薄肉超電導マグネットの問題点

抄録

Large superconducting solenoid magnets are used in high energy physics experiments with colliding beam accelerators. Since solenoids are surrounded with other detectors, they are required to be thin in terms of radiation length in order for particles produced by interactions to pass through the coils with minimal absorption. Various techniques have been developed for construction of large, thin superconducting solenoids. First, heavier materials of solenoid components are substituted by lighter materials of aluminum alloys. Second, the forced flow cooling method of two-phase helium is applied since the pool cooling requires a substantially thicker cryostat arrangement. Since such solenoids are not cryogenically stable and since they hold large stored magnetic energies of about 10×10⁶J or more, it is essential to make them intrinsically safe against quenches.
The CELLO solenoid used pure aluminum as the stabilizer of the NbTi/Cu superconductor, while the TPC solenoid used the conductive bore tube for diverting the magnetic current from the superconducting coil to protect the coil against quenches. The CELLO-type aluminum stabilized conductor can be fabricated intrinsically stable and it has the advantage that the maximum voltage and temperature at the coil during quenches can be easily controlled by optimizing the amount of aluminum stabilizer.
The aluminum-stabilized NbTi/Cu superconductor with the EFT method is used in the CDF solenoid (3mφ×5m, 1.5T, 30×10⁶J). Two solenoids for TRISTAN experiments (TOPAZ and VENUS) also use conductors fabricated essentially with the same method as in the CDF solenoid. All the three solenoids do not possess permanent inner bobbins in order to reduce the coil material thickness. The shrink-fit method is used for the CDF solenoid, while the inner winding method is used for the TOPAZ solenoid.
Various technical difficulties with construction of large, thin superconducting solenoids will be described.