トレーニング防止対策を施した巻枠のない超伝導ソレノイド電磁石

抄録

A superconducting bobbinless solenoid has been constructed and successfully excited without training up to the critical current of the conductor. The central field is 5.86T at the maximum overall coil current density of 300A/mm². The inner diameter of the windings is 19.0cm, the outer diameter is 23.4cm, and the length is 26.7cm. The conductor is a NbTi/Cu keystoned compacted strands cable whose critical current is 4.6kA at 6.3T and 4.2K. The magnet consists of 2 layers with 345 turns of edgewise windings, a GFRP outercylinder and two sheets of cooling channel between them. It has no metallic part except the superconducting cable. Therefore, it is suitable for pulsed operations. Each component has been bonded with epoxy resin and the magnet has been shaped in one body with hydraulic presses under the radial pressure of 14.7MPa and the axial pressure of 29.4MPa. The pressures were determined so that the stresses in the windings become almost the same in the order of magnetudes and directions as thosewhich will be produced by the Lorentz force. The local wire movement considered as one of the causes of training has been reduced in this magnet.
During e ci ation, however, the magnet deforms elastically. These strains of the windings in the axial and circu f rential directions were measured by strain gauges at the center turn of the magnet. At the same time the voltage oscillations considered to synchronize with mechanical vibrations of the magnet were measured by voltage taps. These voltage taps having intervals of 2, 5, 10.5cm were attached to the cable at various positions in the windings. From these experiments, it was found that the change of the axial strain was not uniform but stepwise for the change of the magnet operating current. In addition, the mechanical vibrations were induced by the sudden deformation of the windings.
The thermal stabilities of the magnet were investigated by the heating experiments. A heater was placed on the inner surface of the bobbinless solenoid. The energy dissipated at the heater to quench the magnet and the propagation velocities of normal zone were measured.
The static stresses and deformations in the magnet under the pressurized curing, cooling down and excitation were analyzed by a finite element method, using the program ISAS II which is the Japanese version of NASTRAN. The principal vibrations of the magnet were also analyzed, and the deformation modes were studied. From these data of stresses, strains and vibrations, the magnitudes of mechanical disturbances were estimated. On the other hand, the minimum quench energy of the magnet was estimated by the computer simulations of heating experiments.
As a result, the following things become clear. Even in the case of this magnet, the mechanical disturbances have not been completely removed. These disturbances are induced by the irregular contractions of the windings in the axial direction, and they can trigger the quench, if the cooling condition of the magnet is getting worse.