The use of pressurized bladders for stress control of superconducting magnets

LBNL is using pressurized bladders in its high field superconducting magnet program. Magnet RD3; a 14 T race track dipole; has been assembled and pre-stressed using such a system. The bladder, plated between the coil pack and the iron yoke, can provide 70 MPa of pressure while compressing the coil pack and tensioning a 40 mm thick structural Aluminun shell, Interference keys replace the bladder's functionality as they are deflated and removed leaving the shell in 140 MPa of tension. During cool down, stress in the shell increases to 250 MPa as a result of the difference in thermal expansion between the Aluminum shell and the inner if on yoke. A number of strain gauges mounted onto the shell were used to monitor its strain during assembly cool-down and testing. This technique ensures that the anal and maximum stress in the shell is reached before the magnet is ever energized. The use of a structural shell and pressurized bladders has simplified magnet assembly considerably. In this paper we describe the bladder system and its use in the assembly of a 14T Nb3Sn magnet.