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HFM - Superconducting High Field

Magnets for higher luminosities

and energies (Work package 7)

Objectives:

Magnets with Nb₃Sn conductors are needed to upgrade existing accelerators in Europe such as the LHC on the medium long term and to prepare for new projects on a longer time scale. Their high current density properties in high fields and large temperature margin will be needed to meet the fields and gradient requirements and to withstand the heating due to the radiation in these new and upgraded machines.

On the very long term (> 20 years), an LHC upgrade to 2-3 times the energy is an option to be considered. For such an energy level, dipole magnets with a field of around 20 T would be needed. These accelerator magnets are beyond the possibilities offered by using Nb-Ti or Nb₃Sn conductors alone. A possibility is to use a layered coil with an outer coil of 14 T in Nb₃Sn conductor and an inner coil of HTS conductor, delivering a field contribution of 6 T. High field capabilities are also the limiting parameter for undulators when increasing the central field and reducing the period of the field. These limitations can be overcome using Nb₃Sn conductors also for these devices. The management of this WP has also the role to identify synergies between the various applications of Nb₃Sn. The LHC is the existing infrastructure that will directly benefit from the work in this WP.

Task1. Coordination and Communication

Coordination and scheduling of the WP tasks
monitoring the work, informing the project management and participants within the JRA
WP budget follow-up

Task 2. Support studies

Certify radiation resistance of radiation resistant coil insulation and impregnation
Make a heat deposition and heat removal model for the dipole Nb₃Sn model with experimental validation and determine the thermal coil design parameters for the dipole model magnet.

Task 3. High field model

Design, build and test a 1.5 m long, 100 mm aperture dipole model with a design field of 13 T using Nb₃Sn high current Rutherford cables.

Task 4. Very high field dipole insert

Design, build and test HTS solenoid insert coils for a solenoid background magnet aiming at a field increase up to 6 T to progress on the knowledge of HTS coils, their winding and behaviour. This as in intermediate step towards a dipole insert.
Design, build and test an HTS dipole insert coil for a dipole background magnet aiming at a field increase of about 6 T.

Task 5. High Tc superconducting link

Design of HTS bus: choice of HTS material definition of thermal conditions, requirements for stabilization and quench protection, modelling of quench propagation.
Design. realization and test of electrical joints and electrical terminations.
Mechanical design and assembly of a 20 m long superconducting link (26 pairs of 600 A).

Task 6. Short period helical superconducting undulator

Design, build and test a prototype helical coil undulator magnet with 11.5 mm period, high peak magnetic field in Nb₃Sn technology.

Description of work:

Task 1. Coordination and Communication. The activities of this task are to oversee and co-ordinate the work of all the other tasks of the work package concerned, to ensure the consistency of the WP work according to the project plan and to coordinate the WP technical and scientific tasks with the tasks carried out by the other work packages when it is relevant. The coordination duties also include the organization of WP internal steering meetings, the setting up of proper reviewing, the reporting to the project management and the distribution of the information within the WP as well as to the other work packages running in parallel. The task also covers the organization of and support to the annual meetings dedicated to the WP activity review and possible activity workshops or specialized working sessions, implying the attendance of invited participants from inside and outside the consortium.

Task 2. Support studies. Magnets in accelerators like the upgraded LHC and neutrino factories be subjected to very high radiation doses. The electrical insulation employed on the coils need to be resistant to this radiation. A certification program for the radiation resistance is needed in parallel to the modelling efforts for such magnets. The same radiation is also depositing heat in the coils. The heat removal from the coils needs to be modelled. These models have to be supported with measurements. A thermal design of the dipole model coil can then be made.

Sub-task 1: Radiation resistance certification for radiation resistant coil insulation and impregnation. CERN will lead this activity and provide irradiation time at its accelerators. Other irradiation facilities from the partners might be envisaged. The exact work distribution between the 3 partners PWR, CEA-DSM and CERN still has to be determined.
Sub-task 2: Thermal models and design. PWR will lead this activity. Thermal tests will be done in the various specialized cryogenic facilities at the 3 partner laboratories. All 3 partners will contribute to the modelling efforts aimed at producing a thermal model for the Nb₃Sn dipole model magnet.

