Magnetic field calculations for the KATRIN main spectrometer

The purpose of the KATRIN experiment is to determine the absolute neutrino mass with 0.2 eV sensitivity, by measuring the integral electron energy spectrum near the endpoint of tritium beta decay. Electric retardation with magnetic adiabatic collimation is used for the spectrum measurement, thus one can obtain optimal conditions for the statistics, energy resolution and background. The electrons are guided from the source to detector by magnetic field. The energy analysis of the electrons is made by the main spectrometer, where the magnetic field is only a few Gauss, much smaller than in other parts of the KATRIN system. The quality of the electron spectrum measurement is sensitive to the properties of the magnetic field in the main spectrometer: i, the magnetic field lines coming from the tritium source are not allowed to go to the spectrometer wall, they have to go to the detector, therefore one has to correct the magnetic field of the superconducting coils by axysymmetric air coils around the main spectrometer, and one has to compensate also the Earth magnetic field by an appropriate air coil system; ii, the superconducting coils near the main spectrometer have to fulfill various requirements; iii, magnetic materials close to the spectrometer (for example, the steel in the building) change the magnetic field, and this disturbance could have an important effect to the background. (orig.)