# Presentation: 448936

## Potential Collaborations on NCSX with the Compact Toroidal Hybrid (CTH) group, Auburn University

- Presented at NCSX Research Forum
- Dec. 7-8, 2006
- Stephen Knowlton, Auburn University

## CTH experiment investigates subset of NCSX research areas

- Integrity of magnetic flux surfaces
- Measurement and reconstruction of magnetic equilibrium
- Low- current-driven instabilities and disruptions

**What are areas in which CTH personnel may contribute to NCSX research campaigns in FY09, FY11, and beyond?**
**Are there specific questions for research preparation on NCSX useful to address on CTH?**

## Collaborative Research Interests

- How to determine/confirm best coil model from field-mapping and other magnetic measurements?
- Can we adequately reduce static islands in the vacuum configuration
- What are effects of (controlled) static islands on both equilibrium and stability?
- Can we identify location (phase) of islands in plasmas?
- Compare predicted LCFS with edge profile measurements.
- How well can we reconstruct current profile from external magnetics,  and if we include internal B-measurement (e.g. mm-wave Faraday rotation)?
- Is control of external rotational transform profile robustly effective in avoiding current-driven disruptions?

## E-beam mapping for accurate coil modeling

- ZnO phosphor-coated wire mesh w/ LED’s
- at edge for alignment

- Movable phosphor-coated wand

- Entire set-up (gun, screen, camera) was rotated toroidally by two field periods to test for n>0 deviations in equilibrium.

-  = 36 ̊

-  = 252 ̊

## CTH exhibits good flux surfaces

- Composite photo of measured flux surfaces
- Magnetic axis localized to within several mm

- Mag
-  axis

- Low rotational transform (iota_edge ≤ 0.2) with zero auxiliary toroidal field
-  no low-order rational surfaces => no islands
-  Low aspect ratio configuration achieved

- Comparison of model from IFT (closed symbols) w/ expt. (open symbols)

- 0.6 m

## Small deviations from design investigated by least-squares fitting

- In design, r0 = 0.385 m, a1 = b1 = 0
- Best fit: 	 r0 = 0.384 m, a1 = 2.6 mm, b1 = 0.7 mm
- Results also directed us to find flaw in vertical field configuration (0.4% field imbalance)

- Alter coil model by varying coil parameters in optimizer in IFT line-following program;
-  least squares fit of differences between multiple experimental **magnetic axis** positions and computed results.
- includes BEXT,poloidal coil positions and currents, helical coil winding law
- Example: Helical coil winding law ( tor. Angle;  pol. Angle)
-  = 2/5   + 1 sin  + 2 sin 2
- 		 rCOIL = r0 + a1 cos b1 sin 
- perform SVD analysis to identify likely source(s) of error (Hanson/Muñoz)

- =>  Continuing to validate this scheme

## Static island successfully reduced

- 1/3 island reduced to minimal width by application primary & (orthogonal) secondary corrections

## Contributions to field mapping on NCSX

- Provide analysis & modeling
- Apply validated SVD analysis  procedure to NCSX coil model and measurements
- Use field-mapping results to assist in placement of correction coils, if not already fully designed & emplaced.
- Suppression of vacuum islands
- Provide test-bed for NCSX techniques
- Can we (or should we) map at pulsed high field (B~0.5 T) to reduce effect of remnant fields?
- Test techniques for faster, more accurate mapping
- Possibility of field-mapping at different toroidal locations
- Contribute existing hardware.

## Extra

## CTH program: field-mapping & ECRH plasmas supplemented by ohmic current

- Easily alternate between field-mapping & plasma operation
- Densities up to 4 x 1018  m-3 (ECRH) cutoff attained; higher with ohmic current
- Diagnostics
- Segmented Rogowski coils
- Flux loops
- Diamagnetic loop
- Langmuir probes

- 5 field periods
- Ro =.75 m, R/<a> ≥ 4
- Bo ≤ 0.7 T
- Ip  ≤ 30 kA;  ≤0.5
- Pin=  15 kW ECRH @18GHz

- SXR arrays; 4mm interferometer