Task 3. High field model. The technologies to be used for Nb₃Sn magnets, which are residing with the partners (e.g. high current density conductors, Nb₃Sn wind-and-react coil fabrication, insulation) are to be brought together and tested in short models. Several of these technologies (superconducting cable, insulation, coil design, support structures) were partly developed during the FP6-CARE-NED project.

The proposed dipole model will test these technologies for large accelerator magnets and the model will afterwards be used to upgrade the superconducting cable test facility FRESCA at CERN from 10 T to 13 T. The issues are to reach high fields in large apertures with good temperature margins in the coil, beyond the possibilities of Nb-Ti conductors. As a test bed for high field accelerator magnets a 1.5 m long dipole model will be build with an aperture of 100 mm and a design field of 13 T. For this dipole model, CEA-DSM and CERN will design together the magnet. CERN will do the conductor characterization. PWR will do the thermal design and thermal component tests. CEA-DSM will fabricate the coils and CERN will build the mechanical support structure. Combined teams will integrate the coils into the support structure. The cryogenic test of the model will be done in the CERN test station.

Task 4. Very high field dipole insert. Recent progress has shown outstanding performance on the intrinsic current transport properties of HTS Bi-2212 round wires, well adapted to magnets (Je=450 MA/m2 and Jc=1800 MA/m2 at 4 K under 25 T). This should open the road to higher magnetic fields. This work package is a very first step to prospect for this possibility. The dipole model constructed in task 3 of this WP will serve the role of the outer layer. The development will pass in three steps. The first studies will deal with the specification of several HTS conductors. This will be completed by modelling work focused on stability and quench. The quench of HTS coils with their very often degradation is an identified issue. Due to the difficulty of making in one go a dipole insert coil of HTS conductor, several HTS solenoid insert coils will be made and tested in existing high field solenoid magnets at the partner’s labs. The experience, which will be gained, will be used to construct a dipole insert coil. These sub-tasks are fully interdependent with strong interactions.

Sub-task 1: Specifications, characterizations and quench modelling. The candidate conductors will be specified in this sub-task with as aim to select the best suitable product. The expertise of the partners CNRS, CEA-DSM, FZK, INFN, TUT and UNIGE will be needed for these specifications on electrical, mechanical and thermal behaviour and are of prime interest for our high field objective. Quench behaviour of these HTS magnets will be studied using quench modelling codes. The aim is to propose quench protection and detection strategies to avoid any degradation.
Sub-task 2: Design, construction and tests of solenoid insert coils. This activity will be lead by CNRS-Grenoble with contributions of FZK and INFN for the design and the tests. The design issues for low temperature superconductors and HTS are different. Two major concerns, operating margins and quench protection, are very distinct. Several solenoids will be wound by CNRS-Grenoble with assistance of the partners. The coils will be instrumented to catch the maximum of information. They will be tested at CNRS-Grenoble or at FZK in very high field bores. In particular, the quench behaviour and protection strategies will be studied and analyzed.
Sub-task 3: Design construction and tests of a dipole insert coil. Using the results of the solenoid insert coils, a dipole insert coil will be constructed. CEA-DSM will have the responsibility for this sub-task and will wind the insert coil. As for the solenoids, the partners will bring their know-how for design and manufacturing and the dipole-insert will be instrumented. The coil will be tested at a later stage in the upgraded FRESCA facility of CERN in the dipole model magnet from task 3.

