Magnetic device having a yoke member for generating a magnetic stray field

A homogeneous field magnet used in nuclear spin tomography comprising:

• a) a magnetic circuit which includes a yoke member, said yoke member being made of soft magnetic material having a finite relative permeability;
• b) a magnetic field generating device generating a magnetic flux in said yoke member and also generating a magnetic stray field due to a magnetic dipole moment in said yoke member, which in turn is due to the relative permeability of said yoke member;
• and
• c) a compensating-field winding, said compensating-field winding, surrounding a section of said yoke member, being electrically excited so that, in addition to the magnetic flux produced by the magnetic field generating device in the yoke member, a supplemental magnetic flux of a magnitude is created such that the magnetic dipole moment of the yoke member that produces the stray field is at least partially compensated for.

In a device having

• a yoke member, defining a void of useful volume, and
• at least one excitation coil, coupled with the yoke member, generating a magnetic flux through a path defined by the yoke member, and generating a required magnetic field, having a magnetic field strength in the useful volume, a method for compensating a stray field due to a dipole moment formed in the yoke member due to a relative permeability of the yoke member, comprising the steps of:
• a) determining the length of the path defined by the yoke member;
• b) determining the required magnetic field strength;
• c) providing a compensating winding around the yoke member, carrying a predetermined current and having a number of turns equal to the product of the required magnetic field strength determined in step (b) with the length of path determined in step (a), divided by the predetermined current.