# Presentation: 156110

## Flexible Coils for NCSX:  Status and Near-Term Plans

- S. P. Hirshman (ORNL)
- representing NCSX Coil Design group
- NCSX PAC-4
- August 1-2, 2000

## Topics

- Coil design selection criteria
- Results will be shown for Np = 3 reference design
- Saddle coils (+ 1/R background)
- Modular coils
- Saddle coil + alternate background coils
- Reconstruction from saddles and modulars
- Coil flexibility assessment

## Significant Progress Made Since Previous PAC Meeting

- c82 coil designs were limited by high current coil current density
- limited flat-top: restricted experimental conditions
- limited flexibility: current swings had to be small
- new, improved coils for Np =3 reference plasma
- lower current densities (by factors ~2-3)
- LN2 coil designs which allow current variations for flexibility
- water-cooled copper modular coils almost realized
- improved physics reconstruction

## NCSX (N=3) Coil Reference Design: Goals

- Physics Objectives
- Maintain QA-ness (NC transport) and kink, ballooning, Mercier stability at < = 4% for reconstructed surfaces
- Attain good surfaces and plasma performance for a **range** of < and currents (flexibility)
- Engineering Objectives
- Jmax < 20 kA/cm2 at B = 2T for LN2
- Jmax < 10 kA/cm2 (water-cooled copper) is desirable
- Maximize access and flexibility to achieve physics goals

## Progress in Coil Design: Synergy with Physics Optimization

- Reduced high current density in coils
- Plasma configurations were obtained by the physics
- targeted reduced current-sheet Jmax and coil complexity directly in the optimization process
- Developed, improved tools to lower Jmax for discrete coils
- Used tools to explore alternate coil configurations
- Achieved coils with realizable bend radii
- Codes now target improved accessibility directly
- Coil-coil separation, real-space blocking for ports and diagnostics

## NESVD and GA Used to Lower J for Saddle Coils

- NESCOIL + SVD (Valanju, Pomphrey)
- Scan of SVD coefficients for current potential coefficients yields smoother coils, reduced Jmax and increase coil-plasma separation
- Genetic Algorithm (Miner, Valanju) - UTA GACoil code
- Uses SVD-smoothed current potential contours
- Automated way to select optimized subset of coils with lower Jmax compared to equal contour selection scheme

## Saddles for NCSX N=3

- Jmax < 22 kA/cm2 (2T)

- Jmax > 25 kA/cm2

- GA-reduction of current for li383 to < 17 kA/cm2 has been obtained

## Uniform vs. GA-Cut Saddles

## New Codes for Designing Modular Coils

- SurfOpt Code (Berry)
- Use Levenberg optimizer to generate conformal winding surfaces (initial guess for CoilOpt)
- CoilOpt code (Strickler, Berry, and Whitson)
- Produces desirable modular coils
- Jmax lower than achieved for saddles (so far)
- Relatively smooth coils (realizable bend radii)
- Large plasma-coil separations

## Modular Coils for NCSX (7 coils/per, Jmax < 12 kA/cm2)

## Smoothing Coils Bends with CoilOpt 

- Smoothing Coils Bends with CoilOpt

- Minimum Impact on reconstruction and J

## Alternate Background Field Coils Yield Lower Jmax

- TiltOpt Code (Brooks, Reiersen)
- better fit to helical field reduces conformal coil current
- reduces Jmax in conformal coils for c82 by factor ~2 compared to simple 1/R background field
- background coils are simple (planar) coils
- Some unacceptable properties arose
- large stray magnetic fields - impact NBI
- added complexity: saddles spread to outboard side - reduces access

## Reconstruction of Reference Np=3 Plasma From Coils

- VMEC free-boundary
- First level of reconstruction (done)
- determine how well fixed boundary surfaces are reproduced
- Next check physics targets (done)
- chi-sq values do not exceed fixed boundary values in reconstructed plasma
- PIES
- Next level of reconstruction (focus for near term)
- preserve volume of good surfaces (compared to fixed boundary)
- increase volume of good surfaces (Wooley, Hudson) by reducing low-order island widths (surface, coil modifications)

