Effect of ground insulation contact on stability of epoxy-impregnated superconducting solenoids

To clarify the effect of ground insulation contact on the stability of epoxy-impregnated superconducting solenoids, test coils with ground insulation between a winding and a metal former were fabricated. Aluminium and stainless steel were used as the former materials. The training effects of coils and the circumferential strains of former inner surfaces were measured. The training effects of aluminium former coils were larger than those of the stainless steel former coils. The strains of stainless steel formers became greater as the exciting current increased, but the strains of the aluminium formers did not change. These results show that the insulation of aluminium former coils became a floating state, the insulation of stainless steel former coils maintained a pressed state, and the floating state caused the large training effects.