Abstract: A method for using ferromagnetic shims to reduce the inhomogeneity of the magnetic field in a Diameter Spherical Volume (DSV) in the bore of a Magnetic Resonance Imaging (MRI) device is provided. More specifically, a magnetic field is established within the bore of the MRI device. An operator defines the DSV. The strength of the magnetic field is measured at the surface of the DSV. A Legendre polynomial is derived to model the inhomogeneity in the magnetic field in the DSV. A set of coefficients for the harmonic terms of the polynomial is calculated from the field strength measurements and segregated into three separate groups for evaluation. Each group of terms expresses field inhomogeneity in a unique set of spatial dimensions.

Abstract: A source of coherent radiation. The source includes an evacuated drift tube having an input end for receiving a beam of relativistic electrons and an output end for providing coherent radiation. A relativistic electron supply provides the beam of electrons to the drift tube input end, and a magnetic field is provided for causing the relativistic electrons to spiral. An azimuthal wiggler causes the spiraling electrons to undergo accelerations while an axial magnetic field is provided for controlling the gyroradii of the spiraling electrons acted upon by the wiggler.