Electro-magnetic stress-induced degradation of insulation vacuum of a large cryo-magnetic system. (July 2016)

• Record Type: Journal Article
• Authors: Bhattachryya, Pranab; Gupta, Anjan Dutta; Dhar, S.; Pal, Gautam; Mukherjee, Paramita
• Is Part Of: Cryogenics. Volume 77(2016)
• Page Start: 8
• Page End: 14
• Publication Date: 2016-07
• Subjects: Insulation vacuum -- Residual gas conduction -- Superconducting magnet; Low temperature engineering -- Periodicals; Low temperature research -- Periodicals; 536.56
• DOI: 10.1016/j.cryogenics.2016.04.002
• Languages: English
• ISSNs: 0011-2275

Abstract

Highlights: Insulation vacuum of K-500 SCC cryostat degrades with increasing coil current. Experimentally quantified increment in heat load with insulation vacuum order. Incorporated an additional turbo pump which improved the insulation vacuum. Simulated electro-magnetic stress, using FEM, to diagnose the problem area. Estimated the possible leak size with increased current & simulated the crack zone. Abstract: In superconducting magnets, the cold mass is placed in a vacuum vessel to reduce heat load to the liquid helium system. Helium leaks into the vacuum vessel can degrade the insulation vacuum, which can, in turn, cause an increase in the heat load to the liquid helium system. These leaks are called cold leaks, as they show up when the coil is cooled with liquid helium. K500 superconducting cyclotron magnet at Variable Energy Cyclotron Centre, Kolkata has such cold leaks in the helium vessel that developed during cool down. The leak rate increases with the increase of current in the superconducting coils. This paper describes a series of experiments carried out on the superconducting cyclotron magnet to find the level of degradation of insulation vacuum and measure the increase in heat load with magnet current. The leak rate was also measured and the leak size was estimated analytically. Detail magneto-structural analysis was done using Finite Element Method (FEM) to identify highly stressed zones in the helium vessel and found out that highly stressed zones coincide with the weld zones. The magneto-structural stress was applied on an estimated size of single crack and found that crack tip stress could reach beyond elastic limit of the material. We can predict that the full design current may be unachievable in this situation. Mitigation of increased heat load was also done using an additional vacuum pump for the insulation vacuum space.