Design and stress analysis of in-vessel saddle coils for MHD control in ASDEX Upgrade

A set of in-vessel saddle coils for MHD control in ASDEX Upgrade is described. A conventional coil design is chosen which employs a five turn square cross-section copper conductor with central cooling bore embedded in an epoxy resin. The winding is enclosed in a solid metal casing, made of a thin deep-drawn inconel sheet in order to maximise the electrical resistance and minimise L / R and shielding of the magnetic field. The bending stress in the coil casing due to the mount is reduced by reinforcing ribs perpendicular to the current direction. Fatigue stresses are avoided by application of a pre-stress on the casing exerted by the mounting bolts. Finite element stress calculations are performed to verify the design. Secondary stresses induced by thermal expansion in the casing and the conductor are well tolerable. The structural stiffness of the coil design provides a high resonance frequency, above the operation frequency range f = DC … 1.2 kHz.