# Presentation: 142400

## Development of Superconducting Accelerator Magnets at FermilabA. V. Zlobin 

- SC magnet R&D
- Material and component R&D

## SC Magnet R&D program

- Fermilab has a strong SC magnet R&D program, which is natural for a laboratory with the largest and highest-energy SC accelerator in the world, the Tevatron.
- The goal of our SC Magnet R&D program is the development of new generation SC _*accelerator*_ magnets with high operation fields and high operation margins for different applications.
-  A short list of possible applications includes:
- SC magnets for the Tevatron,
- to replace some present dipoles in order to create space for special devices,
- to replace existing IR quadrupoles with higher-gradient, larger aperture quadrupoles or create a new IR;
- SC magnets for a future Very Large Hadron Collider (VLHC);
- 2nd generation LHC IR dipoles and quadrupoles with larger apertures and higher operation margins for higher luminosity;
- SC magnets for beam transfer lines, etc.

## Superconductor & Technology

- Our present SC Magnet R&D program is focused on the accelerator magnets based on *Nb3Sn superconductor*.
- Present Nb3Sn strands:
- have acceptable, continuously improving properties
- produced on the commercial level
- have reasonable price
- are brittle after reaction heat treatment
- We explore two basic technologies for brittle superconductors:
- wind-and-react
- react-and-wind

## Magnet R&D Infrastructure 

- We have all necessary infrastructure to perform successful short and long model magnet R&D:
- - Cable insulating machine
- - Winding tables and machine: (<2m,<15m)
- - Coil HT oven and retorts (<1m)
- - Epoxy impregnation facility (<6m)
- - Collaring/yoking presses (<15m)
- - Magnet test facilities (<4m,<15m)

## Design approaches

- One design is based on shell-type coils with a cos-theta azimuthal current distribution.
- The other design is based on flat block-type coils arranged in the common-coil configuration.
- In this innovative design approach the coil radii are set by the aperture separation, not the aperture size, and hence, conductor bends are relatively gentle and friendly to brittle conductors.

- Based on these basic design approaches we have developed several innovative dipole magnets for VLHC.

- We are investigating two types of high-field dipole designs for VLHC:

## Cos-theta dipole models

- A series of 1-m long single-bore models (HFDA) of cos-theta Nb3Sn dipole is being fabricated and tested.
- Nb3Sn high Jc superconductor
- traditional 2-layer coil
- Cold iron yoke
- wind-and-react technique
- nominal field of 12 T at 4.5 K
- accelerator field quality
- 43.5-mm diameter bore.
- Four short models were fabricated
- and three of them were tested in FY2001-2002.
- achieved accelerator field quality
- not achieved yet the required field level.
- Focus on the magnet quench performance.

## Magnetic mirror

- We are studying and optimizing the magnet technology and quench performance using half-coils and a magnetic mirror:
- same mechanical structure and assembly procedure
- advanced instrumentation
- voltage taps, spot heaters, thermometers, strain gauges
- short turnaround time, cost effective
- bolted skin, same yoke and spacers
- The magnetic mirror test with old half-coil HFDA03A (January, 2003).
- Splice tests using an old half-coil HFDA03B
- assembly, mechanics, cooling, etc.
- The new half-coil HFM02 was tested (June, 2003):
- technology, quench performance, mechanics.

## Common coil dipole

- Common coil dipole model (HFDC):
- single-layer coil
- cold yoke
- wide pre-reacted Nb3Sn cable
- magnet bore of 40-50 mm
- advanced mechanical structure
- nominal field of 11 T at 4.5 K
- accelerator quality field
- Mechanical and technological models have been fabricated in FY2002.
- The 1st common coil short model has been fabricated and will be tested in August-September, 2003.

## React & wind Nb3Sn racetracks

- Experimental studies of react-and-wind techniques were performed using sub-sized cable and flat 1-m long racetrack coils (HFDB).
- Three react & wind Nb3Sn racetracks have been fabricated and tested in FY2001-2003.
- 2nd and 3rd racetracks reached 75-78% of their short sample limit.

## Production and test plans

- We are planning production and test of 2-3 short model magnets per year.The goals are understanding and improvement of the magnet technologies and quench performance, and optimization of the field quality.
- When basic problems are understood we plan to increase the production and tests of HFM models of different types to 5-6 per year with the goal to study and optimize the performance reproducibility and magnet cost.
- We are also planning to develop infrastructure and start fabrication of long models starting in FY2006-2007.

