The KATRIN experiment will probe the neutrino mass by measuring the -electron
energy spectrum near the endpoint of tritium -decay. An integral energy analysis will be performed
by an electro-static spectrometer (“Main Spectrometer”), an ultra-high vacuum vessel with a length
of 23.2 m, a volume of 1240m3, and a complex inner electrode system with about 120 000 individual
parts. The strong magnetic field that guides the -electrons is provided by super-conducting
solenoids at both ends of the spectrometer. Its influence on turbo-molecular pumps and vacuum
gauges had to be considered. A system consisting of 6 turbo-molecular pumps and 3 km of
non-evaporable getter strips has been deployed and was tested during the commissioning of the
spectrometer. In this paper the configuration, the commissioning with bake-out at 300 C, and the
performance of this system are presented in detail. The vacuum system has to maintain a pressure in
the 10−11 mbar range. It is demonstrated that the performance of the system is already close to these
stringent functional requirements for the KATRIN experiment, which will start at the end of 2016