A cryocooler-cooled magnet consisting of RE-123 coils and an iron return yoke in its cryostat was tested on the beam line of the Heavy Ion Medial Accelerator in Chiba (HIMAC). The magnet has a room temperature beam duct, at which a magnetic field of 2.4 T is generated, whereas the maximum magnetic field to which coated conductors are exposed is approximately 4 T. On the beam line of the HIMAC, at first, beam guiding using the magnet was demonstrated, and then a carbon-ion beam was injected intentionally into the RE-123 coils of the magnet to simulate the incident of uncontrolled beam injection into accelerator magnets. Repeated excitation experiments of the magnet were conducted in order to study its tolerance against AC loss and stability, as well as the reproducibility of the magnetic field generated. The magnet was excited repeatedly with a saw-tooth waveform to examine the tolerance against AC loss. Next, we examined the influence of shielding current on the field stability and reproducibility at the steps during ramping down, where the current was held constant. Prior to such series of tests using the magnet, the feasibility of quench detection using voltage taps and protection using a dump resistor were studied through quench experiments using short pieces of RE-123 coated conductors. Indeed, such quench detection and quench protection schemes were implemented for the test magnet.
A project to develop fundamental technologies for accelerator magnets using high-Tc superconductors (HTSs) was carried out from FY 2009 to 2018 and funded by the Japan Science and Technology Agency under its S-Innovation Program. In this research program, conceptual design studies of fixed—field alternating gradient (FFAG) accelerators for carbon cancer therapy were carried out. In the conceptual magnet design for a full-scale, spiral-sector FFAG accelerator, a rare earth elementbased REBa2Cu3Ox (REBCO) superconducting coated conductor was incorporated to realize a compact, high-field magnet. With the aim of demonstrating the winding technique of REBCO coils that meet the requirements of the FFAG accelerator, we developed a cryocooler-cooled, reduced-size test magnet incorporating multiple REBCO coils, referred to here as the "model magnet". All coils of the model magnet were designed to have a part with a negative-bend shape, and the coils close to the beam duct were designed to have a three-dimensional winding shape. This paper describes the design and fabrication of the model magnet.
A R&D project funded by the Japan Science and Technology Agency under its S-Innovation Program started in 2009. The purpose of this project is to develop fundamental technologies for accelerator magnets incorporating REBCO coils. In this project, there are plans to fabricate a cryocooler-cooled REBCO magnet for evaluating the beam guiding characteristics and stability during beam operation of the Heavy-Ion Medical Accelerator in Chiba (HIMAC). The magnet consisted of a pair of racetrack coil stacks each containing four coils, and a cold iron was incorporated to increase the magnetic field strength. It generated a dipole field of 2.4 T at the beam duct when excited at 200 A. This paper describes the design and fabrication results of the REBCO magnet for tests conducted at the beam-line.
In HTS magnets for accelerator systems, the behaviour of shielding-current-induced fields (SCIFs) is complicated because they are required to generate precise magnetic fields at different load ratios. The behaviour of the SCIF in a designed HTS cosine-theta dipole magnet wound with coated conductors was numerically evaluated using three-dimensional electromagnetic field analyses. To mitigate influence of the SCIF on dipole and sextupole components of the magnetic field, we applied current adjustment and combined with HTS sextupole correction coils. The required operation patterns of the main dipole magnet and correction coils were determined by calculating the time-dependent SCIF in advance.
This paper discusses the demonstration of a stable levitation (SL) system for a magnetic levitation type seismic isolation device using a high-temperature superconducting (HTS) bulk. The SL system utilizes a restoring force between an HTS bulk and permanent magnet (PM) to maintain the stable levitation of a base-isolation object. We first measured the restoring force between the HTS bulk and PM, and then demonstrated the performance of the SL system using a magnetic levitation type seismic isolation model device with a radial arrangement of HTS bulks and a PM rail. The SL system with the HTS bulk can reduce small vibration and displacement during the normal period of operation without experiencing a large disturbance. We also demonstrated that, when a large disturbance was applied, the transmission of horizontal vibration to the base-isolation object via the SL system was eliminated by decoupling the magnetic coupling between the HTS bulk and PM. Furthermore, the vibration transmissibility at any vibration frequency to the base-isolation object was reduced by incorporating an HTS bulk damper and eddy current damper in the model device. These results suggest that a SL system used together with an HTS bulk can realize both the stable levitation of a base-isolation object during the normal period of operation as well as the elimination of the horizontal vibration transmission when a large vibration is applied.