Superconducting High-Field Accelerator Magnets: Status and Perspectives Arnaud Devred CEA/DSM/DAPNIA/SACM & CERN/AT/MAS on behalf of the NED Collaboration SPIE International Congress on Optics and Electronics (Photonics Applications in Industry and Research IV) Warsaw University of Technology 30 August 2005

Contents Why do we need high-field accelerator magnets? State of the art in superconducting accelerator magnet technology Ongoing Nb 3 Sn accelerator magnet programs

The Push Towards Higher Fields For a given tunnel size, the energy of a circular accelerator is limited by the strength of bending (dipole) magnets. For both linear and circular colliders, the luminosity is determined (mainly) by the optics of Interaction Regions (IR’s), which is itself limited by the strength and quality of final-focusing (quadrupole) magnets. Over the years, there has been a constant push from the High-Energy Physics (HEP) community to keep developing higher-field and higher-field- gradient accelerator magnets.

Superconducting Accelerator Magnets The push towards higher fields led naturally to the use of superconductors. One of the pioneers in the development of superconducting accelerator magnets was W.B. Sampson at BNL in the mid 1960’s. 76-mm-aperture, 85-T/m quadrupole magnet model wound from Nb 3 Sn ribbons and cold tested at BNL in January 1966 (Courtesy W.B. Sampson)

The Tevatron The first successful use of sc magnets in an machine took place at the Tevatron at Fermilab, which was commissioned in 1983 and which has been running very reliably since then. The Tevatron was instrumental in demonstrating the feasibility and reliability of superconducting magnet systems and has paved the way to their commercial applications (such as MRI systems) mm-aperture, 4-T Tevatron dipole magnet

The LHC Since the time of the Tevatron, significant progress has been made in the design and production of superconductors and magnets, enabling a gain of a factor ~2 in field. The most advanced superconducting particle accelerator is the LHC presently under construction at CERN. The industrial production of the required 1232 dipole magnets and ~400 quadrupole magnets is well underway and will be completed by the end of mm-twin-aperture, 8.33-T LHC arc dipole magnet

What’s Next? Due to the high radiation doses to which they will be submitted, the life expectancy of LHC IR quadrupole magnets is estimated ~7 years. Hence, it is likely that these magnets will have to be replaced around 2015, thereby offering an opportunity of upgrading LHC IR optics to improve luminosity. Mid-2010’s is also the earliest time frame when one can expect to need final-focusing quadrupole magnets for the International Linear Collider (ILC).

Magnets for LHC IR Upgrade Several scenarios of LHC IR upgrade are presently being considered, e.g. Same layout as presently, but with larger-aperture and stronger final- focusing quadrupoles (Courtesy T. Sen) New layout where beam-separation dipoles are positioned in front of final focusing quadrupoles (Courtesy O. Brüning) The various scenarios call for the development of large- aperture, high-field or high-field gradient magnets.

Magnets for ILC IR’s Magnet requirements are IR-design dependent, e.g. TESLA-type IR requires LHC-type quadrupole magnets to be operated in a 4-T solenoidal background field (Courtesy F. Kircher, CEA) NLC-type IR with large crossing angle requires strong but very compact quadrupole magnets to clear the way for crossing beam (Courtesy B. Parker, BNL)

Roadmap for High-Field Accelerator Magnet R&D A reasonable roadmap for high-field accelerator magnet development appears to be – to get ready for LHC-IR upgrade in 2015 (large-aperture, high-performance dipole and/or quadrupole magnets; cost is not the primary issue), – to develop final-focusing quadrupole magnets for implementation in a linear collider IR in the mid-2010’s (LHC-type quadrupole magnets in a solenoidal background field, or compact quadrupole magnets; cost is not the primary issue), – to promote generic magnet R&D aimed at LHC energy upgrade or a super LHC in the 2020’s (high-performance, low-cost dipole and quadrupole magnets).

Contents Why do we need high-field accelerator magnets? State of the art in superconducting superconducting magnet technology Ongoing Nb 3 Sn accelerator magnet programs

State of the Art in NbTi Since the Tevatron, the most widely used superconductor is the ductile alloy NbTi (world production: ~1500 t/year). The LHC magnet R&D programs have shown that the limit of NbTi at 1.8 K was around 10 to 10.5 T. Hence, to go beyond the 10-T threshold, it is necessary to change the material. Quench performance of 88-mm-aperture MFRESCA dipole magnet at CERN (Courtesy D. Leroy)

Beyond NbTi: Nb 3 Sn At present, the only serious candidate to succeed NbTi is the intermetallic compound Nb 3 Sn (world production: ~15 t/year). (Courtesy P.J. Lee, University of Wisconsin at Madison)

Pros and Cons of Nb 3 Sn Nb 3 Sn has a critical temperature (  C ) and an upper critical magnetic flux density (B C2 ) that are about twice those of NbTi; however, once formed, it becomes brittle are its  C, B C2 and J C are strain sensitive. The brittleness and strain sensitivity require a rethinking of all manufacturing processes and, so far, have limited the use of Nb 3 Sn to specific high field applications (such as high field NMR magnet systems). (Courtesy J.W. Ekin, 1983)