## LARP magnet R&D

- Fermilab actively participated in launching the U.S. LHC Accelerator Research Program (LARP) and will continue play key role in the Program.
- The goal of LARP magnet R&D is to develop 2nd generation IR magnets for LHC to replace the 1st generation magnets.
- Contributions of Fermilab to LARP Magnet R&D include:
- conceptual designs studies of various magnets for 2nd generation LHC IRs
- participation in material and component development
- leading the IRQ short and long model magnet R&D
- design, fabrication and tests of full-scale prototypes of the LHC IR magnets
- Close BNL-Fermilab-LBNL collaboration and strong connection between the LARP magnet R&D program and the base High Field Magnet R&D programs is critical, in order to reduce risks and increase the probability of success.

## Material and component R&D

- The development of new generation accelerator magnets requires advanced superconductors, structural materials and components.
- Fermilab is participating in national programs sponsored by DOE to encourage the development of improved high-field superconductors and materials in U.S. industry.

## Infrastructure

- Fermilab has developed the adequate infrastructure to perform extensive superconductor, cable and material R&D in support of the magnet R&D program.
- Small ovens for NB3Sn strand and cable Heat Treatment
- Sample impregnation fixtures
- Compact 28-strand cabling machine
- Sample compression fixtures (4.2-300K)
- Ic and magnetization sample holders
- compact 25 kA SC transformer
- SEM and optical microscopes
- Short Sample Test Facility
- 15-17 T solenoid,
- 1.5-100 K temperature insert,
- 2 kA power supply

## Nb3Sn strand R&D

- We are purchasing and studying Nb3Sn 0.3-1.0 mm strands produced using different methods:
- “Internal Tin” (IT),
- “Distributed Tin” (DT),
- “Modified Jelly Roll” (MJR),
- “Powder in Tube” (PIT).
- Strand studies include:
- Ic(B)/Jc(B),
- n-value,
- RRR,
- M(B) and deff, magnetic instabilities
- SEM studies and chemical analysis,
- Strand expansion after reaction,
- Heat treatment optimization.

*Heat treatment cycle optimization*

## Cable R&D

- Rutherford-type cables are being developed and studied:
- different Nb3Sn strand types
- rectangular and keystone x-section
- with and w/o SS core
- different packing factor
- one and two stage cables
- Copper stabilizers (Cu strands, Cu tape)
-  The studies include:
- effects of cable design on Ic degradation due to cabling, cable bending and compression,
- cable Ra measurements.

*Nb3Sn strand Ic degradation database.*

## Insulation

- Insulation is one of the most important elements of magnet design, which determines the electrical, mechanical, and thermal performance as well as lifetime of the magnet.** **
- Ceramic and S2-glass insulations are being studied and optimized at Fermilab in collaboration with industry (CTD):
- Insulation: tape and cloth (strong, free of organic components)
- Low-viscosity liquid ceramic binder (improve insulation and coil mechanical properties)
- Ceramic pre-preg (life-time - 2 weeks)
- 1st half-coil with ceramic pre-preg has been built and tested in June, 2003
- React-and-wind technique allows using the traditional insulating materials (Kapton, fiberglass). In order to avoid the Ic degradation during the cable insulation we co-wind the insulation tapes with the cable (instead of wrapping the cable) in our racetracks and common coil magnets.

## Coil impregnation materials

- Traditionally Nb3Sn coils are impregnated with epoxy to improve their mechanical and electrical properties.
- The radiation limit for epoxy is quite low which reduces the lifetime of the magnet.
- The mechanical, thermal and electrical properties of “ten-stack” samples impregnated with polyimide solution Matrimid 5292 have been measured and compared with epoxy-impregnated samples.
- The results are very encouraging.
- These studies will be continued on practice coils and then tested in model magnets.

## End parts

- Complicated end parts, used in traditional cos-theta coils, in case of wind-and-react techniques have to withstand the heat treatment and match the cable shape in the ends to avoid shorts.
- An optimization method for metallic end parts was developed and successfully used at Fermilab together with the rapid prototyping techniques.
- Reduction of the time and the cost of end part development processes.
- Water jet machining was used for end part fabrication.
- reduction of part costs by a factor of 3 (even more in the future) and manufacturing time by a factor of 10 while providing acceptable part quality.

## Passive correction technique

- The use of Nb3Sn conductor typically results in significant coil magnetization effects in high field magnets due to large effective filament diameters.
- In order to reduce this effect a simple passive correction technique based on thin iron strips installed in the magnet bore or inside the magnet coil has been developed and successfully tested.
- 3 tested corrector models confirmed the design parameters
- This approach offers:
- significant increase in the dynamic range of accelerator magnets,
- relaxation of the requirements on the effective filament size in Nb3Sn strands.

## Issues for discussion

- W&R technology for Nb3Sn magnets
- Magnet design and test results
- Nb3Sn strand testing
- Nb3Sn cable testing
- Small racetrack technology
- Collaboration