Epoxy Impregnated Solenoid Coil Training Dependence on Boundary Forces and Peak Field

Abstract

Five solenoid coils are required for the axial component of a Minimum B magnetic field of a confined plasma ion source under construction at NSCL. The design requires an inside radius of 0.15 m and a coil current density of 12 kA/cm² at a B_max of 3.5 T. The initial fabrication scheme was envisaged to be wet wound coils in pockets on the outside of a stainless steel bobbin, (the inner surface being reserved for a six coil superconducting hexapole magnet that completes the magnetic structure). Finite element analysis indicated the possibility of large internal and boundary stresses on the epoxy/coil composite, and we have completed several empirical studies of this geometry. We have measured bobbin to epoxy bond strengths, and have found that the best, most consistent results are obtained by acid etching the stainless steel for good epoxy adhesion. A test arrangement of three coils with differing boundary conditions (potted in pocket, potted in lineal pocket and a free coil) reached, respectively, 40, 48 and 58 kA/cm² after training. Finite element analysis indicate the probable failure mode of the fixed coil to be boundary stress and for the free coils, internal shear is the indicated failure mode.