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A silicon piezo-resistive pressure sensor is investigated for possible in situ pressure measurement in a pressurized superfluid helium environment. A small pressure sensor, PD116, from Toyoda Machine Works Ltd, has been reported as an applicable sensor for in situ pressure measurement up to the maximum pressure of 3 MPa at cryogenic temperatures down to 6 K. It has a low temperature coefficient of sensitivity around 200 ppm/K between 300 and 6 K. In order to evaluate the applicability in a pressurized superfluid helium environment, the PD116 sensor was set in a specific cryostat which can provide superfluid helium environment. The characteristics of the sensor were measured for the pressure range from 100 kPa to 170 kPa at 4.2 K and 2.0 K. A driving current of 1 mA was applied to the sensor in order to limit heat dissipation to less than 1 mW. The pressure dependence of the output voltage of the sensor also indicates high linearity at temperatures lower than 4.2 K. The experimental results of this study clearly indicate a possible use of this sensor for in situ pressure measurement in a pressurized superfluid helium environment.

We are planning to demonstrate a quadrupole magnet with magnetic mirror structure by using 0.7-mm Nb3Al strands. As feasibility studies for this program, we investigated an influence of the diameter reduction of Cu-stabilized Nb3Al strands from 1.0 to 0.7 mm. Wire breakages and Cu separations did not happen with applying the cold die-drawing. The non-Cu Jc and n-values of the 0.7 mm strands did not degrade and could keep the same performance of the 1.0 mm strands. Although irregular deformations of Nb-Al filaments slightly occurred, magnetization properties of the 0.7 mm strands are almost the same as those of the 1.0 mm strands. Ta interfilament matrix of the 0.7 mm strands was also effective to improve the low field instability at 4.2 K. In addition, 27 strands Rutherford cable has been made by using the 0.7 mm F1 strand without any troubles. All of 27 extracted strands taken from the F1 cable showed very uniform Ic performance at 4.2 K.

In A15 superconducting wires, it is known that the critical current has a dependence on the strain in a magnetic field. Therefore, RHQ-Nb3Al wires are being studied to develop a high field magnet. Since the wire is usually composed of three or more materials, residual strain is induced by the different coefficients of thermal expansion in the materials in the cooling process after A15 phase transformation. In neutron diffraction measurements at room temperature, we previously reported that the residual strain of Nb3Al filaments in the wire is tensile. We also reported that the difference of the residual strain in Nb3Al filaments, as obtained at room temperature, has an effect on the tensile strain dependence of the critical current. In this study, we effectively reduced the residual strain by using a thermal cycle method after A15 phase transformation. By applying one thermal cycle process, the tensile residual strain was decreased by approximately 0.08%. This is the first investigation of the thermal cycle effect on the residual strain in the RHQ-Nb3Al wire. Studies on the thermal cycle are necessary to understand the mechanical properties of the wire. In this paper, we report the details of the thermal cycle method, its effect on the lattice structures of Nb3Al and Cu at room temperature, and the analysis results from the perspective of material strength in strain recovery and its hysteresis.

A tension-free rapid heating and quenching (RHQ) simulator was previously employed to demonstrate that a ductile and normal-conducting Cu layer can suppress a magnetic field instability and improve bending strain tolerance. However, a molten Cu barrier caused the filament to be shifted from the original location, and excess heating caused the Cu to locally corrode a Ta layer, through which it diffused into the filament, thereby embrittling the resulting wire. In some cases, the swelling molten-Cu tore the Ta skin. Fabrication of a long piece of Ta/Cu/Ta barrier Nb3Al wire is attempted using reel-to-reel RHQ treatment of a modified precursor with a thicker Ta skin that would tolerate both tension forces in a section of the wire between electrodes and pressure from the swelling of molten-Cu. In the optimum RHQ condition (VRHQ: 9.5 V), a 50-m piece of Ta/Cu/Ta barrier precursor was successfully reel-to-reel RHQ treated to form a bcc supersaturated-solid solution (Nb(Al)ss) without bursting of the Ta skin. Microstructural analysis revealed that the molten Cu, into which Ta dissolved, diffused into the jelly-roll filament, mainly through a region of partially melted Al-rich Nb( Al)ss. The critical current density of the filament Jc (18 T, 4.2 K) was optimized by the deformation of Nb(Al)ss and compared favorably with the conventional precursor.

An upgrade of the low-beta insertion system for the ATLAS and Compact Muon Solenoid experiments is proposed in the high luminosity Large Hadron Collider upgrade project. It includes final beam focusing quadrupoles, beam separation and recombination dipoles, and larger aperture matching section quadrupoles. KEK is in charge of the conceptual design of the large aperture separation dipole D1. The latest design parameters are a main field of ~ 5 T at 1.9 K with Nb-Ti superconducting technology, a coil aperture of 160 mm, and a cos-theta one-layer coil with Large Hadron Collider dipole cable. Because the new D1 is expected to be operated in a very high radiation environment, radiation resistance and a cooling scheme are being carefully considered. The collaring-yoke structure is adopted to provide the mechanical support for the single-layer Nb-Ti coil. We summarize the design study of this magnet, including i) the very large iron saturation effect on field quality due to the large aperture and limited size of the iron yoke, ii) the stray field at the outer surface of the iron cryostat, and iii) the stress management from room temperature assembly to final operation.

