Yukinobu Imamura

Development of a Test Conduction-Cooled MgB 2 Coil

We are now developing conduction-cooled MgB2 coils toward the realization of magnets with 1-MJ magnetic energy. We devised a new fabrication method, which is based on the wind and react method. A bobbin in the react process is made of stainless steel. To improve cooling performance, one of the stainless steel flanges of the bobbin is replaced with an oxygen-free copper (OFC) one after the react process. To prevent winding collapse, these flanges consists of several pieces, and these pieces are replaced in sequence. The OFC flange and the coil winding are bonded with epoxy impregnation. To validate this new method, we fabricated a test coil with an inner diameter of 100 mm and a self-inductance of 8.3 mH by using a multistrand MgB2 wire. We cooled the test coil by a two-stage GM cryocooler and measured its critical current (Ic) at 25 K. The difference between the temperature of the OFC flange and that of the winding surface was within 2 K. The Ic was about 263 A, which had been expected to be 275.8 A in accordance with its load line and short-sample measurements. These results support the validity of our fabrication method.

Evaluation of Overdamping Behavior of a Vibrating Conductive Cylinder in a Strong Magnetic Field

Magnetomechanically coupled problems of conductive structures are generally known, but the focus of interest has been on underdamping, not overdamping. The purpose of this study is to verify the applicability of unidirectional weak-coupling analysis to the overdamping behavior of a vibrating thin conductive plate in a strong magnetic field, which is a necessary procedure for designing magnetic resonance imaging (MRI) equipment. An impact vibration experiment was done with a thin conductive cylinder in the 3-T field of an MRI magnet. Magnetic modal damping ratios and accelerations of the cylinder were calculated by eddy current analysis and frequency response analysis. As a result, specified peaks of the acceleration curves were found to disappear and the corresponding eigenmodes were the same for both the experiment and analysis. Furthermore, relative errors of the acceleration amplitude decrease ratio were in the range of 9.2%-31.9%; hence the calculated acceleration was on the conservative side. Consequently, the analysis method was judged suitable as a practical calculation method for designing MRI equipment that considers the overdamping behavior.