A nuclear magnetic resonance (NMR) apparatus (10) comprises a superconducting main field magnet coil system (14) which generates a homogeneous magnetic field of at least 3T, and a gradient coil system (15) which generates a gradient strength of at least 10 mTm−1, with a slew rate of at least 100 Tm−1s−1, wherein the main field magnet coil system (14) is arranged in a cryostat (12) with liquid helium and a refrigerator (16) in the form of a pulse tube cooler or a Gifford-McMahon cooler, and wherein an evaporation line (17a, 27a, 37a) is provided for helium that might evaporate from the cryostat. In all states of operation of the NMR apparatus (10) without gradient switching, the refrigerator provides a cooling capacity which is at least 0.3 W above the overall power loss of the cryostat, the evaporation line terminates in a buffer container (19, 29, 39, 39′) disposed outside of the cryostat for collecting the evaporating helium, and a return line (17b, 27b, 37b) is provided for returning the evaporated helium that has been collected in the buffer container into the cryostat. For this reason, a considerable amount of evaporating helium is saved, thereby considerably extending the refill cycles of liquid helium, which greatly disturb and obstruct normal work cycles and are disadvantageous for operation, in particular, in hospitals.