Nb3Al superconductors are studied for use in high field magnets. Fine-grained Nb3Al with nearly stoichiometric Al content is obtained by a rapid heating quenching and transformation process. We describe a nondestructive in situ study of the transformation process step of a rapid heating quenching Nb3Al precursor wire with ramp rates of either 120 °C/h or 800 °C/h. High-energy synchrotron X-ray diffraction measurements show the transformation from a Nb(Al)SS supersaturated solid solution into Nb3Al. When heating with a ramp rate of 120 °C/h, a strong reduction of the Nb(Al)SS (110) diffraction peak component is observed when the temperature exceeds 660 °C. Additional diffraction peaks are detectable in the approximate temperature interval 610 °C -750 °C and significant Nb3Al growth is observed above 730 °C.

A 13 T class Nb3Al and Nb3Sn subscale magnet has been in development at the High Energy Accelerator Research Organization (KEK) to establish technology for a high-field accelerator magnet. The magnet was designed to generate a high magnetic field efficiently in a minimum-gap, common coil configuration with 200-mm-long racetrack coils. Two of the five coils in the magnet are Nb3Sn-produced by Lawrence Berkeley National Laboratory. The other three coils are Nb3Al -produced by KEK. The Nb3Al Rutherford cables are comprised of 28 RHQ-Nb3Al wires and were developed at KEK in collaboration with National Institute for Materials Science and Fermi National Accelerator Laboratory. Presently, the fabrication of two double pancake Nb3Al coils with the same configuration of 13 turns per layer was completed using the wind and react method. The magnet assembly and the preparations for excitation tests are currently in progress.

A radiation-resistant superconducting magnet is mandatory to generate intense negative-charged muon beams in the next-generation mu-e conversion experiments, COMET at J-PARC and Mu2e at FNAL. Expected neutron fluence on the superconducting coils in the projects reaches 1021 n/m2. Therefore, irradiation effects in superconducting magnet material are considered in a design of the superconducting magnet system. The pion capture solenoid magnet is designed to utilize aluminum-stabilized superconducting cable to reduce the nuclear heating by radiation from the pion production target enclosed in the magnet. To achieve higher magnetic field on the target, the coil has multiple layers of thick conductor, and need pure aluminum strips in between the coil layers for heat removal. Prototyping of the coil with aluminum stabilized conductor, heat removal strips, and radiation-hard insulator is performed to check the feasibility of the coil winding and basic properties. The conceptual design of the COMET magnet and the prototype coil are reported.

Publisher Summary This chapter describes a new cryogenic test facility developed at KEK for research and development on high field superconducting magnets. It was constructed and tested. A magnet was attached to the vertical double bath cryostat and was cooled down by the refrigeration or liquefier with the automatic computer control system. The cryostat has 8 W of refrigeration power at 1.9 K and it took 6 hours to cool down from 4.2 K to 1.9 K with 450 L of liquid helium and 2 tons of the model magnet. The system could be recovered within 2 hours after a quench at 1.9 K. The fast recovery to 1.9 K after a magnet quench permits several quenches per day and a maximum of five have been demonstrated. It consists of a vertical

We have been developing Nb3Al high-field accelerator magnets with the aim of upgrading the luminosity of the Large Hadron Collider. In this program, four kinds of Ta-matrix Nb3Al strands with slightly different cross-sectional structures (K1, K2, K3 and K4 strands) were designed and fabricated. The non-copper Jc values of these strands were 700-800 at 15 T and 4.2 K. The cabling work of these strands was performed at Fermilab, and the superconducting properties of the cables were studied using the extracted strands from the cables. In this paper, we report on the fabrication experience for the strands and cables. The results obtained from this study are also presented.

A new generation of precision muon conversion experiments is planned at both Fermilab and KEK. These experiments will depend upon a complex set of solenoid magnets for the production, momentum selection and transport of a muon beam to a stopping target, and for tracking detector momentum analysis of candidate conversion electrons from the target. Baseline designs for the production and detector solenoids use NbTi cable that is heavily stabilized by an extruded high RRR aluminum jacket. A U.S.-Japan research collaboration has begun whose goal is to advance the development of optimized Al-NbTi conductors, gain experience with the technology of winding coils from this material, and test the conductor performance as modest length samples become available. For this purpose, a “conductor test” solenoid with three coils was designed and built at Fermilab. A sample of the RIKEN Al-NbTi conductor from KEK was wound into a “test” coil; this was sandwiched between two “field” coils wound from doubled SSC cable, to increase the peak field on the RIKEN test coil. All three solenoid coils were epoxy impregnated, and utilized aluminum outer bandage rings to apply preload to the coils when cold. The design and fabrication details, and results of the magnet quench performance tests are presented and discussed.

