Determination of analysis plane magnetic field in the KATRIN main spectrometer

The Karlsruhe Tritium Neutrino (KATRIN) experiment aims to measure the effective electron anti-neutrino mass with a sensitivity of 0.2 eV (90% C.L.) by investigating the endpoint region of the $\beta$ decay spectrum. The experimental setup of KATRIN consists of a tritium source, from which the decay electrons are magnetically guided through the transport section towards two Mac-E filters (pre- & main spectrometer). The spectrometers act as integrating high pass filters with an energy resolution defined by the magnetic field strength in the spectrometer's center and the Pinch magnet. Only electrons near the energetic endpoint of the decay spectrum are transmitted to the detector. A precise magnetic field model is of high importance for the neutrino mass analysis, as slight magnetic field deviations can significantly influence the spectrometer's energy resolution. This poster presents a method to precisely determine the magnetic field inside of the main spectrometer based on field values measured outside of the vessel, with KATRIN's radial magnetic monitoring system. The systematic uncertainty of the model is analyzed as a function of the applied magnetic field setting. The results of the study are essential for deciding which magnetic field setting to use in the first neutrino mass measurements.