The design of the KATRIN spectrometers is based on a novel electromagnetic concept: The retarding high voltage (HV) is directly connected to the hull of the spectrometer vessel. A nearly massless inner wire electrode at slightly more negative potential than the vessel itself suppresses low-energy electrons emanating from the inner surfaces of the spectrometer walls, representing a potential source of background. In addition this inner electrode fine-tunes the electrostatic field to avoid the occurrence of Penning traps in corners and to optimize the adiabatic transmission properties of the MAC-E filter.

The spectrometer section of KATRIN consists of two electrostatic spectrometers in a ’tandem setup’. Electrons have to pass the smaller pre-spectrometer first, which works at a fixed retarding potential, acting as a pre-filter. In the normal tritium mode of the experiment it will reject low-energy electrons below 18.3 keV, which do not carry information on the neutrino mass. The remaining electrons enter the second, much larger spectrometer, where the energy spectrum close to the β decay endpoint is scanned with an energy resolution of 0.93 eV. The advantage of this tandem setup is the reduction of the total flux of electrons from the tritium source into the pre-spectrometer by a factor of 10⁻⁶. This will minimize the background from ionization of residual gas molecules in the main spectrometer. In addition the pre-spectrometer serves as a test facility to validate the extreme high vacuum (XHV) and electro-magnetic design concepts for the larger main spectrometer. In particular the requirement to reach and to maintain XHV conditions with a pressure of < 10⁻¹¹ mbar in both spectrometers constitute a major technological challenge for the KATRIN project.

The main spectrometer has a length of 23.3 m and a diameter of 10 m at the central part. The dimensions of the pre-spectrometer are 3.38 m and 1.68 m, respectively.