  The PERC (Proton and Electron Radiation Channel) facility is currently under
construction at the research reactor FRM II, Garching. It will serve as an
intense and clean source of electrons and protons from neutron beta decay for
precision studies. It aims to contribute to the determination of the
Cabibbo-Kobayashi-Maskawa quark-mixing element $V_{ud}$ from neutron decay data
and to search for new physics via new effective couplings. PERC's central
component is a 12m long superconducting magnet system. It hosts an 8m long
decay region in a uniform field. An additional high-field region selects the
phase space of electrons and protons which can reach the detectors and largely
improves systematic uncertainties. We discuss the design of the magnet system
and the resulting properties of the magnetic field.
