M. Yoshimoto

Progress of Research and Development of Fundamental Technologies for Accelerator Magnets Using Coated Conductors

A project to develop the fundamental technologies for accelerator magnets using coated conductors is in progress. A coil-dominated magnet and an iron-dominated magnet were designed, based on the conceptual design of spiral sector fixed field alternating gradient accelerator for carbon cancer therapy and that for accelerator-driven subcritical reactor, respectively. The required winding technologies were clarified through designing the magnets. The R&D of winding technologies for coils with three-dimensional shape and those with negative bend have been carried out. The influence of the magnetization of coated conductors on the field quality of magnets was studied experimentally.

Magnetic field design of a Superconducting magnet for a FFAG accelerator

A superconducting magnet for a Fixed Field Alternating Gradient (FFAG) accelerator is proposed. The static magnetic field is required to be proportional to the k-th power of the orbit radius where k is the geometrical field index of the accelerator. The left/right asymmetric coil with elliptical aperture is introduced to maximize the horizontal aperture with a compact magnet design. The 3D coil configuration is designed to meet the requirement in terms of integral magnetic field. The 3D magnetic field is evaluated and the field integral along the beam trajectory satisfies the beam optics requirement of the FFAG accelerator.

Magnetic Field Design of Coil-Dominated Magnets Wound With Coated Conductors

Coil-dominated magnets wound with coated conductors were designed for an FFAG accelerator for carbon therapy, which was designed by applying linear approximation. When designing the coil-end of the magnets, we applied differential geometry. To apply the differential geometry to three-dimensional windings with coated conductors, we introduced the concept of generalized flat-wise bending. Thereby, the combination of flat-wise bending and torsion was considered as bending of developable surface. Generated magnetic field was calculated, and the positions of conductors were optimized to minimize the field error. Three dimensional shapes of magnets were successfully designed with constraints on generalized flat-wise bending and edge-wise bending of coated conductors.

Improvement of the Shift Bump Magnetic Field for a Closed Bump Orbit of the 3-GeV RCS in J-PARC

The four shift bump magnets (BUHS01-04) of the 3-GeV RCS in J-PARC, which are located at the long straight section, produce a fixed main bump orbit to merge the injection beam into the circulating beam. They are realized with four magnets connected in series to form the accurate closed bump orbit. However, the total integrated magnetic field of the four magnets is not zero because of the magnetic field interference between the shift bump magnet and the adjacent quadrupole magnets (Q magnet). In order to measure the magnetic field distribution accurately, the short search coil and the long search coil were used. Furthermore, the imbalance of the total integrated magnetic field has been improvement by inserting 0.3 mm insulators in the median plane of the return yoke of the BUHS02 and 03. The value of the integration has been decreased from 2358.0 G ldr cm to -71.6 G ldr cm.

Progress of Fundamental Technology R&D Toward Accelerator Magnets Using Coated Conductors in S-Innovation Program

We report the progress of an R&D project of fundamental technologies for cryocooler-cooled accelerator magnets using coated conductors funded by the Japan Science and Technology Agency under its S-Innovation Program. Its target applications include carbon cancer therapy and accelerator-driven subcritical reactor. We have been carrying out design studies of HTS magnets for spiral sector fixed-field alternating gradient accelerators to show their feasibility for the target applications and to clarify the requirements of winding technologies. A three-dimensional winding machine has been developed to fabricate a model magnet in which winding technologies required for the designed magnet are implemented. With respect to the large magnetization of coated conductors, which is one of the big concerns on their uses in accelerator magnets, the magnetic field measurements using rotating pick-up coils have been made to clarify its influence on the multipole components of the magnetic field. A method for numerical electromagnetic field analyses of coils with three-dimensional shapes has been developed to predict the influence of magnetization on the field quality of magnets.

