Magnet assembly comprising closed superconducting HTS shims

A magnet assembly for a magnetic resonance apparatus comprising:

• a cryostat;
• a superconducting main field magnet coil system positioned in the cryostat for generating a magnetic field along a z-axis in a working volume centered around z=0 on the z-axis;
• and
• a shim device positioned inside the cryostat for adjusting a homogeneity of the magnetic field generated in the working volume by the main field magnet coil system, the shim device comprising a shim conductor path having a high temperature superconductor (HTS) layer in a single closed loop with a winding number of 0 about the z-axis wherein the HTS layer is geometrically developable such that unwrapping the HTS layer onto a plane changes the geodesic distance between any two points on the HTS layer by no more than 10%, and
• wherein an inner contour or an outer contour of the geometrical development of the HTS layer describes a non-convex curve.