A scheme for the determination of the magnetic field in the KATRIN main   spectrometer
  To determine the magnetic field distribution in the KATRIN main-spectrometer with magnetic field sensors that are placed outside the main-spectrometer vessel one can utilize the absence of magnetic rotation in main-spectrometer volume. There a scalar magnetic potential V(~x) can be defined that fulfills the Laplace equation. Large numbers of magnetic field values on an outer surface of the main-spectrometer can be sampled by moving and fixed magnetic field sensors. These surface samples are used as boundary values in the relaxation of the Laplace equation for V(~x) and the magnetic field components in the volume. In a simulation involving the KATRIN reference solenoid chain, a global magnetic field and an external perturbing solenoid it is shown that with this method the original field can be reconstructed within 2 %. 
A. Osipowicz, U. Rausch, A. Unru, B. Zipfel
physics.ins-det