The FOM-Institute for Plasma Physics Rijnhuizen is preparing the construction of Magnum-psi, a magnetized (3 T), steady-state, large area (80$ cm^2$) high-flux (up to$10^24 H^+ ions m^-2 s^-1$) plasma generator. The aim of the linear plasma device Magnum-psi is to provide a controlled, highly accessible laboratory experiment in which the interaction of a magnetized plasma with different surfaces can be studied in detail. Plasma parameters can be varied over a wide range, in particular covering the high-density, low-temperature conditions expected for the detached divertor plasma of ITER. A vital part of the Magnum-psi experiment is the superconducting magnet system, which generates a magnetic field of 3 T while good diagnostic access to the experiment is guaranteed. In this contribution, we will explain the requirements on the magnet system, which is now in the pre-design phase. The present design consists of 3 cylindrical NbTi coils which generate a plateau shaped field of 3 T in a 1.3 meter room temperature bore. The discrete coils are supported by a 2.4 meter long single cylinder in a shared cryostat with 16 room temperature view-ports of 200 mm diameter. The field will most probably be passively shielded by an iron wall surrounding the experimental area. As background, some elements of the pre-design of the Magnum-psi experiment; i.e. vacuum system, plasma source and diagnostics are presented.