The Cryogenic Pumping Section of the KATRIN Experiment

In order to determine the absolute scale of the neutrino mass with a sensitivity of 0.2 ${\rm eV}/{\rm c}^{2}$ (90% Confidence Level), the Karlsruhe Tritium Neutrino experiment (KATRIN) operates a series of superconducting magnet systems, which guide the electrons adiabatically from the source of tritium beta-decay to the detector within a magnetic flux of 191 ${\rm Tcm}^{2}$. The 7 m long Cryogenic Pumping Section (CPS) is designed as the final barrier of tritium circulation. It has to reduce the tritium partial pressure below $10^{-14}$ Pa in order to limit the background count rate in the measurement. To achieve this, the tritium entering the CPS must be adsorbed onto a pre-condensed argon layer on the inner surface of the beam tube at a temperature of 3 K. The zigzag arrangement of the magnet modules increases the efficiency of tritium retention, but makes the transition of the magnetic flux rather complicated. The solenoids are operated in persistent mode with a central magnetic flux density of 5.6 T. The field drop of the magnet has to be less than 0.1% over one month. This report describes the design of the CPS and the current status of the project.