The Differential Pumping Section
The DPS2F is a pumping section consisting of five 1 m long beam tubes within a superconducting solenoids. To block the line of sight towards the spectrometer for the diffusing, neutral tritium molecules two sections are tilted by 20 degree. Between each section there is a turbomolecular pump with a nominal pumping speed larger than 2000 l/s.
In a series of dedicated test measurements it was possible to show, that the reduction factor of the DPS amounts to 2.5x104 [1]. Besides the tritium flow reduction the beam pipe of the DPS hosts instrumentation to analyze and reduce the ion flow. Ions, mostly produced by the decay of tritium into positive charged Helium, follow as charged particles or in form of positive charged molecules the magnetic fields and are blind against the pumping measures. But again, also the charged tritium is not allowed to enter the spectrometers, in order to keep background levels for the neutrino mass search sufficiently low. One trick is to bias the WGTS source and DPS by a little but known negative voltage, this prevents the positive charged ions or molecules to leave the source and DPS sections. However, one has to get rid of the positive ions, as otherwise by time the source and DPS would charge significantly electrically up. To actively remove positive ions the DPS host with dipole electrodes and Fourier transformation Ion Cyclotron Resonators two kind of instrumentation to analyze the ion species (FTICR) and sweep them out of the beam tube (FTICR abd dipoles).
References:
[1] Measurement of the gas-flow reduction factor of the KATRIN DPS2F differential pumping section
S. Lukic et al.,
Int. J. Mass Spectrom. 288, 1-5 (2009) 
[2] A broad-band FT-ICR Penning trap system for KATRIN
M.Ubieto-Díaz, D. Rodríguez, S. Lukic, Sz. Nagy, S. Stahl, K. Blaum
accepted for publication the Vacuum journal;
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