Shunji Yamamoto

Superconducting bending magnets for compact synchrotron radiation source

A 180 degrees superconducting magnet has been constructed for a synchrotron radiation source. The authors describe the superconducting magnet design and some component test results for the compact synchrotron radiation source. The magnetic field at the electron orbit is 4.5 T. The bending radius is 0.593 m. The magnet has several features such as low liquid helium consumption ratio, persistent current operation, high homogeneous magnetic fields, and leakage flux shielding by the iron core surrounding the cryostat. Component tests of persistent current switches of 15 Omega and 3 Omega and a superconducting joint test have been performed.<<ETX>>

Quench protection of persistent current switches using diodes in cryogenic temperature

Transient characteristics of power diodes at cryogenic temperatures and their application to the protection of persistent current switches (PCS) in superconducting magnet systems are presented. The observed forward voltage drop was 9 V at 4.2 K and was six times larger than that at room temperature. The voltage was quickly reduced to 1 V after the diode was turned on. The transition time was less than 1 ms. A PCS with a normal state resistance of 1.5 Omega was adequately protected from quenching by the diodes.<<ETX>>

Field measurements of superconducting bending magnets for compact storage ring

We have constructed 180 degree superconducting bending magnets for compact electron storage rings and tests of magnetic field qualities of the magnets have been performed. Measured field qualities are; (1) high field homogeneity (normalized field errors are less than 1/spl times/10/sup -3/ over the electron beam aperture), (2) high field stability (the decay of the field is less than 0.1%/year) at persistent current mode of 3.5 T, (3) small gradient of coil axis (the averaged value is 0.5 mrad), and (4) small difference of the fields between two magnets (the difference is less than /spl plusmn/1/spl times/10/sup -3/). From the tests of field qualities, we confirm that the magnets have good performance for electron beam storage. An electron beam of 600 MeV can be stored.<<ETX>>

Super-Stable Superconducting MRI Magnet Operating for 25 Years

This paper describes a superconducting magnet performance continuing persistent current operation for 25 years. The central magnetic field is 0.5 T for whole-body magnetic resonance imaging (MRI). The observed stability of the magnetic field was extremely stable compared with the specification. Because of the ultrafine electrical resistances in persistent current mode operation, it was difficult to observe an accurate magnetic field stability at the factory test. By long-term operation, we are able to observe the magnetic field stability and the joint resistance precisely. The magnet was started to energize on January 1988 and has been operated consecutively without re-excitation. At that time, a superconducting whole-body MRI system was appeared in the world among a normal conducting magnet and a permanent magnet system. We made superconducting joints to connect superconducting conductors between superconducting coils. We carried out the measurement of the central magnetic field by the nuclear magnetic resonance method. Because the device is used for the medical examination at the hospital, the magnetic field data acquisition is in a limited situation. The stability of the last magnetic field becomes 0.0006 ppm/h and gets the good performance that is more than 100 times than the specification. The total resistance of the superconducting joints is 4 ×10-12 Ω and supports good superconducting connection.