## Excellent Reconstruction of Full Beta & Current Design Point Achieved from Saddle Coils (+1/R + multipoles)

- Solid: Original Target   Dashed: Free Boundary Reconstruction

- Good Reconstruction Using Modular Coils(also with multipolar fields)

## Summary of Modular/Saddle Comparison for Li383

## Coil Robustness and Flexibility

- Robustness
- ability of coils to support equilibria with a wide range of assumed internal profiles (pressure, current)
- Flexibility
- ability of coils to produce changes in the plasma shape which correspond to desired changes in physics properties

## Coil Flexibility: Philosophy

- Design coils to fit full beta, full current *design point* as well as possible
- Using these coils, use free-boundary optimizer to vary coil currents to fit physics at other plasma states
- Scope flexibility of these coils + axisymmetric PF (multipole moments)
- If necessary, redesign basis coil set to build in more flexibility
- Use GA to find set of coils matching a variety of plasma states on “average”

## Coil Flexibility: Free-boundary optimizer

- CUROPT code (Brooks)
- Previously, determine coil currents for* a fixed set of coils *to minimize Bnorm on fixed boundary (from full , Ip) at other values of , Ip – plasma physics properties not necessarily optimal this way
- Free-boundary, VMEC-based optimizer
- Find coil currents now by *optimizing plasma properties* directly, rather than matching to a fixed boundary shape
- Indispensable for flexibility studies

## Start-Up Flexibility for Coils

- S1, S2, S3: define *flexibility range*
- S1: zero beta, zero current (vacuum)
- S2: zero beta, full current
- S3: nominal design point, full beta, full current
- Reproduce these states with reference coils
- S3 *by definition* **must** be reconstructed with coils
- use CurOpt code to get initial guess
- apply free-boundary optimizer to match physics criteria for the S1, S2 states
- verify flux surface quality

## S1 (vacuum) Reconstruction with N=3 Saddle Coils

- Full beta Currents

## S1 (vacuum) Reconstruction with N=3 Modular Coils

- Full beta Currents

## Profile Robustness Studies

- Robustness Studies (Pomphrey, Hatcher)
- Used Free-Boundary VMEC
- Fixed coil currents in N=3 modular set
- Considered various pressure and current profiles
- Fixed TOTAL current and beta (results shown here)
- Varied current, beta for fixed N=3 reference profiles
- Results show a marked degree of robustness

## Plasma Profiles for Robustness Studies of NCSX

- current

- pressure

## Robustness Reconstructions (I = 150 kA,  = 4%)

- Modular coil currents held fixed at reference values
- Physics criterion **not** fixed, though

## Demonstration of Flexibility for Modular Coils (+weak 1/R TF)

- Transport “experiment”
- Vary modular coils currents (+4 axisymmetric multipoles) to “dial” a desired value of edge, while optimizing neoclassical 2
- Reference case has nc 2 = 1.7

## Results of Flexibility Experiment

- Modular coil index

- Ij/Ij

- Iota profile vs. flux (s)

## Near Term Plans

- Complete PIES reconstructions for candidate coils
- determine advantages of saddles vs. modulars
- decide if coil modifications are needed to suppress islands for surface quality maintenance
- Complete flexibility comparison for candidate coils
- Support coil effort needed to decide between Np = 2,3 physics candidates
- Initial coils for Np = 2 have higher errors than tolerable for good physics reconstruction

## Summary of Achievements

- Code developments have yielded new capabilities sufficient to design coils for the candidate physics configuration. They preserve the physics properties much better than the coils for c82
- Coils using LN2 cooling have been obtained for two coil configurations. They have lower currents than the c82 saddles and therefore provide an adequate margin for flexibility
- water-cooled copper modular coils may be feasible
- On-track for a December PVR