A Novel Magnet Design Using Coated Conductor for Spiral Sector FFAG Accelerators

A superconducting coil-dominated magnet was designed for a fixed-field alternating gradient (FFAG) accelerator for carbon cancer therapy. The magnet consists of deformed pancake coils with negative bends and a saddle-shape coil with negative bends wound with coated conductors. The minimum flat-wise bending radius and the maximum edge-wise bending strain were considered when designing the coils. The field error was evaluated by using the integrated magnetic field along the beam direction and the local k + 1 value to optimize the shapes of the coils.

Development of a Prototype Superconducting Magnet for the FFAG Accelerator

A superconducting magnet for the Fixed Field Alternating Gradient (FFAG) accelerator has been proposed. The static magnetic field is required to be proportional to the k-th power of the orbit radius where k is the geometrical field index of the accelerator. A left/right asymmetric coil with elliptical aperture is utilized to maximize the horizontal aperture with a compact design. The saddle shaped coil configuration is designed to fulfil the design requirement in terms of integral magnetic field along an arc orbit. In a beam tracking simulation, a particle was circulated and accelerated with the magnetic field generated by the designed coil. A prototype superconducting coil with a cylindrical bore was developed and the field quality was evaluated at warm field measurement

Design and Magnetic Field Analyses of Spiral Sector Magnet in an FFAG Accelerator for Carbon Cancer Therapy

We designed a coil-dominated magnet wound with coated conductors for a spiral sector fixed-field alternating-gradient (FFAG) accelerator for carbon cancer therapy. The magnet consisted of deformed pancake coils with negative bends. Coils were arranged similar to a ladder to generate the required radial profile of magnetic field. The positions and the number of turns of the coils were iteratively adjusted to reduce the magnetic field error. We carried out numerical electromagnetic field analyses of the designed magnet to evaluate the influence of coated-conductor magnetization (screening or shielding current) on its field quality.

Analysis of Hybrid Type Boron-Doped Carbon Stripper Foils in J-PARC RCS

J-PARC (Japan-Proton Accelerator Research Complex) requires a carbon stripper foil to strip electrons from the H− beam supplied by the linac before injection into the Rapid Cycling Synchrotron (RCS) [1]. The foil thickness is about μm (200μg/cm2) corresponding to conversion efficiency of 99.7% from the primary H− beams of 181MeV energy to H+. We have successfully developed the Hybrid type thick Boron-doped Carbon (HBC) stripper foil, which showed a drastic improvement the lifetime without thickness reduction and shrinkage at the irradiated area. We started to study carbon stripper foils microscopically why carbon foils have considerable endurance for the beam impact by boron-doped. At first step, we made a comparison of ion irradiation effect between normal carbon and HBC by the electric microscope, ion-induced analysis. In particular, it seems that grain size of boron-rich area became much larger by irradiation for HBC. It was also observed that the boron-rich grain grew up by taking around material and generated pinholes more than 100 nm near itself consequently.

Field Measurement of DC Magnets at 3-GeV RCS in J-PARC

The J-PARC RCS (rapid cycling synchrotron) is designed to inject the beam from the LINAC, to change their charge from to , to accelerate the beam from 400 MeV to 3 GeV, with protons per pulse at 25 Hz repetition rate, and to extract the beam after the acceleration to the MLF (material and life science experimental facility) and the MR (main ring). Thus the RCS has the beam circulating ring and three beam transport lines. The RCS ring consists of several magnets, and the DC magnets are installed for beam injection and extraction. The four steering magnets and two septum magnets are installed at injection line in order to adjust the beam injection point. One quadratic magnet, two steering magnet and two septum magnets are installed at dump line for the part of the beam with their charge not changed from to at the charge stripping foil. Two deflecting magnets to kick out the beam without exciting the pulse kicker magnets and three septum magnets are installed at the extraction line. Up to now, the DC magnets are developed and manufactured, the field measurements are carried out, and magnets installation into the RCS tunnel are finished. The field distributions of DC magnets are measured by using the new field mapping system with three one-dimensional hall-probes on a 3-axis movable stage. In the case of the quadrupole magnet, the field measurements are not only field mapping but also analyze its magnetic higher order components by the harmonic rotating coil. As a result, the measured field quality meets the design performance requirements of the DC magnets.