A magnet system of a magnetic resonance apparatus for generating a
magnetic field which is homogeneous and temporally highly stable within a
volume under investigation and which extends along a z-axis, with a
superconducting magnet coil (3) which is disposed in a cryostat (1) and
with a periodically operated refrigerator (2) having a magnetic
regenerator material which is disposed in a regenerator housing (5)
within the cryostat (1) in the stray field of the magnet coil (3), and
which is provided with a device (4) for shielding or compensating a
magnetic disturbing field in the volume under investigation generated by
the periodic operation of the refrigerator (2) due to regenerator
magnetization changes is characterized in that the regenerator housing
(5) is surrounded by a compensation coil arrangement (4) which can be
superconductingly short-circuited and which is made from a plurality of
conductor loops (6, 7a, 7b) which are separately short-circuited during
measurement to keep a plurality of magnetic fluxes through these
conductor loops (6, 7a, 7b) constant in the superconductingly
short-circuited state. A magnet system of this type facilitates effective
compensation of the disturbing fields caused by a magnetic regenerator
material at the location of the volume under investigation.