Task 5. High Tc superconducting link. The use of HTS material in buses linking superconducting magnets is of great interest for accelerators such as the LHC. Existing buses use Nb-Ti superconductors, maintained at temperatures below 6 K. The use of HTS enables operation at higher temperatures and offers a convenient gain in temperature margin during operation. In the case of the LHC, the use of HTS links is of specific benefit to an upgrade, in that it provides long distance electrical connections between power converters and superconducting magnets. It links cold magnets electrically. In cases where space is limited and the radiation environment is harsh, it also provides more flexibility in the location of the cryostats supporting the current leads. HTS links of the type required for the accelerator technology do not exist yet, and significant work has to be done to develop a long-length multi-conductor operating in helium gas at about 20 K. Considerable R&D is being done on HTS cables for electrical utilities, and it might be thought that one could simply apply these technologies. However, at present this work is focused on using single or 3-phase AC conductors with high voltage insulation and liquid nitrogen cooling, and it should be noted that this is still development work yet to be concluded. Particle accelerators require high quasi- DC current carrying links with many cables (up to about 50) in parallel and cooled with liquid or gaseous helium. In the LHC there are over 50000 connecting cables with a total length of 1360 km. Thus the need specific to accelerator applications, is for a new type of link with multiple circuits, electrically isolated at around 1 kV - 2 kV, carrying quasi-DC currents. The design study has to cover the option to use MgB2 at a temperature of 20 K as well as the electrical connections between HTS and LTS.

Sub-task 1: Studies on thermal, electrical and mechanical performance. Performance tests on short samples of HTS material. CERN, COLUMBUS, BHTS and SOTON will study together the performance of HTS conductors at low temperatures. Existing test stations at CERN and in SOTON, which are used for measurements at 4.2 K, will have to be adapted to enable measurements of critical currents at 20 K. CERN, COLUMBUS and the SOTON will model the quench propagation in the HTS cables and define the requirements for stabilization and protection. CERN, COLUMBUS and BHTS will perform measurements of mechanical properties of short samples at liquid nitrogen temperature.
Sub-task 2: Design and test of electrical contacts HTS-HTS and HTS-Cu. CERN, COLUMBUS and BHTS will prepare short samples and test their electrical resistance at cryogenic temperature. CERN and DESY will design together the electrical terminations of the HTS link.
Sub-task 3: Design and assembly of a 20 m long HTS multi-conductor 600 A link. CERN, DESY, BHTS, COLUMBUS and SOTON will design together a 20 m long link containing 26 circuits operating at 600 A. The design includes both the superconducting bus and the mechanical envelope providing the vacuum insulation. The cryogenic test will be done at CERN. COLUMBUS and CERN will design and test the electrical insulation of the circuits.

Task 6. Short period helical superconducting undulator. This task is focused on increasing the achievable magnetic field levels in short period magnets through the use of advanced materials (Nb₃Sn conductors) and innovative designs (helical coils). For example, single pass free electron lasers (e.g. X-FEL, FERMI@ELETTRA) could cover a wider wavelength range through field enhancement, or alternatively, operate at significantly lower electron energy. Additionally, short period magnets could be used in the production of positrons for any future lepton collider and increased magnetic field levels will increase the positron yield and also allow for savings. The first part of this task will be a design study of the undulator using an Nb₃Sn conductor. A comparison will be made with existing Nb-Ti. Following this design stage a short prototype (~300mm) will be manufactured and tested magnetically. The results from this prototype will inform the study and the design will be iterated in order to provide the strongest possible field level. This second design will then be prototyped (~500mm) and characterised. The results will be analysed and a full description of the study will be given in a final report.

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ANSYS model of a quarter of the cross-section of the EuCARD high field model (October 2010 design stage, task 7.3). The coils consists of two double pancakes made with 22 mm wide cables with a total of 156 turns per pole. The structure is of the "Shell-Bladder-and-Keys" type. Image Courtesy of A. Milanese.

Cross section of a Niobium-tin (Nb₃Sn) strand. This strand was made with the Powder In Tube (PIT) method by Bruker EAS (D). A similar (but 20% thinner) strand is considered for the 13 T dipole in Task 7.3. Image Courtesy of Bruker EAS and Bruker HTS