A new method for avoiding critical current degradation of YBCO coils using ice impregnation

Abstract

For mechanical reinforcement against the huge Lorentz forces in high magnetic fields, epoxy resins are commonly used to impregnate high temperature superconductor (HTS) coils. Due to the different thermal expansion coefficients between the epoxy resin and the HTS tape, the contraction of the epoxy resin causes critical current degradation in the HTS coils during the cooling process. Alternative techniques, such as coils with no insulation, can avoid current degradation, however their structure has no reinforcement between layers. In view of this, we tried to apply the ice impregnation technique of pulsed magnets to HTS coils for the first time. HTS coils impregnated with ice could avoid critical current degradation after thermal cycles. Also, local heat pulse tests and overcurrent tests showed that the ice impregnated coil had better thermal stability than the epoxy impregnated coil. Moreover, the fabrication process of the ice impregnated coil is simple and economical. Meanwhile HTS tapes can be reused after impregnation. The ice impregnation technique is a promising prospect for high temperature superconducting coils.

Review of materials for HTS magnet impregnation

“…Motokawa et al [84], does this by encasing the coil in a stainless steel shell, which is then packed with alumina filler, evacuated, filled with water, and slowly cooled using liquid nitrogen. Similar processes are described in [79,82,83,85]. The impregnation is made easy by the fact that there is no need to worry about usable life and the viscosity of water is less than 1/10 of typical epoxy [86].…”

“…However, the cracks may still pose a thermal impedance as explained in section 2.4. Wax also has negligible bonding strength to the conductor [79], making it impossible to delaminate the conductor even with large thermal expansion mismatches. Takematsu et al [7] demonstrates this in a study comparing paraffin wax impregnation to epoxy impregnation.…”

“…One common observation, shared by [79,[82][83][84] is that ice impregnation is uniquely easy and economical. Motokawa et al [84], does this by encasing the coil in a stainless steel shell, which is then packed with alumina filler, evacuated, filled with water, and slowly cooled using liquid nitrogen.…”

“…The impregnation is made easy by the fact that there is no need to worry about usable life and the viscosity of water is less than 1/10 of typical epoxy [86]. Additionally, the HTS tapes can be unimpregnated and reused [79].…”

Research on DC overcurrent characteristics and life prediction of YBCO tapes

“…A series of experiments showed that the technique of SFCL has the effect to delete the pulse, while the current distribution is highly peaked at the centre without SFCL [17]. The ice impregnation technique of pulsed magnets was applied to HTS coils for the first time [18]. The fault current limiting performance of YBCO-coated conductors (also called 2G HTS tapes) stabilized with stainless steel laminates was investigated [19].…”