Patent ID: 7235975

Claim:
A magnet system of a magnetic resonance apparatus, the magnet system generating a magnetic field which is homogeneous and temporally highly stable within a volume under investigation and which extends along a z-axis, the magnet system comprising: a cryostat; a superconducting magnet coil disposed in said cryostat; a periodically operating refrigerator, said refrigerator having a magnetic regenerator material which is disposed in a regenerator housing within said cryostat in a stray field of said magnet coil; and means for shielding or compensating a magnetic disturbing field in the volume under investigation which is generated by periodic operation of said refrigerator due to changes in regenerator magnetization, said shield or compensating means comprising a compensation coil arrangement surrounding said regenerator housing, said compensation coil arrangement being structured and dimensioned for superconducting short-circuited operation thereof and comprising a plurality of conductor loops which are separately short-circuited during measurement to keep a plurality of magnetic fluxes through these conductor loops constant.