H. Mitsubori

Cryocooler Cooled Superconducting Magnets and Their Applications

Various types of cryocooler cooled superconducting magnets have been constructed and already used for some applications. An 11 T-52 mm room temperature bore magnet is used for a high-field heat treatment equipment, a 6 T-220 mm room temperature bore magnet is used for a new experiment on the electrochemical reaction in high fields, and a 5 T-50 mm bore with 10 mm gap split type magnet has been combined with an X-ray diffraction apparatus.

A cryocooler cooled 5 T superconducting magnet with a horizontal and vertical room temperature bore

We designed and constructed a cryocooler cooled 5 T superconducting magnet with a horizontal room temperature bore of 50 mm and a vertical room temperature bore of 90 mm without liquid helium. The magnet, which is directly cooled by 4 K Gifford-McMahon cryocooler in vacuum, consists of NbTi coil, Bi(2223) bulk current leads and cryostat. The coil with an inner diameter of 130 mm, an outer diameter of 301 mm, a height of 66 mm and a gap of 80 mm is made using NbTi wires and Cu-plated SUS bobbin. Bi(2223) bulk current leads are thin-walled sintered cylindrical tubes. The outer diameter, height and weight of the magnet are 510 mm, 730 mm and 260 kg, respectively. The magnet is cooled down to 3.8 K in approximately 62 hours. A continuous operation at 5 T, which is generated by an operating current of 122 A, has been performed.

Development of a high current H− ion source for cyclotronsa)

A multi-cusp DC H(-) ion source has been designed and fabricated for medical applications of cyclotrons. Optimization of the ion source is in progress, such as the improvement of the filament configuration, magnetic filter strength, extraction electrodes shape, configuration of electron suppression magnets, and plasma electrode material. A small quantity of Cs has been introduced into the ion source to enhance the negative ion beam current. The ion source produced 16 mA of DC H(-) ion beam with the Cs-seeded operation at a low arc discharge power of 2.8 kW.

High current DC negative ion source for cyclotron

A filament driven multi-cusp negative ion source has been developed for proton cyclotrons in medical applications. In Cs-free operation, continuous H(-) beam of 10 mA and D(-) beam of 3.3 mA were obtained stably at an arc-discharge power of 3 kW and 2.4 kW, respectively. In Cs-seeded operation, H(-) beam current reached 22 mA at a lower arc power of 2.6 kW with less co-extracted electron current. The optimum gas flow rate, which gives the highest H(-) current, was 15 sccm in the Cs-free operation, while it decreased to 4 sccm in the Cs-seeded operation. The relationship between H(-) production and the design/operating parameters has been also investigated by a numerical study with KEIO-MARC code, which gives a reasonable explanation to the experimental results of the H(-) current dependence on the arc power.

Performance Test of Cryogen-Free Bi-2223 HTS Dipole Magnet for Beam Line Switching

We have developed a Bi-2223 high-temperature superconducting dipole magnet for beam line switching at the cyclotron facility of the Research Center for Nuclear Physics, Osaka University. Exit beam lines are periodically switched by increasing and decreasing the magnetic field between 0 and 1.6 T with a switching time of 10 s. The magnet is equipped with two sets of Bi-2223 coil assemblies, which are conduction cooled by two 10-K Gifford-McMahon cryocoolers. We evaluated the superconducting property of the Bi-2223 coil assemblies in liquid nitrogen before installation in the cryostat. There were no degradation in wire performance during the assembly process. Magnetic field, strains, and temperatures of the coil assembly were investigated for performance verification after the magnet fabrication. The magnetic field and the temperatures meet specifications, and the deformation of the coil assembly is considered to be successfully suppressed by reinforcing structure. Furthermore, from a viewpoint of temperature, it is indicated that the magnet can make switching time and cycle time faster than that of the specified operation.

Development of microwave ion source for industrial applicationsa)

A microwave ion source is one of the long-life ion sources. In this paper, we report on the characteristics of the extracted Ar ion beam produced by a microwave ion source under various conditions, in terms of magnetic flux distribution and mass flow, and the stability of the ion beam. The measured spectra show that, under the experimental condition, almost all of produced ions were Ar(+) ions. For more than 6 h, the ion beam was stable.

Development of a 20 mA negative hydrogen ion source for cyclotrons

A cesiated DC negative ion source has been developed for proton cyclotrons in medical applications. A continuous H− beam of 23 mA was stably extracted at an arc power of 3 kW. The beam current gradually decreases with a constant arc power and without additional Cs injection and the decay rate was about 0.03 mA (0.14%) per hour. A feed-back control system that automatically adjusts the arc power to stabilize the beam current is able to keep the beam current constant at ±0.04 mA (±0.2%).

Development of a microwave ion source for ion implantations

A microwave ion source is expected to have a long lifetime, as it has fewer consumables. Thus, we are in the process of developing a microwave ion source for ion implantation applications. In this paper, we report on a newly developed plasma chamber and the extracted P(+) beam currents. The volume of the plasma chamber is optimized by varying the length of a boron nitride block installed within the chamber. The extracted P(+) beam current is more than 30 mA, at a 25 kV acceleration voltage, using PH3 gas.

Experimental apparatus for critical current measurement above 5 K using Bi-based oxide current leads

Experimental apparatus for the measurement of temperature dependence of critical current within a strong magnetic field has been developed. Samples, which are tested by the four probe method are cooled by a Gifford-McMahon (GM) type cryocooler to the lowest temperature of 5 K. The sample holder has a diameter of 70 mm and a height of 70 mm. Transport currents, up to 150 A, are supplied to two samples through Bi-based oxide superconducting current leads. An external magnetic field of up to 15 T, which is applied to the samples perpendicular to the sample axis, is generated by a water cooled single Bitter magnet with a room temperature bore of 82 mm. As a performance test, the critical current properties of a silver clad Bi-based oxide tape have been measured at 5.5 K, 30 K and 50 K within a magnetic field up to 15 T. This experiment has demonstrated the validity of this experimental apparatus.

Improvement of a Large Bore Cryogen-Free Superconducting Magnet for a Hybrid Magnet

A 360-mm room-temperature bore cryogen-free superconducting magnet (CSM), consisting of Nb3Sn coils and NbTi coils, for a hybrid magnet (HM) has generated the maximum magnetic field of 9.5 T. However, the magnetic field of the CSM has been limited to 8.5 T in the hybrid magnet mode because of a cooling problem. As a result, the hybrid magnet composed of a 19-T water-cooled resistive magnet (WM) had the utmost field generation of 27.5 T. Therefore, we improved the CSM to generate higher magnetic fields. For the improvement of the cooling problem, Nb3Sn coils were replaced, and thermal conduction was improved between coils and a 4K-GM cryocooler. Furthermore, support structures with a tensile strength over 80 kN and a spring support were adopted against the magnetic force to support the self-weight of coils and to absorb stress caused by thermal contraction difference between each coil. After the improvement, the CSM generated 9.5 T within 1 h and the maximum magnetic field of 9.7 T in a 360-mm room-temperature bore. The HM succeeded in generating 28 T in a 32-mm room-temperature bore with the CSM operated at 9.0 T.