Superconducting Magnet Developmentfor Ex-Situ NMR

FY04 Plan

G. Sabbi, 3/31/03

FY03 LDRD

We expect the following results from the FY03 LDRD:

• Design, fabrication and test of the first prototype

• Design (field quality) optimization for a second prototype

Goal of the FY03 prototype is to demonstrate:

• Coil configuration (compatible with design optimization)

• Support structure (little/no support on NMR analysis side)

• Training performance, quench protection

• Basic check of calculated vs measured field

Next Steps

1) Fabrication of a cryostat with full access to the sweet spot

• prototype tested in vertical dewar – not a true ex-situ• need a dedicated test facility

2) Fabrication and test of a field quality optimized magnet

• conceptual design in FY03• could be proposed as FY04 LDRD extension • need suitable field measurement technique• may require several iterations (measure/correct)

3) Demonstration ex-situ NMR experiments:

• using warm finger in test dewar• in full ex-situ configuration

Cryostat Development

Goals:

• Full demonstration of the ex-situ magnet configuration

• Facilitate ex-situ NMR experiments

Approach:

• Two options – single magnet or multiple magnet/test facility

• High current leads, LHe supply required with present designs

• First prototype will be available for proof of principle experiment

• Not suitable for LDRD (no serious R&D issues)

• Expensive - look for possible sources of funding

Field Quality Optimization

1. Field Quality Requirements

2. Field Representation

3. Design approach

4. Case studies/performance parameters

5. Field measurements

6. Correction schemes

• Key issue for ex-situ, high resolution NMR spectroscopy

• Main challenge from the magnet development standpoint

Field Quality Requirements

Ex-situ NMR technique aims at reducing FQ requirements with respect to standard NMR

But - presently ~ 10-4-10-5 homogeneity is still needed

Minimal requirement for high-resolution spectroscopy:<10-4 in 3 mm cube

We are setting a design goal of 10-4 in 5 mm radius(10 mm radius may be possible). Then correct to 10-5 using trim coils/magnetic shims)

Field measurement (to set correctors) will be an issue, use of NMR techniques may be best strategy

Best magnet design to date: ~3*10-4 in 5 mm radius~6*10-5 in 3 mm radius

Field Representation/Analysis

1 X [MM] 2 Y [MM] 3 Z [MM] 4 BX [Gauss] 5 BY [Gauss] 6 BZ [Gauss] -2.00000000000 -2.00000000000 -2.00000000000 -0.372910364194E-01 3009.89216170 -0.507796772677 -2.00000000000 -2.00000000000 -1.80000000000 -0.373000572381E-01 3009.89230810 -0.457013607975 -2.00000000000 -2.00000000000 -1.60000000000 -0.373081284638E-01 3009.89243908 -0.406231544570 -2.00000000000 -2.00000000000 -1.40000000000 -0.373152493730E-01 3009.89255466 -0.355450460103 -2.00000000000 -2.00000000000 -1.20000000000 -0.373214213664E-01 3009.89265482 -0.304670232197 -2.00000000000 -2.00000000000 -1.00000000000 -0.373266430920E-01 3009.89273958 -0.253890738490

2. Field tables

3. Field plots

1. Field harmonics

nra

nIrA

nra

nIrB

nnn

nnn

sinsin

,

coscos

,

10

10

Coil Design

Presently following “accelerator magnet” approach:

- coil has a “long” straight section, field is optimized in 2D - 3D design to eliminate end effects with minimal coil length

Motivations:

- our main expertise is in accelerator coils- appears to be the most promising strategy for high field quality- potential for stretching the good field volume along the axis

Alternative design approaches:

Nested solenoids (a design based on normal conducting coils is being pursued in parallel to SC magnet effort).

