Transient Cooldown Stresses in Superconducting Collider Dipole Magnet

Abstract

The three-dimensional thermal transient stress in a collider dipole magnet (CDM) is evaluated and examined. The cooldown rate is one of the factors that determines the availability of the Superconducting Super Collider ring, but it is limited by stress in the magnet during cooldown. The main concerns are the effects of coil stress on stable operation and of shell stress on survival in long-term operation. Cost-effective operation can be attained if fastest cooldown-induced magnet stress would not exceed the safe operating stress of the magnet.

A 50-mm dipole magnet is modeled in the vertical split yoke configuration. Magnet components include shell, yoke, end yoke, collar, coil, and kapton insulation. These components are integrated by 3-D interface elements with preload but without friction. The cooldown condition is established by 4 K helium at 96 grams per second (gps) flowing in four bypass tubes and 4 gps to the beam tube annular area. The cooldown time is about 2 hours in most of the center section of the magnet and about 4 hours for the end sections, assuming that the end plate and the end yoke are also cooled down by the bypass flow.** Purposes of this study are to understand the stress-time history and maximum stress generated by the possible fastest cooldown condition and to demonstrate that non-linear thermal transient stress analysis can be performed in a workstation with limited computing resources. The results can be utilized to evaluate (1) the stress of the shell in long-term operation, (2) the fastest safe cooldown rate in the CDM, and (3) the cooldown time for the specified cooldown rate.