.6% max error**

## Modular coil design issues

**Field errors**
**Current density**
**Radius of curvature**
**Coil twist and overlap**

## Field errorsHSX – safely snuggled between resonances

**HSX reference plasma**
**4 periods**
**No symmetry breaking resonances in plasma**
**No first or second generation resonances in plasma**
**Lowest order resonance is m=11 (the 12/11 at ********=1.09)**
**Monotonic iota, finite shear near edge**
**Vacuum surfaces provide relevant assessment of surface quality **

## Field errorsNCSX – nowhere to hide

**NCSX reference plasma**
**Three periods**
**Lowest order (symmetry preserving) resonance is m=5**
**Three 1st generation (3/5, 3/6, 3/7)**
**Two 2****nd**** generation (6/13, 6/11)**
**Symmetry breaking resonance in plasma (1/2)**
**Zero shear region near edge**
**Wide range of iota flexibility exacerbates field error problems**
**Nonlinear problem with finite beta and I****p****, difficult to assess surface quality (PIES)**

## Strategy for minimizing field errors (1)

**Optimize plasma boundary and modular coil geometry to minimize islands and maximize plasma size for reference plasma configuration**
**Ref. A. Reiman, Fixed Boundary Optimization and Flux Surface Healing in the Design of NCSX**
**Provide trim coils to suppress 1****st**** generation resonances (3/5, 3/6) near edge for other plasma configurations**
**Build the coils to tight enough tolerances such that acceptance deviations will not spoil plasma performance**

## Strategy for minimizing field errors (2)

**Strictly enforce stellarator symmetry in the design of the stellarator core**
**Use low permeability material**
**Coil structure cast with the same SS used on W7-X (********r****~1.01)**
**VV fabricated with Inconel**
**Carefully assess impact of known sources of field errors**
**Bending magnets and iron shielding in NB injectors**
**Building steel**

## Trim coils

**Coupling is best at v=0 cross section, near inboard midplane**
**Closest approach to resonant surface**
**Arrays of saddle coils provided inboard and outboard**
**Effectiveness still being tested**

## Establishing tolerancesTwo paths to follow

**Follow conventional wisdom**
**True position tolerance (TPT) of .001 x R for the current centroid is OK**
**Held for HSX (R=1.2m)**
**No discernable effect with random errors of 2mm**
**Outermost flux surface start behaving stochastically at 5mm level**
**Is NCSX more vulnerable?**
**Develop tolerance requirements from field error studies**
**Problematic for NCSX – surface quality effects can only be assessed directly *****in vacuum***** for for general (asymmetric) perturbations**
**Correlations between field errors on resonant surfaces and island size are not reliable**
**Not clear how to add random errors in setting tolerance requirements**

## Establishing tolerancesNCSX strategy

**Follow both paths**
**Initial tolerance requirements based on conventional wisdom (TPT ~ .001 x R)**
**Tolerance studies will be performed but large uncertainties are expected**
**Assess cost sensitivity**
**Cost sensitivity to tolerance requirements will be assessed by potential suppliers in manufacturing studies during conceptual design**
**Finalize tolerance requirements based on performance and cost considerations**

## Meeting tolerance requirementsDesign concept facilitates achieving tight tolerances

**Winding surfaces and mating surfaces are on the same machined part**
**Minimizes tolerance build-up**
**Shimming of winding and mating surfaces possible, if necessary**
**Mating surfaces are on simple, radial planes**
**Easy to precisely machine**
**Easy to position and align**
**Completed assembly forms integral shell**
**Robust structure **
**Minimizes deflections**
**Penetrations provided where access is needed**

## Meeting tolerance requirementsUse state-of-the-art equipment and techniques

## Current densityA problem for NCSX

**Reference scenario**
**J****Cu,max**** > 13 kA/cm****2**** **
**t****ESW**** ~ 1.2s (0.5s flattop)**
**Contributing factors to high J**
**Large toroidal excursions (low toroidal fill factor)**
**Centroid needs to be close to plasma (small radial build)**
**Wound with flexible conductor (75% p.f.) for ease of winding**
***Pulse length will be limited by temperature rise -> thermal stresses***

## Current densityPre-cooling to LN2 temperature

**Reduces adiabatic temperature rise **
**Reduces power supply requirements**
**Avoids failure modes of water cooling**
**15 minute cool-down possible**
**Small operating cost penalty**
**No refrigerator required**
**LN2 shipped in on trucks**
**Requires a cryostat**
***R&D to be conducted to establish allowable temperature rise***

## Coil geometryMore radical than on HSX or W7-X

## Sharp bend radii

**Limit appears to be 5x the conductor thickness**
**Smaller values have negative consequences**
**Insulation crinkles**
**Keystoning**
**Conductor locks (cannot bend further) when winding**
**Paths forward...**
**Locally eliminating sharp bends in the winding path appears promising**
**18-coil solutions have larger minimum  bend radii**

## Coil twist and overlapCoils cannot see shell structure

**Problem traceable to bumpy winding surface**

**Paths forward... **
**Smooth winding surface in toroidal direction**
**Penalize azimuthal separation instead of center-to-center separation**
**Manually adjust twist angle**

## Vacuum vessel / PFC design issues

**350****oC b****akeout**
**Accommodating a divertor**
**Access for in-vessel maintenance**
**Field assembly**

## 350° bakeoutStand-alone liner adopted

**Plasma facing components are carbon-based, requiring a 350****°C bakeout**
**Operating the vessel at 350°C creates difficulties**
**Diagnostic windows**
**In-vessel diagnostics**
**Vacuum seals**
**Stand-alone liner adopted**
**Vessel baked to 150****°C**
**Temperature-sensitive components mounted on vessel**
**Stand-alone liner baked to 350 °C**
**CFC panels on poloidal ribs**
**Ribs registered toroidally to VV, free to grow radially and vertically**

## Accommodating a divertor

**Divertor requirements**
**Divertor required for density control, shielding impurities**
**Long (L>100m) connection lengths required to decouple temperature at last closed magnetic surface (LCMS) and divertor**
**Requires a slot connected to a (pumped?) plenum**
**Path forward...**
**Provide expanded stay-out zone for divertor operation**
**Incorporate a pumped slot**
**Create plenum by “sealing” PFCs to vessel near slots**

## Access for in-vessel maintenance

**NCSX is an awkward size**
**Too small to *****comfortably***** crawl inside**
**Too big to reach *****everywhere***** from outside**
**Path forward...**
**Pursue 18-coil options**
**Provide large ports **
**Pull conformal shell back to vessel to maximize space inside vessel**
**Provide features to facilitate machine disassembly**

## Field assembly

**Interferences are encountered during field assembly **
**Exacerbated by expanded vessel **
**Inboard RF**
**Divertor stay-out zone**
**Paths forward...**
**Locally increase coil-to-vessel separation**
**Segment device in more (6?) pieces**

- Interference

## Industrial involvement

**Early industrial involvement in design of the modular coils and vacuum vessel planned **
**Manufacturing studies in conceptual design**
**Investigate different manufacturing processes**
**Assess feasibility and cost**
**Identify design improvements for easier fabrication and assembly, improved performance, or reduced cost**
**Multiple vendors engaged**
**R&D and full-scale prototypes in preliminary design**
**Develop manufacturing processes**
**Two vendors engaged**
**Single vendor for final articles**

## Summary

**Critical design issues have been identified**
**Most of them are familiar to stellarator designers**
**Potential solution paths have also been identified and are being pursued**
**Early industrial involvement is planned to make NCSX easier to fabricate and to minimize its cost**