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Description
Accelerator magnets based on Nb3Sn superconductor are often prone to severe training. Previous works in literature found that stored strain energy in the coil’s impregnant, usually epoxy resin, is released as heat during cracking, debonding and subsequent friction. This heat affects locally the temperature margin of Nb3Sn eventually leading to a quench, and it can contribute to elongated magnet training. However, the relation between the mechanical strain energy release rate of the magnet impregnant and thermal heat generation during cracking at cryogenic temperatures is not well known and it is investigated in this study.
To quantify this relation, a compact tension double cantilever beam (CT-DCB) type of experiment at 77 K was chosen. This type of mechanical experiment allows to measure the strain energy release rate from the load-displacement curve for a certain initial crack under cryogenic conditions. The CT-DCB sample is made of CTD-101K, a typical epoxy magnet impregnant these days often used in accelerator magnets. It is further prepared to mimic cracks in the impregnant near the Nb3Sn wire in the coil windings. Therefore, a copper tube insulated with S-2 fiberglass mimicking the Nb3Sn wire is embedded in the CT-DCB sample. After fracture, a deposited thermal energy of 100 +/- 10 J/m2 was measured with a thermocouple positioned inside the copper tube. This energy matches closely half the strain energy release rate measured to be at the level of 220 +/- 20 J/m2. The latter is a value reported in literature. This let us to believe that the mechanical strain energy is almost integrally converted into thermal energy on the two fracture surfaces.
In combination with a descriptive model for the minimum quench energy of the Nb3Sn superconductor, this experimental result can help to better understand and quantify the relation between the locally stored strain energy in the impregnant and causes of training in Nb3Sn magnets.