Progress on Nb 3 Sn Over the last decade, significant progress has been made on Nb 3 Sn, thanks to the ITER Engineering Design Activities (EDA) and vigorous R&D programs carried out in the USA. Progress on non-Cu J C (at 4.2 K and 12 T) of multifilament composite wires Progress on maximum quench field of dipole magnet models

Main Challenges for High-Field Accelerator Magnet R&D Although Nb 3 Sn technology is far from being mature, it seems at hand for the high-field and high-field-gradient accelerator magnets that may be needed for LHC IR upgrade and ILC IR’s. However, we do need to keep working hard if we want to turn the few successful demonstrator magnets built so far into accelerator-class devices that can be implemented in a machine in a 10-year time frame.

Contents Why do we need high-field accelerator magnets? State of the art in superconducting accelerator magnet technology Ongoing Nb 3 Sn accelerator magnet programs

Ongoing Nb 3 Sn Accelerator Magnet R&D Programs At present, two main, complementary, Nb 3 Sn accelerator magnet programs have been launched – the US-LHC Accelerator Research Program (LARP) in the USA, – the Next European Dipole (NED) Joint Research Activity (JRA), within the framework of the Coordinated Accelerator Research in Europe (CARE) Integrated Activities. Note that CEA, Fermilab and LBNL are also pursuing (independently-funded) “base” programs.

US-LARP (1/2) The US-LHC Accelerator Research Program is aimed at supporting US efforts in LHC commissioning and at designing and developing equipment for LHC upgrade (such as advanced beam instrumentation and Nb 3 Sn magnets). It is carried out by a collaboration made up of BNL, Fermilab, LBNL and SLAC. Serious things will start in FY06, with a budget of 11 M$ (5 for magnets, 4 for accelerator-related R&D and 2 for management); this budget is expected to be maintained at a constant level for a few years.

US-LARP (2/2) The magnet part of LARP is aimed at building by 2009 one or two 4-m-long, 90-mm-aperture, 200 T/m quadrupole magnet prototypes, so as to demonstrate the feasibility of “long”, accelerator- class Nb 3 Sn magnets. It will also support the continuous development of high-performance Nb 3 Sn wires in US industry.

EU-CARE/NED (1/2) The CARE/NED JRA is a more modest program, that was re-scoped in 2004 to match the allocated EU funding (1 M€ total over ). At present it is articulated around three main tasks – high-performance Nb 3 Sn conductor development and characterization (in collaboration with EU industry), – insulation development and characterization, – design studies for a large aperture (88 mm) high field (15 T conductor peak field) dipole magnet.

EU-CARE/NED (2/2) In spite of the limited funding, NED is supported by a very active collaboration made up of eight institutes – CCLRC/RAL (UK), – CEA (France), – CERN (International), – CIEMAT (Spain), – INFN-Genoa and Milano (Italy), – Twente University (The Netherlands) – Wroclaw University of Technology (Poland).

NED Status: Conductor Contracts for the Nb 3 Sn conductor development (aiming at a non-Cu J C of 1500 A/mm 2 at 4.2 K and 15 T) have been awarded last fall to Alstom/MSA in France and SMI in the Netherlands. The first rounds of R&D wires are expected in the oncoming month. The final conductors should be produced by the end of 2006.

NED Status: Insulation The insulation characterization task includes heat transfer measurements in He-II environment. To perform these measurements, a new double-bath cryostat has been designed by CEA and built in Poland under Wroclaw University supervision. The cryostat will be delivered to CEA mid- September. Radiati on shields Vacuum containe r Heat exchanger piping Heat exchang er Expansi on valve Pumping He IIp He IIs LH e GHe Insert Cryogen ic vessel He I Experimen tal volume NED Cryostat Design (Courtesy B. Baudouy, CEA) Elements of NED Cryostat (Courtesy M. Chorowski, WUT)

NED Status: Design Studies Several magnet configurations are being compared so as to determine which one is the most efficient in terms of manufacturability, performance and costs. Cos  layer design (courtesy D. Leroy, CERN) Block design (courtesy H. Felice, CEA) Motor-type design (courtesy F. Toral, CIEMAT)

NED Outlook The NED collaboration is working hard at finding the money to complete the R&D program and build the proposed model magnet.

Conclusion Superconducting NbTi magnets have been used successfully in particle accelerators for close to 25 years. The LHC magnet R&D program has demonstrated that the performance limits of NbTi have been achieved and that to go beyond we need to switch to Nb 3 Sn. The US-LARP and the EU NED JRA are assessing the feasibility of accelerator-class Nb 3 Sn magnets, so as to open new ranges of options for LHC-IR upgrade and ILC IR’s. Bringing a new technology to maturity is always a challenge that requires dedicated resources; let us hope that both programs will be given the opportunity to achieve their goals in a timely manner.