# Presentation: 800765

## NCSX Physics Mission and Requirements

Hutch Neilson
for the NCSX Team

Princeton Plasma Physics Laboratory
Oak Ridge National Laboratory

NCSX Construction Feasibility Review
Princeton, NJ
October 31, 2007

- Hutch Neilson
*for the NCSX Team*
*Princeton Plasma Physics Laboratory*
*Oak Ridge National Laboratory*
**NCSX Construction Feasibility Review**
**Princeton, NJ**
**October 31, 2007**

## Topics

- NCSX mission
- Physics design
- Field error requirements
- Controlling field errors in design and fabrication.
- Tolerances, trim coils.
- Experimental verification plans
- Summary

## Role of Compact Stellarators in Fusion R&D

**Stellarators solve critical problems for magnetic fusion.**
- Steady state without current drive.
- Stable without feedback control or rotation drive. No disruptions.

**Compact Stellarators (CS) improve on**
**previous designs.**
- Quasi-axisymmetric magnetic field.
- Good confinement.
- Link to tokamak physics: can build on ITER burning plasma R&D.
- Lower aspect ratio.

**NCSX Plasma**
*3D geometry has benefits and costs.*

**NCSX Mission**
- Assess attractiveness of compact stellarators for MFE.
- Advance 3D plasma physics.

## NCSX Machine Design

- Major radius:  1.4 m
- Magnetic field (pulse length)	2.0 T  (0.2 s)	1.2 T  (1.7 s)
- Flexible coil set
- Modular, TF, PF, trim
- Coils cooled to LN2 temperature
- Vacuum vessel bakeable to 350 C.
- Accommodates 12 MW of plasma heating
- Neutral beams, RF.

## Stellarator Configuration is Optimized for Attractive Physics Properties

**Plasma Cross Sections**

- Aspect ratio R/a = 4.4; 3 periods.
- Quasi-axisymmetric; ripple ≤ 1.5%
- MHD stable at high  (4.1%).
- Good magnetic surfaces at high .

**Plasma and**
**Modular Coils**
**18 coils**
**3 types (A, B, C)**
**(TF and PF coils also included in analysis)**

**A**

**B**

**C**

**C**

**B**

**A**

## NCSX Coils Are Designed to Produce Good Surfaces at High 

**NCSX Coils Are Designed to Produce Good Surfaces at High ******

**radial coordinate**

**Rotational transform profile**
**(Magnetic field line “twist”)**

- Field-line puncture plot from PIES equilibrium code calculation.

## Fundamental Requirement: Minimize Islands 

**General Requirements Document**
- 3.2.1.5.1 Field Error Requirements
- Field error correction (trim) coils shall be provided to compensate for fabrication errors.
- The toroidal flux in island regions due to fabrication errors, magnetic materials, and eddy currents shall not exceed 10% of the total toroidal flux in the plasma (including compensation).
**To minimize islands, minimize resonant field perturbations.**

## Project Strategy for Controlling Resonant Field Errors in Design and Fabrication

**1.****	****System Requirements, Component Specifications**
- Tolerances (±1.5 mm on completed modular coil system).
- Low magnetic permeability (typically <1.020)
- Low eddy currents. (materials, insulating breaks).
- Low stray fields from leads, coil feeds, crossovers.
- Stellarator symmetric design.
- Minimize deflections under load. (robust structure)
- 	**Implementation: P. Heitzenroeder**
**2.****	****Provide Trim Coils**
- 24-coil array.
- 	**Performance Asessment: A. Brooks**

## There is Flexibility in Implementing the Requirements

- Physics performance impact is a “soft limit”.
- Island width scales as (perturbation)1/2.
- Plasma shielding effects are predicted to reduce islands.
- Effects of assembly errors on other physics properties (effective helical ripple, ballooning stability) are negligible.
- Deviations or non-conformances can be accepted as long as fundamental requirements (i.e., island widths) are satisfied. This can occur if the condition:
- Is very localized or sufficiently remote from the plasma.
- Is stellarator-symmetric.
- Can be compensated by small changes in the coil geometry.
- Can be compensated with trim coils.

## Efficient Tools Were Developed to Evaluate Island Widths

- Existing equilibrium codes have limitations as design tools:
- VMEC: assumes good surfaces, stellarator symmetry.
- PIES: assumes stellarator symmetry. Slow.
**Solution: small-perturbation approximation (A. Brooks)**
- Field perturbations are superposed on an island-free (VMEC) plasma equilibrium.
- Perturbed field = VMEC field + perturbation field.
- This is an approximation (plasma response neglected).
- An analytic predictor (VACISLD) was developed to evaluate island width.
- A field line tracing routine (TraceBrtp) was developed to examine effects of both symmetric and symmetry-breaking field errors.

## Trim Coils Can Compensate for Fabrication Errors

- 24-coil set controls low-order (n = 1, 2, 3) resonant errors.
- Moderate currents (20 kA-turns) can compensate for coil displacements within the ±1.5 mm tolerance envelope.
- Islands reduced to <<10%.
- Performance margin provides capability to compensate for non-conforming conditions.

## Trim Coils Can Compensate for Fabrication Errors

**m=2/n=1 island due to construction errors within tolerance**

**Island suppressed by trim coils**

**Trim Coils Can Compensate for Fabrication Errors**

- radius

- radius

- poloidal angle

- poloidal angle

## Stellarator Field Error Effects Are Measured

**Large Helical Device (LHD), Japan**

- Vacuum magnetic surfaces mapped with e-beam and fluorescent mesh.
- Islands seen at  = 1 and 1/2 surfaces in LHD.
- Same technique will be used in NCSX.

- electron gun

- fluorescent mesh

- CCD

- island
- divertor

- probe array

## Trim Coils Control LHD Islands

## Magnetic Island Control on CTHAuburn Univ.

- Detailed island studies yielded “as-built” model for coils
- Exp’ts in correcting single, multiple islands with 5 trim coils
- Will increase to 15 trim coils.

## Summary 

- Field error requirements, including tight tolerances, are driven by the sensitivity of magnetic surfaces to resonant magnetic perturbations. Islands result.
- Larger errors can be accepted under many conditions as long as fundamental requirement (island width <10%) is satisfied.
- Engineering has developed efficient tools to evaluate island widths due to field errors.
- Trim coils can compensate for construction errors, with margin to mitigate field-error risks.
- Experimental verification tests are planned.