A large superconducting thin solenoid magnet for TRISTAN experiment(VENUS) at KEK

A 30 Gev e+e- collider TRISTAN is now under construction at KEK. The detector system VENUS in TRISTAN requires a large space of 7.5 kG magnetic field for the tracking of the particles while keeping the material thickness of the magnet as thin as possible. The superconducting thin solenoid magnet which is in the middle of the construction has 3.4m in warm bore diameter and 5.24m in usable length with material thickness of 0.52 radiation length. The geometrical thickness of the magnet is as thin as 208 mm while the outside dimension is as large as m. Since the electro-magnetic force is toward outside, the coil is supported by an aluminum \´case\´ covering outside the coil instead of a bobbin in the coil. The water cooled welding technique and an expandable mandrel were developed to form the coil-case composite. The conductor is made of NbTi/Cu and pure aluminum stabilizer is extruded so as to contain the conductor in the stabilizer. The cooling of the magnet is provided by the forced flow of two phase helium in the cooling channel of the \´case\´. The cryostat uses CFRP (Carbon Fiber Reinforced Plastics) to reduce the material thickness of the magnet. The construction method and the design were confirmed by superconductive model and real size non-superconductive model. Major components such as conductors and cryostat have been completed and the magnet will be excited next spring.