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The normal operation of various superconducting magnets is greatly influenced by the thermal stress of the insulating system, commonly made of epoxy resins with vacuum pressure impregnation (VPI) technology, throughout the curing and cooling processes. In this work, we developed a Fiber Bragg Grating (FBG) strain measurement method to monitor the real-time strain responses of an epoxy resin IR-3 and its glass fiber-reinforced composite during both curing and cooling processes. Then, we also monitored the same process of a Nb–Ti superconducting magnet coil prepared by VPI technology. With the help of FBG strain sensors, the residual strains of the coil at various positions and directions were investigated. The results show that the large residual strain occurred near the end of the coil in the axial direction. In addition, the interlaminar shear stress properties were measured at both room temperature and liquid nitrogen temperature. The strain characteristics of the epoxy resin IR-3 and the insulating system can provide useful guidance for the design and construction of high-field Nb–Ti superconducting magnets. [ABSTRACT FROM AUTHOR]

• The tree structures, inception probability, growth rates at 77 K were studied. • The Raman spectra of the degradation products within the tree channels at room temperature and 77 K were analyzed. • The effects of oxygen and graphitic carbon on the tree characteristics of epoxy resin were analyzed. The insulation structure which is mainly composed of epoxy resin is a key part of the superconducting magnet system. Due to the special operating environments, superconducting coils must address the challenges of cryogenic liquids and various kinds of high voltages. In this paper, the electrical tree characteristics of epoxy resin at 77 K were investigated. An experimental setup with a partial discharge (PD) device and a cryostat filled with liquid nitrogen (LN 2) was set up. Optical microscopy and confocal Raman spectrometer were applied to investigate the accumulation of degradation products within the tree channels. Each group of samples was tested at a range of AC voltages (50 Hz) from 8 kV rms (root mean square) to 20 kV rms and the PD experiments were carried out at both room temperature and 77 K. The results indicated that the color of electrical trees was darker and the tree structures were more complicated at 77 K compared with those at room temperature. The weak reaction between oxygen and the degradation products within the tree channels was responsible for the darker tree channels at 77 K. The density of electrical trees increased and the damage to materials caused by electrical trees was more serious at 77 K. Meanwhile, the tree inception probability decreased and the tree growth was inhibited obviously at 77 K. It's also revealed that the degradation products within the tree channels were confirmed to be graphitic carbon based on the corresponding Raman spectra. The size of graphitic domains within the tree channels at 77 K was more than 2 times of that at room temperature, which contributed to the tree growth acceleration in the later growth stage at 77 K. [ABSTRACT FROM AUTHOR]