Conductor in groove for more freedom in conductor placement (may also be good for correctors)

Design “algorithm”

1. Optimize coil cross section (efficiency, 2D field quality)

2. 2D iron design (shielding, saturation, stray field)

3. Iterate on cross-section to adjust systematics

4. Find minimal coil length for no significant 3D effects

5. Try to further reduce coil length (3D coil, iron geometry)

6. Estimate random errors (tolerances, sweet spot distance)

7. Design corrector package (trim coils or magnetic shims)

Main challenge: 2D cross-section optimization for

- High efficiency (sweet spot field vs. coil peak field)

- Low systematic harmonics (esp. octupole, decapole)

• Based on SM coil design

• Optimize upper layer, then correct for lower layer

• 2D issues:- blocks are too wide – limits on 2D harmonics- narrow island decreases efficiency- narrow island can result in vertical forces

• 3D issues:- insufficient ratio of coil length to sweet spot distance- decreasing sweet spot distance makes design less attractive

& makes 2D optimization more difficult (high order systematics, random errors)

Coil cross-sections (Phase I)

Phase I – 2D cross-sections

Four blocks/layer (2D):

B1 = 2600 GaussB1/Bpk = 0.025

Six blocks/layer (2D):

B1 = 3300 Gauss B1/Bpk = 0.037

Coil cross-sections (Phase II)

Still based on RD coils (double-pancake racetrack) but:

- RD3c type design with central spacer- drop restrictions on length & top/bottom distance - NbTi assumed, with same cable geometry as SM

Advantages:

- doubled degrees of freedom for same # coils & splices- better efficiency (main field to peak field ratio) - better control on conductor positioning- standard approach to 3D effects

Phase II – 2D cross-sections (1 layer)

RDopt3

RDopt5

RDopt4

Phase II Performance Parameters (2D)

Parameter Symbol RDOPT3 RDOPT4 RDOPT5

Ref. Current [kA] Iref 10.0 10.0 10.0

Ref. Radius [mm] Rref 5.0 5.0 5.0

Main Field [Gauss] B1 2780 2800 2570

Skew Quadrupole a2 -0.2 0.3 0.2

Normal Sextupole b3 -0.2 0.3 0.3

Skew Octupole a4 -9.8 -5.5 -2.9

Normal Decapole b5 2.1 0.8 0.2

Main/Peak Field B1/Bpk 0.073 0.066 0.053

Case study: RDOPT4

Starting from the single layer RDOPT4 cross-section, thefollowing calculations were performed:

•Requirements on layer-to-layer separation (18 cm)•Field enhancement due to iron shield (10%)•Iron saturation effect (none)•Cross section iteration to correct harmonics (ok)•Coil length (upper bound) for no end effect (80 cm)

RDOPT4 Parameters (NbTi)

Param. RDOPT4_V4

Iref Iss [kA] 10.0

Rref [mm] 5.0

B1 [Gauss] 3052

a2 -0.3

b3 0.2

a4 -5.3

b5 0.8

B1/Bpk 0.061

Correction Schemes

• Systematic errors can be understood/corrected in a prototype series• Fabrication tolerances still generate random errors at the 10-4 level

Need to measure field and correct

Active correction (trim coils) Passive correction (mag. shims)

AML “conductor-in-groove” plates may be used above main coils

Demonstrated during the RHIC and LHC IR quad development

Field Measurement

•Rotating coils are used to measure accelerator magnets•Integral measurement along the axis of the magnet•Accuracy depends on radius and length of the probe•~10-5 is achieved in typical accelerator applications •Not suitable for present application

Need to develop a new system for the ex-situ NMR magnet

Hall probe arrays: some experience, but unlikely to get accuracy

Use of NMR tecniques, with accurate positioning system, appearsthe most promising option

Access through warm bore can be provided

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Ex-situ NMR Experiments

Evaluate possibility of a proof of principle experiment using the first prototype (but: field homogeneity is limited)

Future experiments can be performed using a warm bore or a dedicated test facility with full access to the analysis volume.

5 mm : sweet spot to sample edge 5 mm : antenna and its support, plus the RF mirror thickness 5 mm : clearance to warm finger (allows a 3 cm wide RF mirror) 5 mm : cold/warm finger clearance10 mm : coil mechanical support structure 5 mm : correction coil plate 5 mm : coil position optimization (coils are not all on the same plane) 5 mm : design margin

Space budget in 49 mm warm bore (45 mm coil to sweet spot)

Summary

Discussion/Feedback is needed in the following areas:

1. Cryostat development

2. Field quality requirements

3. Field representation/analysis

4. Distance from sweet spot

5. Optimized magnet development

6. Field measurement/correction schemes

7. Ex situ NMR experiments planning

8. Overall strategy

For #1, #5, #6, #7, we need to locate possible sources of funding and prepare proposals