The development of a large-aperture (120-180 mm) dipole magnet is proposed in the framework of the CERN-KEK cooperation program. The application target is the D1 magnet (separation dipoles) replacement for the HL-LHC (High Luminosity - Large Hadron Collider) upgrade. The Cos-theta type coil cross section and the shell-based structure are adopted in the conceptual design of this magnet. The nominal field is estimated to be 6-10 T at 1.9 K with a 30-mm-width coil arranged in two layers. The candidates of superconductor are Nb3Al, Nb$_{3}$Sn and Nb-Ti. We present the analytical estimation of the key parameters of this magnet, and the magnetic & mechanical simulation results of the actual design, including the field quality in the aperture, the stray field of the magnet, and the stress distribution in the coil.

KEK has emphasized efforts to develop the RHQ-Nb3Al superconductor and a 13 T sub-scale magnet towards the LHC upgrade in last years. In addition, relevant R&D regarding radiation resistance has been carried out. For realization of higher field magnets beyond 15 T, thorough understanding of strain behavior of HTS and A15 superconductors is truly desired. KEK has launched a new research subject on stress/strain sensitivity of HTS and A15 superconductors in collaboration with the neutron diffraction facility at J-PARC and the High Field Laboratory in Tohoku University.

To suppress the low-magnetic-field instability (flux jumps in low magnetic fields) of a rapid-heating, quenching and transformation (RHQT) processed Nb3Al superconductor, we had previously modified the cross-sectional design of an RHQT Nb3Al by adopting a Ta filament-barrier structure. Unlike Nb barriers, Ta barriers are not superconducting in magnetic fields at 4.2 K so that they electromagnetically decouple filaments. However, small flux jumps still occurred at 1.8 K, which is a typical operating temperature for the magnets used in high-energy particle accelerators. Furthermore, poor bonding at the Ta/Ta interface between neighboring Ta-coated jelly-roll (JR) filaments frequently caused precursor wires to break during drawing. To overcome these problems, we fabricated a new RHQT Nb3Al wire with a Ta/Cu/Ta three-layer filament-barrier structure for which an internal stabilization technique (Cu rods encased in Ta are dispersed in the wire cross section) was extended. Removing the Ta/Ta interface in the interfilamentary barrier (JR filament/Ta/Cu/Ta/JR filament) allowed precursor wires to be drawn without breaking. Furthermore, the Cu filament barrier electromagnetically decoupled filaments to suppress flux jumps at 1.8 K. The ductile Cu layer also improved the bending strain tolerance of RHQT Nb3Al.

We have been developing Nb 3 Al wires processed by rapid heating and quenching for a number of years as promising candidates for use in future high-field accelerator magnets. These wires have better strain and stress tolerances than Nb 3 Sn wires do, but to meet the demands of future accelerator magnet designs, it is necessary to further improve their performance. In particular,it is necessary to increase their non-copper critical current density in 12-20T fields. To pursue this goal, we introduced double rapid heating and quenching (DRHQ) treatment into the fabrication process for Nb 3 Al wires, and studied the mechanical and superconducting properties of the resulting DRHQ-processed wires.

The overall performance of accelerator magnets strongly relies on electrical and mechanical robustness of their components. With an increase of the energy, future particle accelerators will have to withstand integral doses of ionizing radiation of up to several tens of MGy. Initially developed for the components of the D1 separation dipole magnet, designed and manufactured by KEK and part of the HL-LHC Project, this study was enlarged to characterise a spectrum of Glass Fiber Reinforced Polymers and resins potentially interesting for superconducting magnets. As a collaboration between CERN, KEK and QST Takasaki, an irradiation campaign was held with gamma-ray doses going from 10 MGy to 100 MGy. This paper describes the different methodologies applied to perform mechanical and chemical tests, both at room and cryogenic temperatures on a wide range of materials and resins. The results and the analysis of these tests are presented with the goal to provide some guidance on the choice of specific material or resin in HEP applications.

Development of low-/spl beta/ quadrupole magnets for the LHC beam interaction regions has been carried out. The magnet is designed with a field gradient of 240 T/m in a coil aperture of 70 mm, and is to be operated at /spl les/215 T/m with absorption of beam loss heating. Three 1-m models with the final design have been developed and tested with satisfying requirements of field quality and training characteristics. A full-scale (6.3-m) prototype has been developed, and is being tested. Progress of the LHC low-/spl beta/ quadrupole development at KEK is reported.

The conceptual design study of a high gradient super conducting insertion quadrupole magnet has been carried out in collaboration between KEK and CERN for the Large Hadron Collider (LHC) to be built at CERN. A model magnet design has been optimized to provide a nominal design field gradient of 240 T/m with a bore aperture of 70 mm and an operational field gradient of 225 T/m at 1.9 K under radiation environment with a beam energy deposit of several watts per meter in the superconducting coils. The design and its optimization process are discussed.