My invention relates to superconducting magnets, and more particularly to support structures for ring-shaped superconductive coils.
A plurality of ring-shaped superconductive coils having a common longitudinal axis are useful in establishing a large magnetic field, aligned with such longitudinal axis, in the vicinity of the axis. Such a magnetic field is required, for example, in nuclear magnetic resonance (NMR) imaging apparatus. In such apparatus, the subject of inquiry, such as an entire human body, is placed in the region of high magnetic field and exposed to selected radio-frequency electromagnetic waves. The atoms of the human body re-emit radio-frequency electromagnetic waves at different frequencies depending on the identity of the atoms. Detection of such different frequencies reveals the identity of the atoms, and this information is used to generate images of internal body structures.
Superconductive coils must be maintained below a critical, extremely low temperature, typically about 4.degree. Kelvin (i.e., 451.degree. Farenheit below zero) in order to be superconducting. When superconducting, these coils are capable of carrying electric current at an extremely high density, permitting the attainment of extremely high magnetic fields. Any heating of a superconducting coil that raises the temperature of even a single, localized part of the coil above the critical temperature results in the entire coil losing superconductivity due to rapidly spreading I.sup.2 R or resistive heating of the coil.
A ring-shaped superconductive coil typically comprises a winding of superconductive wire impregnated with epoxy so as to form a monolithic structure. The epoxy-impregnation in large measure prevents relative movement between the individual wires of the winding that would give rise to undesirable frictional heat generation.
Heat generation in a superconductive coil can also result from interaction of the coil and its support structure which maintains the coil in a desired position. A first component of such heat generation is the frictional heat generated from relative sliding movement between the coil and the support structure. A second component of such heat generation results from relative movement of internal coil strands, typically transitory and of minute magnitude, which is induced from high stress in the winding, either compressional or tensile in nature. These transient movements give rise to a phenomenon known in the art as "training" of the coil. A coil in the process of training experiences interior movement during coil energization, even though it is epoxy-impregnated. Such movement causes the coil to become frictionally-heated above the critical temperature and lose superconductivity at a lower than rated current density, and also requires cessation of current therethrough to avoid excessive resistive heating of the coil. Upon cooling of the coil below the critical temperature and reenergization thereof, it typically experiences further interior coil movement, although usually less pronounced, again resulting in frictional heating of the coil above the critical temperature; however, this occurs at a higher current density if the interior coil movement is less. Cycles of cooling and reenergization of the coil continue, with the coil attaining higher current densities as interior coil movement subsides. In this way, coils are "trained" to withstand the high stresses imposed upon them, although such training is not always permanent.
Training of a coil is not without considerable costs arising, for example, from the required cycles of coil cooldown and reenergization. Additionally, the container in which the cooling medium for the coil (i.e., the cryostat) is located must be designed to withstand repeated heating and pressure cycles, so as to prevent loss of cooling medium. Thus, it would be desirable to provide a coil support structure that interacts with a coil in such a way as to reduce stresses in the coil that would require training of the coil.
A commercially-available superconducting magnet including ring-shaped superconductive coils utilizes a support structure comprising an aluminum body of generally-cylindrical shape and which has outwardly-opening recesses extending around the circumference thereof. Ring-shaped superconductive coils are manufactured in the recesses by winding superconductive wire directly into the recesses and then impregnating the resulting winding with epoxy. These coils, however, are undesirably subject to two different sources of heating, both arising from the fact that a ring-shaped superconductive coil tends to expand in diameter upon energization. One potential source of heating constitutes relative sliding movement between the axial sides of the coil and the adjacent walls of the recess in which the coil is formed. Another potential source of heating results from the high tensile stresses that arise in the expanded coil and undesirably may require that the coil be trained.