Superconducting Magnet Development for 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 ~ homogeneity is still needed Minimal requirement for high-resolution spectroscopy: <10 -4 in 3 mm cube We are setting a design goal of in 5 mm radius (10 mm radius may be possible). Then correct to 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 2. Field tables 3. Field plots 1. Field harmonics

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): B 1 = 2600 Gauss B 1 /B pk = Six blocks/layer (2D): B 1 = 3300 Gauss B 1 /B pk = 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) ParameterSymbolRDOPT3RDOPT4RDOPT5 Ref. Current [kA]I ref 10.0 Ref. Radius [mm]R ref 5.0 Main Field [Gauss]B1B Skew Quadrupolea2a Normal Sextupoleb3b Skew Octupolea4a Normal Decapoleb5b Main/Peak FieldB 1 /B pk

Case study: RDOPT4 Starting from the single layer RDOPT4 cross-section, the following 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 I ref  I ss [kA] 10.0 R ref [mm]5.0 B 1 [Gauss]3052 a2a b3b3 0.2 a4a b5b5 0.8 B 1 /B pk 0.061

Correction Schemes Systematic errors can be understood/corrected in a prototype series Fabrication tolerances still generate random errors at the 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, appears the most promising option Access through warm bore can be provided

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 clearance 10 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