Influence of gas dynamics in the tritium source on the neutrino mass measurement of KATRIN

The Karlsruhe Tritium Neutrino Experiment (KATRIN) aims to measure the neutrino mass with a sensitivity of 200 meV/c² (90% C.L.) in a direct approach using the beta decay of molecular tritium. The neutrino mass is extracted from a fit of modelled beta decay spectra to the measured electron spectrum. Hence, it is important to include modifications from systematic effects in the simulated spectrum. Especially the gas dynamics, density and velocity distribution, in the windowless gaseous tritium source (WGTS) play a key role for accurate modelling. Since in most cases this can not be measured directly, the modelled beta spectrum relies on gas dynamics calculation as well as on monitoring of operation parameter changes. A comprehensive pseudo-3D model has been developed. The accuracy of the gas dynamics model in the spectrum simulation including the monitoring of operation parameters is reviewed and implications on the systematics budget for the neutrino mass measurement are described.